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PIRT Report for NuScale Design-basis Boron Dilution Transients
	ML24064A042
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Issue date:	10/31/2022
From:	Stephen Bajorek, Robert Gladney, Peter Lien, Shanlai Lu, Jacqueline Thompson, Peter Yarsky; NRC/RES/DSA/CRABII
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	Yarsky P
References
	ML22318A183, UN NRR-2021-019
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PHENOMENON IDENTIFICATION AND RANKING TABLE (PIRT) REPORT FOR NUSCALE DESIGN-BASIS BORON DILUTION TRANSIENTS UN NRR-2021-019 Task 7

PIRT Panel Members:

Stephen M. Bajorek Peter Lien Shanlai Lu Jason Thompson Peter Yarsky

PIRT Facilitator:

Jason Thompson Robert Gladney

October 2022

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Division of System Analysis

This page was intentionally left blank.

2 0 Executive Summary

Under NRR-2021-019 (Ref. 1) and by an email (Ref. 2) dated April 28, 2022, the Office of Nuclear Reactor Regulation requested that the Office of Nuclear Regulatory Research staff convene a panel to identify the importance of the phenomena affecting the core integrity and coolabiltiy during postulated design-basis events with significant boron dilution and redistribution in the NuScale US460 small modular reactor (SMR) in accordance with the process described by Regulatory Guide 1.203 (Ref. 3). The panels findings, which built upon previous work (Ref. 4, Ref. 5, Ref. 6), are documented in this report as a Phenomena Identification and Ranking Table (PIRT).

The panel convened several meetings to reach consensus on the limiting scenarios, figures of merit, identification of phenomena, the importance ranking for those phenomena, and the ranking of the associated knowledge level for each phenomenon. At the conclusion of these meetings, the panel compiled the PIRT.

The panel identified the following three scenarios as potentially limiting: a small break loss of coolant accident (SBLOCA) in the liquid space of the Chemical and Volume Control System (CVCS) line, inadvertent actuation of one Reactor Recirculation Valve (RRV), and loss of offsite power (LOOP). The panel identified the average boron concentration in the downcomer and the average boron concentration in the lower plenum as the figures of merit. In total, the panel considered nearly 300 phenomena occurring in various components through the primary, secondary, Decay Heat Removal System (DHRS), Emergency Core Coo ling System (ECCS),

and containment (CNV) systems of the NuScale Power Module. The importance and knowledge level rankings for the phenomena are summarized in Table 4 of this report.

After reviewing the final consensus rankings, the panel identified four phenomena with high importance rankings but low knowledge levels. They are:

Species stratification in the CNV,

Boron dissolving in the dissolver basket located in the CNV,

Boron plate out on the Reactor Coolant System (RCS) pressure bo undary surfaces, and

Boron dissolving in the RRVs,

The panel identified 22 phenomena with high importance rankings but medium knowledge levels:

Internal natural circulation in the CNV that is driven by the temperature difference between the Reactor Pressure Vessel (RPV) and CNV bulk fluid temperatures,

Thermal stratification in the CNV,

Vertical/radial natural circulation in the liquid in the CNV,

Boron precipitation in the core/subchannel,

Vertical/radial natural circulation in the core/subchannel,

Heat transfer on the pool-side of the DHRS condenser tubes,

Condensation heat transfer on the inside of the DHRS condenser tubes,

Natural circulation within the DHRS flow loop,

Two-phase pressure drop in the DHRS tubes,

3

Flow oscillation and reversal in the RPV,

Species stratification in the downcomer,

Thermal stratification in the downcomer,

Vertical/radial natural circulation in the downcomer,

Riser hole flow,

Orificing/pressure drop in the riser holes,

Two-phase level swell in the hot leg riser,

The flow regime on the core/riser-and downcomer-sides of the riser holes,

Vertical/radial natural circulation in the hot leg riser,

Species stratification in the lower plenum,

Thermal stratification in the lower plenum,

Vertical/radial natural circulation in the lower plenum, and

Condensation heat transfer on the uncovered steam generator (SG) tubes.

The panel identified eight phenomena with at most medium importance ranking but a low knowledge level:

Boron dissolving in the CNV,

Boron dissolving on the RCS pressure boundary surfaces,

Boric acid volatilization in the subchannel,

Turbulent mixing in the subchannel,

Boron plate out in the downcomer,

Boric acid volatilization in the hot leg riser,

Boron plate out in the hot leg riser, and

Boron plate out in the lower plenum.

These phenomena will be evaluated in a follow-on task to this P IRT to determine the applicability of TRACE to evaluate boron dilution transients for the NuScale US460 design. In the follow-on task, similar to what has been done previously in Ref. 6, the phenomena will be evaluated based on the following criteria:

Category A - TRACE is considered fully applicable and can be expected to accurately simulate associated processes and phenomena.

Category B - TRACE is expected to be applicable and should be capable of simulating the associated processes and phenomena with only limited code development or additional assessment.

Category C - TRACE models/correlations are not designed to simulate these phenomena. However, the effects of the phenomena can be bounded using existing TRACE capabilities (i.e., code work-around) and the appropriate well scaled integral effects data representing the SMR geometry.

Category D - Additional TRACE model development and assessment is considered necessary to demonstrate TRACE applicability for the phenomena in this category.

Also, the use of well-scaled integral and/or separate effects data representing the SMR geometry is required.

In addition, under the follow-on task (Ref. 2), RES staff will perform a detailed literature review and develop a TRACE Assessment Matrix and Gap Report based on the results of this

4 PIRT. The TRACE Assessment Matrix will establish a set of assessment or validation activities needed to confirm that TRACE can reliably model the key phenomena identified in this PIRT to perform the subject boron redistribution analyses. The assessment matrix will likely rely on a combination of separate effect tests (SETs) and integral effects tests (IETs). The Gap Report will identify assessment requirements that are not currently co vered by the existing TRACE assessment experience and that need to be performed to determine the applicability of TRACE to perform the subject analysis. It is possible that the gap report may include a recommendation to gather data that is not currently available in the open literature. In such an instance, the gap report will include recommendations for an experimental program (SETs and/or IETs, as appropriate) to gather the necessary data or other work-around approaches if possible. If the RES staff identifies such a gap, then RES and NRR will coordinate next steps, as necessary.

5 Contents 0 Executive Summary............................................................................................................. 3 1 Introduction.......................................................................................................................... 8 2 Plant Description.................................................................................................................. 9 3 Figures of Merit................................................................................................................. 13 4 Scenario Identification....................................................................................................... 14 5 Scenario Period Identification............................................................................................ 15 6 Systems, Subsystems, and Components............................................................................ 16 7 PIRT Membership and Methodology................................................................................ 17 8 Phenomena Ranking and Discussion................................................................................. 20 8.1 Containment................................................................................................................................ 21 8.1.1 Break and Valves................................................................................................................ 21 8.1.2 Containment Vessel (CNV)................................................................................................ 21 8.1.3 Dissolver Basket................................................................................................................. 26 8.1.4 RCS Pressure Boundary Surfaces....................................................................................... 2 6 8.2 Core............................................................................................................................................ 27 8.2.1 Barrel / Baffle...................................................................................................................... 27 8.2.2 Fuel Rods............................................................................................................................ 28 8.3 Decay Heat Removal System (DHRS)....................................................................................... 3 4 8.3.1 DHRS Tubes....................................................................................................................... 34 8.3.2 Piping from Condenser Tubes to Feedwater Inlet............................................................... 38 8.3.3 Piping from SG to Condenser Tubes.................................................................................. 38 8.4 Emergency Core Cooling System (ECCS)................................................................................. 39 8.4.1 Reactor Coolant System (RCS) and Containment Vessel (CNV)....................................... 39 8.4.2 Reactor Pressure Vessel (RPV)........................................................................................... 39 8.5 Instrumentation and Controls..................................................................................................... 42 8.5.1 Pressurizer Control Systems............................................................................................... 42 8.5.2 Primary and Secondary Control Systems............................................................................ 42 8.6 Primary....................................................................................................................................... 43 8.6.1 All Components.................................................................................................................. 43 8.6.2 Baffle Plate.......................................................................................................................... 43 8.6.3 Break................................................................................................................................... 45 8.6.4 Downcomer......................................................................................................................... 46 8.6.5 Hot Leg Riser...................................................................................................................... 48

6 8.6.6 Lower Plenum..................................................................................................................... 52 8.6.7 Pressurizer........................................................................................................................... 54 8.6.8 Riser Holes.......................................................................................................................... 56 8.6.9 Steam Generator (SG) Annulus.......................................................................................... 57 8.6.10 Upper Plenum..................................................................................................................... 59 8.7 Reactor Pool............................................................................................................................... 59 8.7.1 Reactor Pool........................................................................................................................ 59 8.8 Secondary................................................................................................................................... 60 8.8.1 SG Tubes............................................................................................................................. 60 9 Phenomena Importance / Knowledge Issues..................................................................... 73 10 Summary and Conclusions................................................................................................ 75 11 References.......................................................................................................................... 78 12 Appendix A: Reference LOCA PIRT Rankings (Ref. 4).................................................. 79 13 Appendix B: List of Acronyms.......................................................................................... 82

7 1 Introduction

Under User Need NRR-2021-019 (Ref. 1), the Office of Nuclear Reactor Regulation (NRR) requested support from the Office of Nuclear Regulatory Research (RES) to perform pre-application topical report review activities for the NuScale US460 small modular reactor (SMR) standard design approval application (SDA), which is an uprated version of the approved NuScale design certification application (DCA). Under Task 7 of this User Need, NRR requested RES staff support for NRRs review of the NuScale Long-term Cooling, Reactivity Control, and Boron Analysis Methodology Topical Report. Under this task, NRR requested (Ref. 2) that RES staff convene a panel to identify the importance of the phenomena affecting the core integrity and coolabiltiy during postulated design-basis events with significant boron dilution and redistribution in the NuScale US460 in accordance with the process described by Regulatory Guide 1.203 (Ref. 3). The panels findings are documented in this report as a Phenomena Identification and Ranking Table (PIRT).

This PIRT report supports two efforts. First, it provides support to NRRs review of the Long-term Cooling, Reactivity Control, and Boron Analysis Methodolog y Topical Report by identifying, ranking the importance, and assessing the current state of knowledge of phenomena related to boron redistribution in the NuScale SDA design. Second, this effort is part of the Evaluation Model Development and Assessment Process (EMDAP), as defined in Regulatory Guide 1.203. At a high level, the EMDAP process is used to develop a reliable evaluation model to perform analysis of a scenario (e.g., a loss of coolant accident (LOCA)) for nuclear reactor applications. The PIRT is part of the first step of the EMDAP process to determine the requirements for the evaluation model. Therefore, this report is the first of a few steps required for RES staff to develop an evaluation model capable of reliably performing confirmatory analysis of boron redistribution transients in the NuScale US460 design.

One of the key results from this PIRT report is the identification of phenomena that are important for modeling boron dilution transients in the NuScale SDA design but require additional assessment to gain confidence in the modeling methodology. Under the follow-on task (Ref. 2) to this report, RES staff will perform a detailed literature review and develop a TRACE Assessment Matrix and Gap Report based on the results of this PIRT. The TRACE Assessment Matrix will establish a set of assessment or validation activities needed to confirm that TRACE can reliably model the key phenomena identified in this PIRT to perform the subject boron redistribution analyses. The assessment matrix will likely rel y on a combination of separate effect tests (SETs) and integral effects tests (IETs). The Gap Report will identify assessment requirements that are not currently covered by the existing TRACE assessment experience and that need to be performed to determine the applicability of TRACE to perform the subject analysis. It is possible that the gap report may include a recommendation to gather data that is not currently available in the open literature. In such an instance, the gap report will include recommendations for an experimental program (SETs and/or IETs, as appropriate) to gather the necessary data or other work-around approaches if possible (e.g., application of computational fluid dynamics (CFD)). If the RES staff identifies such a gap, then RES and NRR will coordinate next steps, as necessary.

8 The RES staff have previously developed PIRTs for LOCA (Ref. 4, Ref. 5) and instability-driven transients (Ref. 6) for the NuScale Power Module (NPM) design defined in the DCA. These PIRTs were used to determine the applicability (Ref. 7) of TRACE/PARCS for the NuScale DCA review. In addition, RES staff have performed analysis of boron dilution transients for the DCA review (Ref. 8, Ref. 9). The current work builds on this previous experience.

Section 2 of this report provides a brief description of the NuScale US460 design. In Section 3, the figures of merit (FOMs) are identified and discussed. The limiting scenarios are identified in Section 4. In Section 5, the scenarios are divided into periods. Section 6 discusses the subdivision of the plant into systems, subsystems, and components. Section 7 describes the PIRT panel membership and overall PIRT methodology. Section 8 provides the phenomena rankings and associated discussions. Section 9 identifies and briefly discusses phenomena that have importance rankings greater than their knowledge level. Section 10 provides a summary and conclusions.

2 Plant Description

The reactor coolant system (RCS) of the NPM (as defined in the DCA) is provided in Figure 1.

During normal operation, water is passively circulated through the primary loop via natural circulation; water is heated in the core, rises within the riser due to buoyancy forces, transfers heat through the steam generators, travels through the downcomer, and then returns to the core.

During normal shutdown, decay heat is generally removed from the reactor via the Decay Heat Removal System (DHRS) (Figure 2). The DHRS is a closed, passive flow loop between the steam generators and an array of pipes (i.e., a heat exchanger) connected to the outside of the containment vessel (CNV) and submerged in the reactor pool. The DHRS transfers decay heat to the reactor pool via natural circulation. During accident conditions, the Emergency Core Cooling System (ECCS) may actuate resulting in reactor trip and opening of the reactor vent valves (RVVs) at the top of the RPV and reactor recirculation valves (RRVs) on the side of the RPV. The schematic for the ECCS system is provided in Figure 3. In general, water boils in the core, travels through the RVVs as steam, condenses on the containment wall, and returns to the RPV via the RRVs during steady state ECCS operation. Of course, the overview provided in this paragraph is highly simplified; multiple heat transfer and flow phenomena always occur, as will be discussed in more detail later in this report.

The RCS may experience postulated events that could allow the addition of relatively cold water with diluted boron concentrations to accumulate in the downcome r. For instance, during a LOCA and following ECCS actuation, diluted or unborated water can accumulate inside containment due to steaming from the RVVs (which will concentrate boron in the core and riser). The diluted or unborated water can then return to the RPV via the RRV ECCS recirculation path, mix with the downcomer fluid, and reduce the downcomer boron concentration. As another example, boron dilution may occur in the downcomer during an event where the riser uncovers and the DHRS is the primary heat sink. When the riser uncovers, the normal natural circulation flow pattern in the RCS is broken and the RCS flow will significantly decrease. The core decay heat will heat the coolant and cause some amount of coolant to boil-off. The core will act as a distiller where the boric acid is concentrated in the core and/or riser sections. Some of the steam that leaves the core may condense on the steam generator tubes during DHRS cooling. The

9 condensation of the steam will result in a flow of relatively pure water from the steam generator annulus to the downcomer. Over time, the boiling in the core and the condensing on the steam generator tubes creates a distillation effect whereby the boron becomes more and more concentrated in the core/riser region and more and more dilute in the downcomer region.

The NPM (as defined in the DCA) includes features designed to help mitigate boron dilution events. For example, the ECCS has a low CNV level setpoint and low RCS pressure actuation mechanism (and associated interlock). The CNV level setpoint is designed such that the CNV pressure and level will be less than RPV pressure and level upon ECCS actuation, which should preclude an initial surge of diluted water from entering the RPV upon ECCS actuation. The low RCS pressure actuation setpoint is designed such that the ECCS should actuate relatively early during small break LOCA (SBLOCA) transients to limit the amount of boron dilution time within the RPV. In addition, the NPM (as defined in the DCA) has small holes in the riser near the mid-point of the steam generator. These riser holes provide a bypass flow path between the downcomer and riser in the instances where there RPV level drops below the top of the riser.

The riser holes will delay and reduce boron dilution in the dow ncomer region while there is riser hole flow.

Notwithstanding these mitigative features, it may be postulated that both dilution examples discussed above could be accompanied by an event that causes a diluted slug of water to return to the core, displace the borated water, and potentially result in re-criticality. This possibility was analyzed for the DCA design review in Ref. 8 and Ref. 9.

For the NuScale US460 design, which is the subject of the SDA, RES staff anticipate evaluating similar boron dilution transients as the DCA review. NuScale indicated that they will make multiple design changes from the DCA to SDA that may affect bor on dilution transients. These changes are generally preliminary and subject to change but are considered in the present analysis to develop the rankings in the PIRT. Here is a summary of the expected design changes that are relevant to boron dilution transients:

The power will increase by 52% (to 77 MWe, 250 MWt) per module.

A boron dissolver basket will be added to containment vessel. This is a passive system where water from condensation on the containment vessel wall is funneled into a basket containing boron pellets. The pellets may then slowly dissolve and add boron to the liquid inventory in the containment. There is significant design uncertainty in this component.

Additional riser holes will be added in the riser. Generally, the riser hole locations are expected to span the riser height, including locations around the core.

Reactor pool level will decrease.

The number of RVVs will decrease from three to two.

The Inadvertent Actuation Block (IAB) valves were removed from the RVVs.

The ECCS setpoints are expected to change, but the new ECCS setpoints are not final. In general, ECCS is expected to actuate due to any of the following:

o actuation based on ECCS setpoints, o actuation at 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months after loss of offsite power, and o actuation if the differential pressure between the containment and primary is less than or equal to 5 psia.

10 Figure 1: NuScale Reactor Coolant System (Ref. 6). The arrows indicate the primary system flow path during normal operating conditions.

11 Figure 2: Schematic of DHRS, from Ref. 10.

12 Figure 3: Schematic of NuScale Power Module ECCS from Ref. 10.

3 Figures of Merit

The figures of merit (FOMs) for the PIRT are the average boron concentration in the lower plenum and the average boron conc entration in the downcomer. These FOMs were selected because they indicate the amount of positive reactivity that ca n be inserted in the core if the diluted water in the downcomer and lower plenum were inserted i nto the core and displaced the heavily borated core inventory.

13 These FOM were selected based on the assumption that the core will remain subcritical for the duration of these design-basis events following reactor trip. Under this assumption, reactor re-criticality is precluded if the boron concentration in the downcomer and lower plenum is above the critical boron concentration. If the boron concentration in the downcomer and/or lower plenum is less than the critical boron concentration, then these FOMs indicate the amount of positive reactivity available in criticality analysis.

Furthermore, if recriticality is precluded, there is no potential for severe consequences vis--vis fuel overheat; therefore, FOMs related to fuel damage are not considered (e.g., peak cladding temperature, cladding oxidation, and fuel enthalpy).

4 Scenario Identification

The staff identified three transients that may be limiting for boron dilution in the NuScale SDA design. These transients are (1) a small break LOCA (SBLOCA) in the liquid space of the CVCS line, (2) inadvertent opening of one RRV, (3) and loss of offsite power (LOOP). Each of these scenarios are assumed to occur at beginning of cycle (BOC), because reactor boron inventory is greatest at BOC. Phenomena in the PIRT are ranked for each of these transients.

The staff identified (1) and (2) as potentially limiting for boron dilution in the downcomer prior to ECCS actuation, and (3) is the limiting event for boron dilution following ECCS actuation.

The ECCS actuation timing is important when considering (1), (2), and (3) because actuation of ECCS is expected to cause a level swell and flush in the core/riser, which could result in significant inventory transfer from the downcomer and lower ple num to the core (Ref. 8, Ref. 9).

Thus, ECCS actuation was identified as a time in the transient when downcomer and lower plenum boron concentrations may be of particular interest from a safety perspective.

The LOOP transient will progress roughly as follows. The reactor will trip after loss of offsite power and switch to DHRS cooling. The LOOP will cause the CNV vacuum pumps to trip and CNV pressure will increase to roughly atmospheric. The RPV inventory will shrink due to reactor trip and DHRS cooling. Eventually, the riser will uncover due to shrinkage and the primary flow path will be interrupted. At this point, decay heat will be transferred to the steam generator (SG) tubes (i.e., the DHRS) via (A) liquid convection to the core-side of the riser wall, conduction through the riser wall, and liquid convection in the steam generator annulus between the riser wall and the steam generator tubes and (B) condensation heat transfer on the SG tubes in the vapor space. (B) will contribute to dilution in the dow ncomer, while (A) will not. The ECCS will actuate during the LOOP scenario after either 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months (i.e., when the ECCS valves actuate due to loss of DC power) or when the differential pressure between the CNV and RPV is less than or equal to 5 psia. In either case, the differential pressure between the CNV and RPV is expected to be small for the LOOP scenario compared to the SBLOCA and inadvertent opening of the RRV scenarios.

Similar to the LOOP scenario, the SBLOCA may be limiting by max imizing the amount of dilution in the downcomer prior to ECCS actuation. The RPV level in the SBLOCA scenario will decrease more rapidly than in the LOOP scenario due to loss of inventory through the break.

As a result, more of the SG will be exposed prior to ECCS actuation. This will cause (B) to be

14 more dominant in the SBLOCA scenario than the LOOP scenario. It is hard to say if the SBLOCA or LOOP transient would cause more downcomer dilution prior to ECCS actuation, because the LOOP scenario will have more time prior to ECCS act uation but a slower dilution rate due to the greater competition between (A) and (B).

The SBLOCA in the liquid space was selected because the RPV pressure will decrease more slowly than a SBLOCA in the vapor space. In addition, more bor on mass would be released to containment if the break is in the liquid space. The panel could not identify if a SBLOCA of the discharge or charging line of the CVCS is more limiting. Withi n the panel ranking justifications, the panel included discussion of how the location of the break would affect the importance of various phenomena where applicable.

The inadvertent opening of the RRV may be limiting for dilution after ECCS actuation because this scenario maximizes the amount of boron released to the CNV. The borated water released through the RRV prior to ECCS actuation may become trapped beneath the RRV valve in the CNV, and unable to return to the downcomer/lower plenum via nominal ECCS flow. Following ECCS actuation, the downcomer and lower plenum may continue to dilute as steam generated by decay heat travels through the RVVs, condenses on the CNV wall, and returns to the RPV via the RRV. This transient is not expected to be limiting prior to ECCS actuation because the RRV will quickly release a large amount of inventory to the CNV. As a result, the ECCS will actuate relatively early in the transient, which minimizes dilution time via (B).

Last, note that the scope of this analysis is limited to design-basis accidents. Therefore, operator actions are not credited, and the recovery phase is not evaluated. In addition, the panel assumed that deflagration and detonation of radiolytic gas are out of scope.

5 Scenario Period Identification

As stated in (Ref. 6), not all phenomena that occur are of uniform importance throughout the duration of the transient. For example, critical/choked flow may be of high importance at the break location early in the transient, while differential pressure between the containment and reactor vessel is high, but it may be inactive once the containment and reactor vessel pressures equalize. Therefore, it is generally necessary to consider the importance of phenomena in operationally characteristic time periods in which dominant processes and phenomena remain essentially constant. To this end, the panel considered and agreed to the period definitions in Table 1. Each of the three scenarios are evaluated for each of the periods defined below.

15 Table 1: Period Definitions

Period Period Name Definition This period is from scenario initiation to when the RPV level 1 Draw Down / Shrinkage decreases to the top of the riser (i.e., the moment before the nominal primary flow path is completely interrupted).

High Pressure This period is from the end of period 1 until the moment before 2 Steam Generator actuation of the ECCS valves Condensation This period starts with ECCS actuation and ends when RPV and 3 Blowdown CNV pressure are approximately balanced (i.e., when blowdown forces no longer dominate).

Low Pressure This period begins once RPV and CNV pressures are 4 Containment approximately balanced and ends when the CNV level reaches the Condensation RRV.

5 Long Term This scenario begins once CNV level reaches the RRV and Cooling (LTC) terminates at 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months .

6 Systems, Subsystems, and Components

As stated in (Ref. 6), not all phenomena that occur are of uniform importance throughout the NPM. For example, boiling may be of high importance in the sub channel, but it will be less important inside the SG annulus where the primary coolant is being cooled. Therefore, it is generally necessary to consider the importance of phenomena based on the system, subsystem, or component where they occur within the NPM. To this end, the RES staff relied on previous PIRT experience for LOCA and stability and subdivided the NPM into smaller systems and components.

Table 1 summarizes the division of the NPM into the subsystems and components. The divisions listed in Table 1 track closely with the convention adopted in the LOCA and stability PIRTs.

One exception is that the ECCS includes two catch-all components: Reactor coolant system and containment vessel and Reactor pressure vessel. These compo nents were used to capture system wide phenomena that are generally expected to be independent of system and component.

16 Table 2: Systems and Components

System Component

Containment Vessel (CNV) Breaks and valves, CNV, dissolver basket, RCS pressure boundary surface Core Barrel/baffle, fuel rods, subchannel, control rods and guide tubes, kinetics Decay Heat Removal System (DHRS) DHRS tubes, piping from DHRS tubes to steam generator (SG) tubes, piping from SG tubes to DHRS tubes Emergency Core Cooling System (ECCS) Reactor coolant system (RCS) and CNV, reactor pressure vessel (RPV), reactor recirculation valves (RRVs), and reactor vent valves (RVVs)

Instrumentation and Controls Primary and secondary side control systems, primary pressurizer control system, reactor protection, secondary side control systems Primary Baffle plate, break, downcomer, hot leg riser, lower plenum, pressurizer, riser holes, steam generator annulus, and upper plenum Reactor Pool Reactor pool Secondary SG tubes

7 PIRT Membership and Methodology

The staff relied on the process described in Step 4 of Element 1 of RG 1.203 (Ref. 3) to develop the PIRT. The PIRT panel is comprised of Dr. Stephen Bajorek, Dr. Peter Lien, Dr. Shanlai Lu, Mr. Jason Thompson, and Dr. Peter Yarsky. The PIRT panel was facilitated by Mr. Robert (Lee) Gladney for approximately the first half of the process, and by Jason Thompson during the second half.

As a starting point, the panel referred to the previous PIRTs developed for LOCA (Ref. 4, Ref. 5) and stability (Ref. 6). The phenomena identification and ranking began by compiling the phenomena documented in Ref. 4, Ref. 5, and Ref. 6. From this starting point, the PIRT panel convened and held a meeting to update the list of phenomena. The purpose of meeting to update the list of phenomena was to ensure that the existing list did not lack phenomena that are specifically related to boron dilution or redistribution characteristics of the NuScale US460 design (and therefore, possibly not considered previously). This resulted in the addition of several phenomena and clarification of other phenomena. The final PIRT included approximately 300 phenomena (counting duplicates across components and systems but not periods or scenarios). Therefore, there are approximately 4500 importance rankings in the final PIRT (300 phenomena, 5 periods, 3 scenarios).

17 The staff used the LOCA PIRT in Ref. 4 to simplify the present effort using the following approach. The LOCA PIRT in Ref. 4 evaluates an inadvertent opening of an RRV, which is one of the scenarios evaluated in the present PIRT and is similar to the SBLOCA and LOOP scenarios. Therefore, the LOCA PIRT in Ref. 4 will have many of the same importance rankings and knowledge level rankings as the present PIRT, especially for phenomena specific to the LOCA aspects of the present study. The LOCA PIRT in Ref. 4 did not consider boron dilution phenomena, and therefore, the panel carefully reviewed the rankings in the LOCA PIRT in Ref.

4 to determine if the rankings (for both importance level and knowledge level) were appropriate for the present study. To simplify this approach, the panel only evaluated if importance rankings needed to be increased. This is appropriate because reducing the importance would not introduce additional importance level-knowledge level conflicts, and therefore would not affect the assessment process in later steps. For example, if a phenomenon was ranked with high importance in the LOCA PIRT but only medium importance in the p resent study, the phenomenon does not need to be reevaluated in the present study because any issues relating to a knowledge level and importance ranking conflicts would have been previously addressed in the TRACE applicability assessment for the NuScale DCA review (Ref. 7). The panel reviewed the rankings in the LOCA PIRT in Ref. 4 and found that all but phenomena in Table 3 were ranked appropriately or with greater importance than needed in the present study. The full list of phenomena from the LOCA PIRT in Ref. 4 are included in the Appendix of this document.

Also, as suggested above, note that the importance level and knowledge level conflicts in Ref. 4 were evaluated in the TRACE applicability assessment (Ref. 7).

After applying the LOCA PIRT in Ref. 4, approximately 180 phenomena needed to be ranked.

The panel members individually ranked each phenomenon for each period of each scenario on a scale of zero through nine. A numerical value of zero indicates that the phenomenon is inactive.

A value of one indicates the phenomenon is highly unlikely to occur, and so on. Generally, values of one through three correspond to low importance, four through six correspond to medium importance, and values of seven through nine correspond to high importance. The panel used numerical values to more finely differentiate between the broader importance rankings of low, medium, and high to facilitate subsequent discussions.

18 Table 3: Phenomena from Ref. 4 that needed re-ranking for the present study.

System Component Phenomenon Containment Containment Vessel Thermal Stratification

Containment Containment Vessel Vertical/Radial Natural Circulation (liquid)

Containment Containment Vesse l Non-condensable Gas Effects Core Subchannel Cross Flow / Mixing Core Subchannel Flashing Core Subchannel Turbulent Mixing

Core Subchannel Vertical/Radial Natural Circulation*

Emergency Core Cooling System Reactor Vent Valves Critical/Choked Flow Emergency Core Cooling System Reactor Vent Valves Single-phase Pressure Drop Emergency Core Cooling System Reactor Vent Valves Two-phase Pressure Drop

Primary Downcomer Vertical/Radial Natural Circulation

Primary Hot Leg Riser Turbulent Mixing (e.g., hot streaking)

Primary Hot Leg Riser Two-phase Level Swell

Primary Hot Leg Riser Vertical/Radial Natural Circulation

Primary Lower Plenum Vertical/Radial Natural Circulation

Primary Steam Generator Annulus Vertical/Radial Natural Circulation

This phenomenon was titled Natural Circulation in the SBLOCA PIRT but renamed in the present PIRT to be more consistent with other phenomena.

In subsequent meetings, the panel discussed individual importance and knowledge level rankings to reach consensus. In the consensus importance ranking, the phenomena are ranked as being:

(1) Low (L) importance - having only a small influence on the figure of merits, (2) Medium (M) importance - having a moderate influence on the figure of merits, (3) High (H) importance - having a significant or dominant influence on the figure or merit, or (4) Inactive (I) - that the phenomenon is not possible or highly unlikely.

The knowledge level was ranked according to a similar scale, namely:

(1) Low (L) - the phenomenon is not well understood. Modeling the phenomenon is currently either not possible or is possible only large uncertainty, 19 (2) Medium (M) - the phenomenon is understood, however, can only be modeled with moderate uncertainty, or (3) High (H) - the phenomenon is well understood and can be accurately modeled.

8 Phenomena Ranking and Discussion

This section provides the rationale for the PIRT panels consensus rankings on the importance and the knowledge level of each phenomenon as it relates to the scenarios of interest. These rationales are organized in this section by system, component, and phenomenon. In the discussions below, the maximum ranking and knowledge level are listed next to each phenomenon for convenience. Table 4 provides a summary of the importance and knowledge level rankings.

Note that most of the phenomena in this PIRT are considered self-explanatory and are not defined in detail. For example, it is assumed that the reader is familiar with the definition of conduction. There is a subset of phenomena definitions that ar e less obvious, however, and these phenomena are generally defined in the ranking discussion. Some mixing and fluid transport phenomena are in this subset. These phenomena are generally of medium or high importance in this PIRT, and they are therefore defined again here to further minimize confusion:

Natural Circulation Flow / Bulk Flow is defined as flow that results in an average transport of inventory from one component to another (e.g., flow from the CNV to the RPV).

Vertical / Radial Natural Circulation, by contrast, is limited to relatively local flow phenomena within one or a few connected subsystems (e.g., mixing in the core and riser),

which are generally governed by buoyancy forces.

Cross Flow / Mixing is defined in the subchannel and refers to subchannel-to-subchannel flow behavior (i.e., highly localized behavior). The integral effect of the subchannel-to-subchannel flow is captured by the turbulent mixing and radial/vertical natural circulation phenomena for the subchannel.

Turbulent Mixing captures the homogenization of fluid properties (i.e., mixing) and viscous losses in a component due to chaotic flow behavior (i.e., turbulence). In the core, this phenomenon captures the integral effect of turbulent mixing in or between subchannels.

Thermal and Species Stratification captures the propensity of fluid within a specific component to settle (or stratify) due to negative buoyancy forces (i.e., buoyancy forces that are in the direction of the gravity vector) generated by the fluid temperature or species concentration, respectively.

Vortexing is the fluid formation characterized by a whirling flow field that may cause localized mixing of the surrounding fluid.

Natural Circulation in the DHRS tubes refers to loop wise circulation; it does not refer to local natural circulation within a single DHRS tube.

20 8.1 Containment 8.1.1 Break and Valves

Breaks and Valves - Boric Acid Volatilization (L, L)

During the first three periods of the SBLOCA and inadvertent RRV opening scenarios, the flow from the valves or breaks will flash. Due to the rapid pressure decrease, boron volatilization may occur. However, the amount of volatilization is expected to be relatively small and have a small impact on FOMs. Volatilization may also occur during periods 4 and 5, but it is expected to be even less significant. As a result, this phenomenon is ranked with low importance during all periods of the SBLOCA and inadvertent RRV opening scenarios. For the LOOP scenario, this phenomenon is expected to be inactive for the first two periods because there will not be valve/break flow. For periods 3 through 5, this phenomenon is ranked with low importance for similar reasoning as the SBLOCA and inadvertent RRV opening scenarios. The panel agreed that the knowledge level for this phenomenon is low.

Breaks and Valves - Flashing (H, H)

In general, flashing will occur in or near the break (for the SBLOCA scenario) and the RRVs (while open) during periods 1 through 3 because the RPV pressure will be significantly greater than the CNV pressure and the inventory on the RPV side of the boundary is liquid. Flashing will occur for slightly longer than the critical flow phenomenon in period 3 because flashing can persist after the Mach number in the throat of the valves or break decreases below unity. Note that the definition of this phenomenon includes flashing in flow from the break or valves that occurs away from the valve or break (i.e., in the CNV vapor space). The panel agreed that flashing is of high importance during these periods because it will have a large impact on inventory (and therefore boron) transfer between the RPV and CNV and scenario progression.

The panel agreed the knowledge level is high.

8.1.2 Containment Vessel (CNV)

CNV - Boron Dissolving (M, L)

For the SBLOCA and the inadvertent RRV opening scenarios, the panel agreed that there may be some plate out on the CNV wall due to jet flashing from the break or ECCS valves prior to RCS-CNV pressure equalization. However, the panel also agreed that plate out would be mitigated because condensate flow on the CNV wall will tend to re-dissolve boron. As a result, the panel agreed that boron dissolving on the CNV wall is of medium importance for all periods. For the LOOP scenario, the panel agreed that there will be significantly less plate out on the CNV wall due to the reduced differential pressure between the CNV and RPV during period 3. Therefore, this phenomenon is of low importance in periods 3 through 5. The panel agreed this is not of high importance because the total mass of boron plating on the CNV wall is expected to be small. The panel agreed that the knowledge level for this phenomenon is low based on the limited modeling experience.

CNV - Boron Plate Out (L, L)

This phenomenon is limited to plate out on the CNV vessel wall; plate out on the outer surface of the RCS pressure boundary is discussed below. The panel agreed that significant plate out on the CNV wall below the liquid level is unlikely to occur because voids are unlikely to be generated 21 on the CNV wall due to its relatively cold temperature. Furthermore, plate out that may occur during blowdown will likely be re-dissolved by condensate flow. The panel agreed that the importance ranking for boron plate out in the CNV is low for all periods and scenarios. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level low.

There is additional uncertainty in the knowledge level for plate out in the CNV due to the uncertainty in design information for the dissolver basket component. If the dissolver basket increases the boron concentration in the fluid to near the solubility limit, plate out may be more important.

CNV - Boron Precipitation (L, M)

The panel agreed that boron precipitation in the CNV is expected to be limited because (1) the boron concentration in the CNV is expected to remain below the solubility limit and (2) the panel does not expect blowdown to be a significant source for boron precipitation. Therefore, this phenomenon is ranked with low importance for all periods when liquid inventory is expected in the CNV and otherwise inactive. A knowledge ranking of medium was selected because precipitation is generally well understood in laboratory experiments, but data may not be available for all thermal-hydraulic conditions of interest. I n addition, the state of practice for modeling boron precipitation and precipitate transport in nuclear reactors is not fully mature.

CNV - Containment Flooding and Drain System (I, M)

The scope of the PIRT is limited to design basis accidents. In design basis accidents, inventory changes via the containment drain and flooding system are not credited. Therefore, the operation of this system is not necessary in modeling the scenario, and this system is expected to be inactive for all periods. The panel discussed the state of knowledge of the Containment Flooding and Drain System and agreed the knowledge level is medium because there is some uncertainty in the system design and the applicable operating procedures.

CNV - Convection Heat Transfer to the Vessel (M, H)

The panel discussed that there are three dominant heat transfer pathways for the heat generated in the core to transfer to the ultimate heat sink (i.e., the reactor pool). Those pathways are (1) heat removal via the DHRS, (2) condensation heat transfer on the CNV wall, and (3) the conduction-convection pathway. The conduction-convection pathway includes both liquid convection and conduction through the vessel walls. It is defined as conduction through the riser wall, RPV wall, and CNV wall, where heat is passed between these structures via convection from the liquid coolant. Thus, liquid convection on the CNV wall is integral to (3). The panel considered that (1) will dominate during the first two periods, and (2) is likely to dominate during blowdown and after. The conduction-convection pathway is expected to be low importance during the first two periods because CNV inventory w ill be small. Period 3 is expected to be relatively brief and dominated by (2). Therefore, (3) is of low importance.

During period 4 and 5, (3) will contribute to the total heat removal and is ranked with medium importance. The level in CNV will be greater in period 5 than period 4. Therefore, this phenomenon is most important (although still medium importance) in period 5. It is important to note that (3) is the only pathway that does not directly contribute to boron dilution, where (1) and (2) produce dilution in the downcomer and CNV, respectively. As a result, it is important to capture the relative heat removal of each heat transfer pathway to accurately capture boron redistribution. The panel agreed that the state of knowledge of convection heat transfer is high.

22 CNV - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations are unlikely to be important in the CNV because there is not a source for significant void production. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in conditions similar to those expected in the CNV.

CNV - Entrance Effects / Developing Length (L, M)

The entrance and developing length effects are ranked with low importance for all periods (except the first two periods of the LOOP scenario where it is inactive because the RPV is sealed). Prior to blowdown, CNV inventory is small and entrance effects are insignificant.

During blowdown, liquid in the CNV will be mixed, which will erase any history of entrance effects. During period 4 and 5, entrance effects and developing length are expected to have a relatively small impact on boron mixing compared to other phenomena, and is therefore considered of low importance. While entrance effects are generally well understood, the panel agreed that the knowledge level was medium because the CNV has multiple flow paths, some with unique flow geometries.

CNV - Flashing (L, H)

This phenomenon is limited to flashing in the liquid pooled at the bottom of the CNV vessel; flashing from valve and break components that may occur in the CNV vapor space is considered under other phenomena (e.g., flashing from the breaks/valves, choked/critical flow, boron plate out, etc.). The panel discussed and agreed that there is not a clear mechanism for flashing in the liquid pooled at the bottom of the CNV, and therefore this phenomenon is of low importance.

The panel agreed the knowledge level for flashing is high.

CNV - Interfacial Drag / Relative Motion of the Phases (L, M)

The panel considered this phenomenon in a few different ways. First, there is a phase interface between the pooled liquid at the bottom of the CNV and the vapor space. This interface is expected to be relatively inactive and have a negligible impact on the progression of the transient. Second, there is the possibility for some two-phase flow in the stratified liquid as a result of voiding on the RPV wall. Voiding on the RPV wall is considered unlikely, and therefore this phenomenon is considered of low importance. Third, there will be a phase interface between the condensate (droplet or film) and the surrounding steam. This interaction is expected to be between relatively pure water and steam. Therefore, it will not affect the mixing of boron or the progression of boron dilution. Finally, there will be some interfacial effects between the condensate from the CNV walls flowing to the liquid pooled at the bottom of the CNV. This is also expected to only have a minor impact on the mixing of borated water at the bottom of the CNV, and is therefore of low importance. The panel considered the knowledge level of interfacial drag and the relative motion between phases and agreed with a ranking of medium.

23 CNV - Internal Natural Circulation (RPV-CNV wall driven) (H, M)

The temperature difference between the RPV wall and the CNV wall (or bulk liquid in the CNV) will drive natural convection in the liquid pooled at the bottom of the CNV. Natural convection will help mix the water in the CNV and therefore promote homogenization of the boric acid concentration in the liquid. The degree of mixing vs. stratification of liquid in the CNV will directly influence the boric acid concentration of the fluid flowing through the RRVs to the downcomer and lower plenum, and therefore has a direct impact on the figures of merit. This phenomenon is considered of low importance in periods 1 through 3 because liquid will not flow from the CNV to the RPV. Furthermore, during blowdown, the inventory in the CNV is expected to be well mixed, which will erase any history of stra tification or mixing during periods 1 and 2. This phenomenon is considered of medium importance in period 4 because it may affect the boric acid concentrations in the liquid during period 5. During period 5, this phenomenon is of high importance because it directly affects the boric acid concentration of the flow returning to the RPV. In LOOP, since the ECCS actuates after a relatively long time, the temperature difference between RPV and CNV is less than the other two transients. Therefore, the importance ranking is low and medium in periods 4 and 5, respectively. The panel considered the state of knowledge with respect to this phenomenon and agreed the knowledge level is medium.

CNV - Non-condensable Gas Effects (H, M)

This phenomenon was originally included in the 2011 LOCA PIRT (Ref. 4) discussed above. In Ref. 4, this phenomenon was ranked with high importance for all periods and a low knowledge level. This phenomenon is included again in the present study in part to include increased discussion on the effects of non-condensable gases in boron dilution transients. The importance rankings are inherited from the LOCA PIRT and are not re-evaluated. The knowledge level is reranked here as medium based on recent developments in correlations for modeling non-condensable gas effects (e.g., Ref. 11). In addition, the long-term effects and numerical simulation of radiolytic gas, non-condensable gas, and boron transport for SMRs is discussed in Ref. 12.

Non-condensable gases will be present in the CNV due to (1) the initial non-condensable gas inventory in CNV at nominal conditions, (2) the potential for increased inventory due to loss of CNV vacuum due to turbine trip or LOOP, and (3) radiolysis in the primary. Non-condensable gases in the containment may reduce the condensation rate in the CNV. This is significant because condensation in the CNV is a dilution mechanism; reduci ng the efficiency of condensation heat transfer in the CNV may result in increased heat transfer via liquid convection in the CNV or condensation on the SG tubes. The panel agreed that the loss of containment vacuum is likely to be a greater source for non-condensable gases than radiolysis.

CNV - Pressure Wave Propagation (L, H)

The panel agreed that this phenomenon is of relatively low importance for all periods. The dynamic scale of pressure wave propagation, which is related to the speed of sound, is much quicker than the dynamic scale of boric acid concentration changes. Pressure changes could be treated as occurring instantaneously and it would not noticeably affect the evolution of the boron content. The panel agreed that the knowledge level of pressure wave propagation is high.

24 CNV - Radiation Heat Transfer between RPV and CNV (L, H)

The panel agreed that radiation heat transfer is of relatively low importance for the transients of interest. The system temperatures and temperature differences are expected to be small enough such that this phenomenon is not a significant heat transfer pathway. The panel considered that radiation heat transfer could be more important in severe accident analysis, but the present analysis is limited to design basis accidents. The panel considered the state of knowledge of radiation heat transfer and agreed the knowledge level is high.

CNV - Single-phase Pressure Drop (L, H)

Single phase pressure drop in the CNV is expected to be small and have minimal impact on the boron dilution. Flow in the CNV will be driven by steam condensation, local gradients in fluid density, and the differential pressure between the CNV and the RPV; single phase pressure drop in the CNV will have a relatively small influence on the figures of merit compared to these drivers. The panel considered the state of knowledge of single-phase pressure drop and agreed the knowledge level is high.

CNV - Species Stratification (H, L)

The reasoning for the ranking for the Internal Natural Circulation (RPV-CNV wall driven) phenomenon also applies to species stratification. CNV inventory will not transfer to the RPV during the first three periods, and history of CNV inventory mixing or stratification will be erased during blowdown. This phenomenon is important during periods 4 and 5 because it will affect the boron concentration of the flow returning to the RPV during period 5. Note that the CNV liquid level will be below the RRV in period 4, but the degree of stratification in period 4 will directly affect the boron concentration of the water flowing through the RRV in period 5. In the LOOP scenario, the ECCS actuates after a relatively long time compared to the other two transients. As a result, the temperature difference between RPV and CNV is small compared to other two transients, which results in less species stratificat ion inside CNV. The panel considered the state of knowledge of this phenomenon and agreed it is low for two reasons.

First, the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models. Second, there is significant uncertainty in the behavior of the dissolver basket component and the amount of dissolved boron that will be present in the CNV liquid.

CNV - Thermal Stratification (H, M)

The basis for the importance rankings for this phenomenon is the same as the basis for species stratification in the CNV (above). The panel agreed that the knowledge level for thermal stratification is medium because the interplay between stratification and natural convection forces are not fully validated in modeling space. In addition, while the temperature distribution has been studied well in some integral effect tests, the complex geometry of dissolver basket layout reduces the state of knowledge.

CNV - Two-phase Level Swell (L, H)

The panel agreed that the liquid space in the CNV is expected to have very little voiding or flashing. Therefore, the two-phase level is expected to be approximately equal to the single-phase level. As a result, this phenomenon is of relatively low importance. The panel agreed that the knowledge level for two phase level swell is high.

25 CNV - Two-phase Pressure Drop (L, H)

The panel agreed that the liquid space in the CNV is expected to have very little voiding or flashing. Therefore, the two-phase pressure drop is of relatively low importance. The panel agreed that the state of knowledge for two phase pressure drop is high.

CNV - Vertical / Radial Natural Circulation (Liquid) (H, M)

The basis for the importance rankings for this phenomenon is similar to the basis for the importance rankings for the thermal stratification, species str atification, and Internal Natural Circulation (RPV-CNV wall driven) phenomena. This phenomenon is considered unique from Internal Natural Circulation (RPV-CNV wall driven) because it captures natural convection resulting from sources other than RPV heating (e.g., condensate flow from the vapor space, natural convection driven by temperature gradients from the bottom of the CNV and the vapor space, etc.). In the LOOP scenario, the ECCS actuates after a relatively long time compared to the other two transients. As a result, the temperature difference between RPV and CNV is small compared to other two transients, which results in less natural circulation inside CNV. The panel agreed the importance rankings for this item are the same as the thermal stratification phenomenon. The panel agreed that the knowledge level for this phenomenon is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models.

8.1.3 Dissolver Basket

Dissolver Basket - Boron Dissolving (H, L)

Similar to other phenomena in the CNV, the dissolver basket is expected to be inactive for the first two periods of the LOOP scenario because the RPV will not transfer inventory to the CNV until blowdown. For the SBLOCA transient, dissolving boron in the dissolver basket is expected to be of medium importance during periods 1 and 2 because there will be relatively small CNV inventory. For all other periods, CNV inventory is expected to be large. Therefore, dissolving boron in the dissolver basket is of high importance because it adds boron to the system and will have a direct impact on the FOMs in period 5. The panel agreed that the state of knowledge for the dissolver basket system is low due to design uncertainty and the lack of separate effect tests.

8.1.4 RCS Pressure Boundary Surfaces

RCS Pressure Boundary Surfaces - Boron Plate Out (H, L)

During periods 1 through 3 of the SBLOCA and inadvertent RRV opening scenarios, jet flashing is expected to occur. The panel expects that plate out will occur on the RCS boundary (especially in the vicinity of the valves and break location) as the jet flashes. As condensation on the CNV walls will likely only occur below the CNV head based on the reactor pool level, the panel does not expect condensate to drip on the RCS boundary and re-dissolve the boron on the RCS surfaces above the CNV liquid level (there will be boron re -dissolving below the CNV liquid level). As a result, the panel ranks this phenomenon with high importance because boron may plate out above the RRVs, which will effectively remove boron from the system. The panel agreed that the knowledge level for this phenomenon is low based on the uncertainty in the

26 location of plate out (i.e., above or below the RRVs), the extent plate out is likely to occur, and the limited modeling experience.

RCS Pressure Boundary Surfaces - Boron Dissolving (M, L)

During periods 1 through 3 of the SBLOCA and inadvertent RRV opening scenarios, jet flashing is expected to occur. The panel expects that plate out will occur on the RCS boundary (especially in the vicinity of the valves and break location) as the jet flashes. As condensation will likely only occur below the CNV head based on the reactor pool level, the panel does not expect condensate to drip on the RCS boundary and re-dissolve the boron on the RCS surfaces above the CNV liquid level. As a result, boron dissolving above the RRVs should be limited.

However, there will be boron re-dissolving below the top of the liquid level at the bottom of the CNV. As a result, the panel ranks this phenomenon with medium in periods 3 through 5 of the SBLOCA and inadvertent RRV opening scenarios. The panel agreed that the CNV liquid level will generally be below the RCS boundary in periods 1 and 2. Therefore, this phenomenon will generally not occur. For the LOOP scenario, plate out is expected to be minimal because there will be limited flashing due to the small differential pressure between the RPV and CNV during period 3. Therefore, boron dissolving will be inactive for the first two periods (while the RPV is sealed) and of low importance in periods 3 through 5. The panel agreed that the knowledge level for this phenomenon is low based on the limited modeling experience.

8.2 Core 8.2.1 Barrel / Baffle

Barrel / Baffle - Bypass Block Conduction (Effect on Bypass Fluid Temperature Distribution)

(M, H)

This phenomenon will affect the local temperature distribution in the core, which will affect internal recirculation in the core. Core mixing will determine the boron concentration of the flow through riser holes and in the lower plenum, which will affect the FOMs. This phenomenon is ranked with low importance during the periods 1 and 3 because bulk flow and flashing phenomena dominate core flow behavior, respectively. During periods 2, 4, and 5, this phenomenon is ranked with medium importance because it will aff ect core flow, but other phenomena are more influential to core flow. The panel agreed that this phenomenon is well understood and assigned a knowledge level of high.

Barrel / Baffle - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

27 Barrel / Baffle - Entrance Effects / Developing Length (L, M)

Entrance effects may affect flow distributions and mixing in the core, which may have a small impact on FOMs, but these effects are expected to be small in magnitude compared to other mixing phenomena. Therefore, this phenomenon is ranked with low importance for all periods.

The panel discussed and agreed that the state of knowledge for this phenomenon for this component is medium.

Barrel / Baffle - Water Thermal Expansion (M, H)

In general, water thermal expansion may impact the transient in a couple ways. First, the amount of shrinkage will affect level. The level impacts the riser uncovery timing, riser hole uncovery timing, and the pressure gradients over riser holes (because th e level in the downcomer and riser may be different). Second, water thermal expansion governs the natural circulation driving head.

For the barrel/baffle region, however, there is limiting heating and therefore these effects are generally small. During the long term (period 5), heat transfer from the barrel to the downcomer via conduction can contribute to the internal recirculation pattern. As a result, this phenomenon can become more important. Therefore, the panel agreed that this phenomenon is of low importance in periods 1 through 4 and medium importance in period 5 for all three scenarios.

The panel considered the state of knowledge of water thermal expansion and agreed the knowledge level is high.

8.2.2 Fuel Rods

Fuel Rods - Boron Plate Out (M, M)

Throughout each scenario, boron will concentrate in the core/riser due to the distillation process described above. The panel agreed that boron plate out is most likely to occur in period 5 because system pressure and temperature will be the lowest (i.e., the solubility limit will be smallest in period 5) and the boron concentration will be great est. However, the panel also agreed that there is significant uncertainty on whether there will be sufficient boron in the core to cause boron plate out. The panel noted that the boron solubility limit increases dramatically as the water approaches its boiling point (Ref. 13), and saturated conditions are expected in the core. Therefore, the panel concluded that a significant amount of boron would need to be added to the system by the dissolver basket and transported to the core region to cause boron plate out, and that there is significant uncertainty in whether this will occur. If there is adequate boron to cause boron plate out, plate out may occur on the fuel rods as voiding in the core occurs. In addition, the panel agreed that the accumulation of plated boron should be small and will not affect core coolability. Given that there is uncertainty in whether boron plate out will occur but that it could potentially directly impact the amount of dissolved boron in the primary, the panel agreed to rank this phenomenon with medium importance in period 5 for all scenarios. The panel agreed that this phenomenon is of low importance during all other periods. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level is medium. The knowledge level for this phenomenon for the fuel rods and subchannel (M) is higher than the other components (L) because there is more oper ational and modeling experience for boron plate out in the core, which reduces the degree of uncertainty.

28 Fuel Rods - Cladding Strain (L, H)

The thermal time constant of the fuel is relatively short (Ref. 8). Given the relatively slow progression of the scenarios of interest, FOMs will be relatively insensitive to small variations in fuel properties. As a result, the panel ranked phenomena relating to fuel properties as low importance for all periods. There is an extensive existing database with respect to cladding strain; therefore, the knowledge level is considered high.

Fuel Rods - Core Pin-by-Pin Burnup Distribution (L, H)

The thermal time constant of the fuel is relatively short (Ref. 8). Given the relatively slow progression of the scenarios of interest, FOMs will be relatively insensitive to small variations in fuel properties. The fuel burn-up distribution may also affect the core power distribution.

However, the effect of the core power distribution on the decay heat distribution is captured in kinetics phenomena. As a result, the panel ranked phenomena relating to fuel properties as low importance for all periods. There are sufficient radiochemical assays (RCA) data related to this phenomenon; therefore, a knowledge level of high is selected.

Fuel Rods - Fuel Heat Capacity (L, H)

The thermal time constant of the fuel is relatively short (Ref. 8). Given the relatively slow progression of the scenarios of interest, FOMs will be relatively insensitive to small variations in fuel properties. As a result, the panel ranked phenomena relating to fuel properties as low importance for all periods. There is an extensive existing database with respect to fuel heat capacity; therefore, the knowledge level is considered high.

Fuel Rods - Pellet Radial Power Distribution (L, H)

The thermal time constant of the fuel is relatively short (Ref. 8). Given the relatively slow progression of the scenarios of interest, FOMs will be relatively insensitive to small variations in the pellet radial fuel power distribution. As a result, the panel ranked this phenomenon as low importance for all periods. There is sufficient numerical and experimental evaluation with respect to pellet radial power distribution; therefore, the knowledge level is considered high.

8.2.3 Kinetics All of the kinetics phenomena listed in Table 4 except for Steady-state Assembly-to-Assembly Radial Peaking are of low importance for all periods of all scenarios because the reactor will trip and become subcritical in the first period (and the panel assumes that it will remain subcritical, as discussed above), before there is any chance for boron dilution to occur.

Therefore, the reactor kinetics phenomena do not affect the boron dilution processes. Similar reasoning applies for reactivity effects related to Xenon and Samarium, which are not included in Table 4. There has been extensive research and/or operating experience on each of these phenomena. Given this extensive state of knowledge, the knowledge levels for these phenomena are considered high.

Kinetics - Steady-state Assembly-to-Assembly Radial Peaking (M, H)

The local boron concentration near the riser hole locations (on the core-side of the riser) will govern the boron concentration of the flow through the riser holes and to the downcomer. In addition, mixing in the core would mitigate or preclude stratification of boron in the core/riser or

29 lower plenum sections. Therefore, mixing in the core is an important phenomenon due to its potential impact on the figures of merit. In previous work (see Ref. 7 and Ref. 8), staff found that a radial power profile in the core promotes internal recirculation (i.e., boron mixing) in the core and riser once bulk flow is interrupted. As the steady state radial power profile will govern the decay heat radial power profile, the steady-state assembly-to-assembly radial peaking is generally important. During period 1, this phenomenon is ranked with low importance because bulk flow will be intact. This phenomenon is ranked with medium importance in periods 2, 4, and 5 because it will affect the core mixing but is still secondary to the total core power. This phenomenon is considered of low importance during blowdown because core flashing will dominate core flow behavior. The panel discussed and agreed that the knowledge level for this phenomenon is high.

8.2.4 Subchannel

Subchannel - Boric Acid Volatilization (M, L)

During periods 1 and 2, the panel agreed that boric acid volatilization was unlikely because RPV pressure is expected to be high, which will suppress volatilization. The panel agreed that the importance ranking should be low for these periods. During periods 3 through 5, system pressure is expected to decrease significantly, and volatilization is more likely as a result. The panel agreed that the importance ranking for these periods is medium. The panel consider the state of knowledge with respect to boric acid volatilization and agreed it is low.

Subchannel - Boron Entrainment (Steam) (M, M)

Before blowdown, the void fraction in the core should remain relatively low; therefore, the rate of boric acid entrainment should be relatively small. The panel agreed that the importance of entrainment in the subchannel prior to blowdown is low. During blowdown, flashing in the core will enhance entrainment and this phenomenon is more important. In periods 4 and 5, system pressures will reduce resulting in increased voiding and more potential for entrainment. The panel agreed that entrainment was of medium importance in perio ds 3 through 5 instead of high importance because entrainment will be mitigated by the torturo us flow path through the baffle plate near the pressurizer. The baffle plate will act as a flow separator to reduce the amount of entrained fluid removed from the core/riser region over time. The panel considered the knowledge level and agreed that there is some uncertainty in the mechanism for entrainment as the flow regime transitions between periods. The panel agreed on a knowledge level of medium.

Subchannel - Boron Plate Out (M, M)

Throughout each scenario, boron will concentrate in the core/riser due to the distillation process described above. The panel agreed that boron plate out is most likely to occur in period 5 because system pressure and temperature will be the lowest (i.e., the solubility limit will be smallest in period 5) and the boron concentration will be great est. However, the panel also agreed that there is significant uncertainty on whether there will be sufficient boron in the core to cause boron plate out. The panel noted that the boron solubility limit increases dramatically as the water approaches its boiling point (Ref. 13), and saturated conditions are expected in the core. Therefore, the panel concluded that a significant amount of boron would need to be added to the system by the dissolver basket and transported to the core region to cause boron plate out, and that there is significant uncertainty in whether this will occur. If there is adequate boron to

30 cause boron plate out, plate out may occur on structures in the subchannel such as the control rod guide tubes, which are relatively cold. Given that there is uncertainty in whether this phenomenon will occur but that it could potentially directly impact the amount of dissolved boron in the primary, the panel agreed to rank this phenomenon with medium importance in period 5 for all scenarios. The panel agreed that this phenomenon is of low importance during all other periods. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level is medium. The knowledge level for this phenomenon for the fuel rods and subchannel (M) is higher than the other components (L) because there is more operational and modeling experience for boron plate out in the core, which reduces the degree of uncertainty.

Subchannel - Boron Precipitation (M, M)

Throughout each scenario, boron will concentrate in the core due to the distillation process described above. The panel agreed that boron precipitation is most likely to occur in period 5 because system pressure and temperature will be the lowest (i.e., the solubility limit will be smallest in period 5) and the boron concentration will be great est. However, the panel also agreed that there is significant uncertainty on whether there will be sufficient boron in the core to cause boron precipitation. The panel noted that the boron solubility limit increases dramatically as the water approaches its boiling point (Ref. 13), and saturated conditions are expected in the core. Therefore, the panel concluded that a significant amount of boron would need to be added to the system by the dissolver basket and transported to the core region to cause boron precipitation, and that there is significant uncertainty in whether this will occur. If boron precipitation does occur in the core, it is possible that the precipitate will sink to the lower plenum region and increase the boron concentration in the lower plenum. Given that there is uncertainty in whether this phenomenon will occur but that it c ould potentially directly impact a figure of merit, the panel agreed to rank this phenomenon with medium importance in period 5 for all scenarios. The panel agreed that this phenomenon is of low importance during all other periods. The panel agreed that the boron solubility limit is well understood. However, the panel agreed to a medium knowledge l evel because the state of practice for modeling boron precipitation and precipitate transport in nuclear reactors is not fully mature.

Subchannel - Cross Flow / Mixing (L, M)

This phenomenon is defined by the panel as subchannel-to-subchannel flow behavior (i.e., highly localized behavior). The panel agreed that the local flow behavior from subchannel-to-subchannel flow is not important, but the integral effect of the local behavior is important because it will affect the overall mixing behavior in the core. The integral effect of the subchannel-to-subchannel flow is captured by the turbulent mixing and radial/vertical natural circulation phenomena for the subchannel. With that understanding, the panel agreed that the cross flow/mixing phenomenon is of low importance because the localized mixing is averaged out compared to the integral behavior. The panel agreed the knowledge level is medium.

Subchannel - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that affect figures of merit. While the oscillations may affect

31 mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

Subchannel - Dissolving Boron (L, M)

The panel agreed that there may be some precipitation in the subchannel, especially in period 5.

However, if precipitation occurs, it is unlikely that there will be significant boron redissolving because the solubility limits are unlikely to increase, and the boric acid concentrations in the subchannel are unlikely to decrease as the transient progresses. Therefore, the panel agreed that the importance ranking for this phenomenon is low. The panel considered the knowledge level and agreed it is medium for similar reason as the boron precipitation phenomenon.

Subchannel - Entrance Effects / Developing Length (L, M)

Once the riser uncovers, these effects could have a modest impact on the natural circulation flow rate and pattern, which in turn would affect convective mixing of the boron. However, this phenomenon is expected to only have a small impact on the total mixing of the core and riser region. Other effects, including natural circulation forces, radial power peaking, etc. are expected to dominate core mixing. As a result, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed the knowledge level is medium.

Subchannel - Flashing (H, H)

For the LOOP scenario, flashing is not expected to occur in the first two periods because the RPV is sealed. For the SBLOCA scenario, some small amount of flashing may occur in the first two periods, but the amount is expected to be small because the break is small and in the liquid space, which results in a slow decrease in RPV pressure. The panel agreed that flashing is of low importance in the first two periods of the SBLOCA transient. In contrast, the break for the inadvertent RRV actuation scenario is relatively large, and the panel agreed that flashing is of medium importance for the first two periods. For all scenarios, flashing is of high importance in the blowdown period because flashing in the RPV can be substantial. Flashing during blowdown will cause boron concentration in the core/riser and relatively pure water to flow to the CNV.

Flashing is expected to occur to a less extent in period 4, and the panel agreed to a medium importance ranking. Flashing is unlikely in the long-term cooling phase, and the panel agreed to a low importance ranking. The panel considered the state of knowledge with respect to flashing and agreed on ranking the knowledge level high.

Subchannel - Primary Natural Circulation Flow / Bulk Flow (H, H)

The panel agreed that the bulk flow is of medium importance for period 1 and 2 of the SBLOCA and Inadvertent RRV scenarios because there will be significant fluid transport to the CNV, which results in potential changes in the FOMs in the RPV. For the LOOP scenario, bulk flow has a minimal impact on FOMs, and inventory is trapped in the RPV. Bulk flow is of high importance during periods 3 through 5 in all scenarios because it will result in significant boron dilution or concentration in different components. The panel considered the knowledge level of primary natural circulation and bulk flow behavior and agreed that the knowledge level is high.

32 Subchannel - Radiolysis (M, M)

Radiolysis will be a source of non-condensable gases in the primary and CNV. Prior to ECCS actuation, non-condensable gases will generally accumulate in the pressurizer and have a small impact on condensation rates in the steam generator annulus. Following ECCS actuation, non-condensable gases will accumulate in the CNV and, coupled with non-condensable gases that are present due to loss of CNV vacuum, may reduce the condensation rate in the CNV. This is significant because condensation in the CNV is a dilution mechanism; reducing the efficiency of condensation heat transfer in the CNV may result in less dilution in the CNV. Presence of non-condensable gases in RPV may reduce condensation on SG tube and the dilution in downcomer.

The panel agreed that the loss of containment vacuum is likely to be a greater source for non-condensable gases than radiolysis. The panel agreed that this phenomenon is of medium importance in all periods because it is a source for non-condensable gases that may affect dilution rates. The panel agreed that the knowledge level for this phenomenon is medium based on the current state of practice for modeling radiolytic gas formation and transport.

Subchannel - Radiolytic Decomposition of Boric Acid (L, L)

Radiolytic decomposition is expected to be insignificant because (1) it does not remove boron from the system, (2) core decay heat (and therefore heat fluxes) will be very small, and (3) there has not been operational evidence that radiolytic decomposition is a significant sink for boron.

The panel agreed that the importance rating for radiolytic decomposition is low for all periods.

The panel considered the state of knowledge of this phenomenon and agreed the knowledge level is low.

Subchannel - Species Stratification (L, M)

In all periods and for all transients, stratification is expected to be of low importance in the subchannel because natural convection mixing driven by decay heat will dominate stratification phenomena. The panel agreed that the knowledge level for species stratification in the subchannel is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models. Note that the knowledge level for species stratification in the containment is low due to additional uncertainty in the dissolver basket design.

Subchannel - Thermal Stratification (L, M)

In all periods and for all transients, stratification is expected to be of low importance in the subchannel because natural convection mixing driven by decay heat will dominate stratification phenomena. The panel agreed that the knowledge level for thermal stratification in the subchannel is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models.

Subchannel - Turbulent Mixing (M, L)

Turbulence in the subchannel is expected to occur in all periods due to a combination of structural flow agitators in the subchannel (e.g., spacer grids) or voiding. The panel agreed that turbulence is of low importance in the first period because it cannot contribute to changes in the boron concentration until after riser uncovery. For periods 2 through 5, steam flow from the core/riser will be replaced with relatively dilute coolant from the downcomer. Turbulence is important during these periods because turbulence (and other mixing phenomena) will mix the

33 relatively dilute flow and the relatively highly borated coolant and prevent stratification. The panel agreed that the importance ranking for this phenomenon is medium in periods 2 through 5, instead of high, because natural circulation phenomena are expected to dominate turbulence.

The panel discussed the state of knowledge of this phenomenon in this context and agreed the knowledge level is low, primarily because additional data would need to be obtained to effectively quantify turbulence for the flow conditions of interest.

Subchannel - Vertical / Radial Natural Circulation (H, M)

The basis for the importance rankings for this phenomenon is similar to the basis for turbulent mixing in the subchannel, but vertical/radial natural circulation is expected to be more prevalent (and therefore more important) than turbulent mixing. Note that this phenomenon is considered within the full core region; it is not limited to just the subchannel. The panel considered the knowledge level of vertical/radial mixing and agreed it is medium.

8.3 Decay Heat Removal System (DHRS) 8.3.1 DHRS Tubes

DHRS Tubes - Asymmetric Loading (L, M)

Asymmetric loading may occur due to plugging, etc., but they should have a low impact on dilution rates and RPV behavior. The asymmetric loading of a secondary system is typical in PWR operation. However, due to the complex heat transfer modes in the DHRS to remain in balance with the primary side, some uncertainties are expected. The panel considered the state of knowledge and experience and agreed on ranking the knowledge level as medium.

DHRS Tubes - Heat Transfer (Reactor Pool Side) (H, M)

The basis for the importance rankings for this phenomenon is the same as the basis for the condensation inside the DHRS tubes (below). The heat transfer on the pool-side of the DHRS tubes, which is generally expected to be nucleate boiling heat transfer, is in series with the other thermal resistances that make up the DHRS heat removal pathway. The panel agreed that the knowledge level for this phenomenon is medium.

DHRS Tubes - Condensation Heat Transfer (Inside DHRS Tubes) (H, M)

Condensation on the primary side of the SG tubes, which is part of the DHRS heat transfer pathway, is one of the primary dilution mechanisms. Therefore, it is important to capture the relative thermal resistance and heat removal capacity of the DHRS heat transfer pathway to accurately capture the amount and location of dilution compared to the other heat transfer pathways. Condensation inside the DHRS tubes is an important component of the DHRS thermal resistance. Note also that the initial DHRS inventory will affect the DHRS heat removal capacity; the allowable range for DHRS inventory will be dictated by technical specifications and is not explicitly evaluated as a phenomenon.

For period 1, the primary flow loop will be intact, and dilution will not occur. As a result, the panel agreed that this phenomenon is of relatively low importance in period 1 due to its low direct impact on dilution. Dilution will begin in period 2, an d the DHRS is expected to be the primary heat removal pathway (at least for the SBLOCA and LOOP scenarios; RRV flow will be competitive in the inadvertent RRV opening scenario). For this reason, the panel agreed that this 34 phenomenon is of high importance in period 2. During periods 3 through 5, DHRS cooling is expected to be active but secondary to other heat transfer pathways. As a result, the panel agreed that this phenomenon is of medium importance in periods 3 through 5. The panel considered the state of knowledge of condensation inside the DHRS tubes and agreed that it is medium because (1) condensation is highly geometry specific and (2) surface co nditions such as fouling may affect condensation.

DHRS Tubes - Conduction through the Tube Wall (H, H)

The importance rankings and the basis for the importance rankings for this phenomenon are the same as the rankings and basis for the condensation inside the DHRS tubes. The conduction through the DHRS tube wall is in series with the other thermal resistances that make up the DHRS heat removal pathway. The panel agreed that the knowledge level for conduction through a tube wall is high.

DHRS Tubes - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations are unlikely to occur in the DHRS tubes. If they were to occur, their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average quantities, they should not affect the heat transfer capacity of the DHRS. Thus, their importance was rated low for all periods. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities.

DHRS Tubes - Entrance Effects / Developing Length (L, H)

Entrance effects may affect both the flow rate and the heat transfer coefficient inside the DHRS tubes. These effects will also affect the heat removal capacity and thermal resistance of the DHRS heat removal pathway. However, these effects are believed to be small, and the panel agreed that other phenomena will dominate. In addition, these effects are generally captured by correlations derived for other phenomena (e.g., the condensation heat transfer correlation for condensation inside tubes includes flow regime information based on film thickness). As a result, the panel agreed that this phenomenon is of relatively low importance for all periods. The panel considered the state of knowledge this phenomenon and agreed the knowledge level is high.

DHRS Tubes - Flashing (L, H)

Flashing is not expected to occur in the DHRS tubes because condensation will dominate.

Therefore, the panel agreed to rank this phenomenon as inactive. The panel considered the knowledge level and agreed with a ranking of high.

DHRS Tubes - Inlet Losses / Orificing (H, H)

Like the DHRS heat transfer phenomena (e.g., condensation heat transfer in DHRS tubes), the dominant pressure loss phenomena in the DHRS are important because they will affect the DHRS thermal resistance and heat removal capacity. The pressure drop phenomena affect the heat transfer properties by affecting the loop flow rate. Inlet/orifice losses for the DHRS tubes are expected to be a significant contributor to DHRS loop pressure drop. During period 1, the

35 DHRS system cannot meaningfully affect boron dilution concentrations because the riser is still covered. Therefore, the importance ranking for this phenomenon is low. During period 2, DHRS heat transfer is dominant, and loop pressure drop is important for determining dilution rates via the DHRS. The panel agreed that this phenomenon is of high importance for period 2.

For periods 3 through 5, DHRS heat transfer is expected to be active but secondary to other heat transfer pathways. As a result, this phenomenon is ranked with medium importance. The panel considered the state of knowledge of this phenomenon and agreed it is high.

DHRS Tubes - Interfacial Drag / Relative Motion of the Phases (L, M)

While interfacial drag and the relative motion between phases may affect loop pressure drop in the DHRS, this phenomenon is considered secondary to other pressure drop phenomena.

Therefore, the panel agreed to rank this phenomenon with low importance. The panel considered the state of knowledge of this phenomenon and agreed it is medium.

DHRS Tubes - Ledinegg Instability (L, M)

While some instabilities may occu r in the DHRS or SG tubes, they are not expected to affect the average heat removal capacity of the DHRS. Since the boron dilution phenomena that may be affected by DHRS cooling are relatively slow moving, only the time-averaged heat removal capacity of the DHRS is important. Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level for this phenomenon is medium.

DHRS Tubes - Minimum Stable Film Boiling (I, M)

This phenomenon is not expected to occur in the DHRS tubes because condensation heat transfer will dominate. As a result, the panel agreed the importance ranking is inactive for all periods.

The panel considered the knowledge level and agreed with a ranking of medium.

DHRS Tubes - Natural Circulation (H, M)

This phenomenon is defined as the loop wise natural circulation in the DHRS, as opposed to local natural circulation within the DHRS condenser tubes. Like the DHRS heat transfer phenomena (e.g., condensation heat transfer in DHRS tubes), loop flow in the DHRS, which is driven by natural circulation with phase change, is important because it will affect the DHRS thermal resistance and heat removal capacity. During period 1, the DHRS system cannot meaningfully affect boron dilution concentrations because the riser is still covered. Therefore, the importance ranking for this phenomenon is low. During period 2, DHRS heat transfer is dominant, and flow is important for determining dilution rates via the DHRS. The panel agreed that this phenomenon is of high importance for period 2. For periods 3 through 5, DHRS heat transfer is expected to be active but secondary to other heat transfer pathways. As a result, this phenomenon is ranked with medium importance. The panel considered the state of knowledge of this phenomenon and agreed it is medium.

DHRS Tubes - Parallel Channel Effects (L, H)

Parallel channel effects may cause local flow variations, but they should not affect time averaged DHRS performance. Therefore, this phenomenon should have a negligible impact on boron dilution during all periods. The panel agreed that the importance rating for this phenomenon is low and the knowledge level is high. Note that the knowledge level for parallel channel effects 36 in the SG tubes (M) is less than the knowledge level in the DHRS tubes (H) because (1) there is less reliable experimental data for the pressure losses through the helical geometry of the SG tubes and (2) the pressure losses in the SG tubes is expected to vary from tube to tube due to differences in coil diameter (and therefore length) of the SG tubes.

DHRS Tubes - Pressure Wave Propagation (L, H)

The panel agreed that this phenomenon is of relatively low importance for all periods. The dynamic scale of pressure wave propagation, which is related to the speed of sound, is much quicker than the dynamic scale of boric acid concentration changes. Pressure changes could be treated as occurring instantaneously and it would not noticeably affect the evolution of the boron content. The panel agreed that the knowledge level of pressure wave propagation is high.

DHRS Tubes - Single-phase Heat Transfer to Liquid (L, H)

Condensation heat transfer is expected to be the dominant heat transfer regime in the DHRS tubes. While there may be some single-phase heat transfer near the bottom of the DHRS tubes, where the void fraction may be zero, the total heat transfer will be dominated by upstream condensation. Note that the initial DHRS inventory may affect the liquid level in the DHRS tubes; the allowable range for DHRS inventory will be dictated by technical specifications and is not explicitly evaluated as a phenomenon. Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed the knowledge level is high.

DHRS Tubes - Single-phase Heat Transfer to Vapor (L, H)

Condensation heat transfer is expected to be the dominant heat transfer regime in the DHRS tubes. While single phase heat transfer from the wall to the vapor may occur near the DHRS inlet, this regime will be small, and condensation will dominate the total heat transfer.

Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed the knowledge level is high.

DHRS Tubes - Single-phase Pressure Drop (L, H)

The pressure drop in the DHRS tubes will primarily be in the two-phase flow regime because condensation heat transfer will dominate. While there may be some single-phase pressure drop due to the accumulation of condensate near the bottom of the DHRS tubes, this is expected to be minor compared to other pressure drop phenomena. Therefore, the panel agreed that this phenomenon is of low importance during all periods. The panel agreed that the knowledge level for this phenomenon is high.

DHRS Tubes - Stored Energy of the Tubes (L, H)

The energy stored in the tubes is relatively small and will have an insignificant impact on the progression of boron dilution. The panel agreed that the importance ranking for this phenomenon is low over all periods and the knowledge level is high.

DHRS Tubes - Transition Boiling (I, M)

This phenomenon is not expected to occur in the DHRS tubes because boiling will not occur in the tubes. As a result, the panel agreed the importance ranking is inactive for all periods. The panel considered the knowledge level and agreed with a ranking of medium.

37 DHRS Tubes - Two-phase Heat Transfer (I, H)

Here, two phase heat transfer is defined as transfer of heat from the wall to the two-phase fluid within the DHRS tube (e.g., flow boiling). This phenomenon is expected to be inactive for all periods because heat will be removed via the reactor pool during all periods. The panel agreed the knowledge level is high.

DHRS Tubes - Two-phase Pressure Drop (H, M)

Two phase pressure drop is expected to be the dominant pressure drop term in the DHRS tubes and of a similar magnitude to the pressure drop from inlet/orificing losses and the DHRS inlet.

Therefore, the basis for the two-phase pressure drop rankings are similar to the basis inlet/orificing phenomenon for the DHRS tubes above. The panel considered the state of knowledge for two phase pressure drop and agreed the knowledge level is medium.

DHRS Tubes - Void Distribution (L, H)

The void distribution will have a secondary effect on loop flow and heat transfer compared to the other phenomena in this section. The panel agreed that other phenomena will dominate, and that void distribution is of relatively low importance. The panel agreed that the knowledge level for void distribution is high.

8.3.2 Piping from Condenser Tubes to Feedwater Inlet

Piping from Condenser Tubes to Feedwater Inlet - Flow Resistance (M, M)

The flow resistance in the piping from the DHRS tubes to the SG tubes (not including inlet/orificing effects at the SG tubes or DHRS tubes) is expected to be small compared to other pressure loss terms (i.e., two phase pressure losses in the SG and DHRS tubes and inlet/orifice losses). As a result, the panel agreed that this phenomenon is of low importance for most periods. The panel agreed that this phenomenon is of medium importance in period 2 of each transient, because period 2 is when the DHRS heat transfer is most significant. The panel agreed that the knowledge level for flow resistance in this piping is medium because there is some uncertainty in the piping design (i.e., piping diameter, piping length, types and number of form losses).

8.3.3 Piping from SG to Condenser Tubes

Piping from SG to Condenser Tubes - Flow Resistance (M, M)

The flow resistance in the piping from the DHRS tubes to the SG tubes (not including inlet/orificing effects at the SG tubes or DHRS tubes) is expected to be small compared to other pressure loss terms (i.e., two phase pressure losses in the SG and DHRS tubes and inlet/orifice losses). As a result, the panel agreed that this phenomenon is of low importance for most periods. The panel agreed that this phenomenon is of medium importance in period 2 of each transient, because period 2 is when the DHRS heat transfer is most significant. The panel agreed that the knowledge level for flow resistance in this piping is medium because there is some uncertainty in the piping design (i.e., piping diameter, piping length, types and number of form losses).

38 8.4 Emergency Core Cooling System (ECCS) 8.4.1 Reactor Coolant System (RCS) and Containment Vessel (CNV)

RCS and CNV - Molecular Diffusion (L, M)

The panel agreed that molecular diffusion is expected to be insignificant compared to convective mixing throughout the CNV and RPV, and that this phenomenon is of low importance for all periods. The panel considered the state of knowledge of molecular diffusion of boron is agreed the knowledge level is medium.

8.4.2 Reactor Pressure Vessel (RPV)

RPV - Flow Oscillation and Reversal (H, M)

During period 1, the riser will be covered and dilution will not occur. Therefore, the panel agreed flow oscillations and reversals are of low importance. At the initiation of blowdown (period 3), flashing will occur preferentially in the core/riser due to the higher coolant temperature. As a result, the inventory in the core/riser will rapidly expand and swell, causing the lower core inventory to briefly reverse and flow from the core to the lower plenum and downcomer (Ref. 8). As the pressure and level between the downcomer and core/riser equalize, flow oscillations will occur. These oscillations will promote mixing between the downcomer/lower plenum and core/riser volumes. As these volumes are likely to have significantly different boron concentrations, the panel agreed that flow oscillations and reversal are of high importance in period 3. During period 5, CNV and RPV pressure will decrease significantly and flashing or geysering instabilities become likely. These instabilities will promote mixing between the core/riser and downcomer/lower plenum. As a result, the panel agrees flow oscillation and reversal are of high importance in period 5. During periods 2 and 4, RPV pressure will remain relatively high, which should dampen flow oscillations or reversals driven by flashing or geysering. However, because some flow oscillations are still possible due to minor density wave propagation or other instabilities, the panel agreed that flow oscillation and reversal is of medium importance in periods 2 and 4. The panel agreed that the knowledge level is medium because there are some challenges in accurately modeling flow oscillation and instabilities, and there is some phenomenological uncertainty in geysering.

8.4.3 Reactor Recirculation Valves (RRVs)

RRVs - Boron Dissolving (H, L)

During the first three periods for all transients, the RRVs will either be closed or flashing and plate out will dominate. Therefore, this phenomenon is of low importance (or inactive, as appropriate) during the first three periods. During periods 4 and 5, RPV and CNV pressure will equalize, flashing will be minimal, and the boron concentration throughout the CNV and RPV inventory is expected to be below the solubility limit. As a result, flow through the RRVs will tend to dissolve plated out boron. The panel agreed that this phenomenon is of high importance during periods 4 and 5 for SBLOCA and RRV opening but medium for LOOP since the pressure difference between the RPV and CNV is expected to be relatively small. The panel agreed that the knowledge level for this phenomenon is low given the limited modeling experience and limited data on boron re-dissolving following plate out.

39 RRVs - Boron Plate Out (L, L)

Boron plate out may occur when there is flashing in the RRVs. Flashing is expected to occur in the RRVs during blowdown for the SBLOCA transient and the first three periods of the inadvertent RRV opening transient. However, most of the plate out is expected to happen in the vicinity of the RRV, which is captured in a different phenomenon, instead of within the RRV.

Boron plate out is expected to be limited within the RRV becaus e the large valve flow rate will discourage boron plating. Furthermore, any plate out that does occur within the first three periods will likely be re-dissolved during periods 4 and 5. As a result, the panel agreed that boron plate out is of low importance during these periods for these scenarios. This phenomenon is inactive in periods 1 and 2 of the SBLOCA and LOOP scenarios because the RRVs are closed.

During periods 3, 4 and 5 of the LOOP scenario, plate out is even more unlikely because flashing will be more limited. As a result, this phenomenon is ranked w ith low importance. The panel agreed that the importance ranking is low over these periods. The panel also agreed that the knowledge level for boron plate out is low based on the limited experimental database and modeling experience.

RRVs - Inlet Losses / Orificing (M, H)

The RRVs will be closed in periods 1 and 2 of the SBLOCA and LOOP transients. Therefore, this phenomenon is inactive. In all other periods, inlet/orifice pressure losses will have a modest effect on the rate of inventory transfer between the RPV to CNV, which will affect the evolution of the boron redistribution. The panel agreed that the importance ranking is medium over these periods. The panel also agreed that the knowledge level for this phenomenon is high based on the assumption that valve design information will be available.

RRVs - Venturi Effects (M, M)

Venturi effects are expected to behave similarly to inlet losses/orificing. Therefore, the panel agreed to the same rankings for both phenomena for the same bas is. The panel agreed the knowledge level for this phenomenon is medium.

RRVs (CNV-side) - Vortexing (L, L)

The panel agreed that vortexing in CNV will be localized and small in magnitude relative to other phenomena. Therefore, the panel agreed the importance ranking is low. The mixing behavior is localized and easily affected by the azimuthal flows and two-phase flow inside CNV, thus it requires higher resolution in modeling. The knowledge level is ranked as low since computational fluid dynamics (CFD) for applications with complicated two-phase environments is not mature.

RRVs (Primary-side) - Vortexing (M, M)

The panel agreed that vortexing is of low importance in periods 1 and 3 because bulk flow and/or blowdown phenomena would dominate. In periods 2, 4, and 5, the panel agreed to rank vortexing in the downcomer with medium importance. In general, the panel agreed that other mixing phenomena may dominate vortexing, but the panel agreed with a medium importance during these periods as a conservative assessment because vorte xing may affect boron mixing.

The panel agreed that the knowledge level is medium because sin gle phase CFD modeling is mature and can be expected to model this phenomenon in the downcomer region.

40 8.4.4 Reactor Vent Valves (RVVs)

RVVs - Boron Plate Out (L, L)

Flow through the RVVs is expected to be primarily steam because the RVVs are in the steam space and entrainment should be small due to flow separator effects produced by the baffle plate.

As a result, boron plate out on the RVVs should be minimal. The panel agreed that the importance ranking is low for all periods. The panel also considered the state of knowledge of boron plate out and agreed it is also low.

RVVs - Critical / Choked Flow (H, H)

Critical/choked flow will only occur in the RVVs during the blowdown period for the SBLOCA and inadvertent RRV opening scenarios. However, in the LOOP scenario, choked flow is not likely to happen due to the small pressure difference across the valve. The panel agreed that critical/choked flow is of high importance during the blowdown period because it will have a large impact on inventory (and therefore boron) transfer between the RPV and CNV and scenario progression for the SBLOCA and inadvertent RRV opening scenarios. The panel agreed that this phenomenon is of low importance for period 3 in the LOOP scenario. The panel agreed that this phenomenon is inactive in all other periods for all three scenarios. The panel discussed the state of knowledge of choked/critical flow and agreed it is high because there is sufficient data to calculate or bound this phenomenon in the ranges of interest.

RVVs - Inlet Losses / Orificing (M, H)

The RVVs are closed (i.e., inactive) for periods 1 and 2. The panel agreed that inlet/orifice losses are of medium importance during the blowdown period of the SBLOCA and inadvertent RRV opening scenarios because they could have a modest impact on the rate of inventory transfer from the RPV to the CNV. However, the panel agreed that this phenomenon is of only low importance during period 3 of the LOOP scenario since the pressure difference between the CNV and RPV is smaller. During periods 4 and 5, the steam flow rate from the RRV to the CNV is expected to be relatively insensitive to pressure losses due to inlet/orifice losses because the pressure losses will be small. The panel agreed the importance is low during these periods for all three scenarios. The panel considered the state of knowledge of this phenomenon and agreed it is high based on the assumption that valve design information will be available.

RVVs - Single-phase Pressure Drop (M, M)

The RVVs are closed (i.e., inactive) for periods 1 and 2. The panel agreed that single phase pressure losses are of medium importance during the blowdown period of the SBLOCA and inadvertent RRV opening scenarios because they could have a modest impact on the rate of inventory transfer from the RPV to the CNV; the critical/choked flow phenomenon will be dominant. However, the panel agreed that this phenomenon is of only low importance during period 3 of the LOOP scenario since the pressure difference between the CNV and RPV is smaller. During periods 4 and 5, the steam flow rate from the RRV to the CNV is expected to be relatively insensitive to pressure losses due to single-phase pressure losses because the pressure losses will be small. The panel considered the state of knowledge of this phenomenon and agreed it is medium.

41 RVVs - Two-phase Pressure Drop (L, M)

The RVVs may have some entrained water droplets during blowdown, potentially resulting in two-phase pressure drop, but it is expected to be minimal due to flow separator effects in the baffle plate. As a result, the panel agreed that this phenomenon is of low importance for all periods. The panel considered the state of knowledge for two phase pressure drop and agreed the knowledge level is medium.

RVVs - Venturi Effects (M, M)

The venturi is expected to affect flow similar to the inlet losses/orificing phenomenon in the RVV. Therefore, the importance rankings and their basis are the same for both phenomena. The panel considered the state of knowledge of the venturi effects and agreed the knowledge level is medium.

8.5 Instrumentation and Controls 8.5.1 Pressurizer Control Systems

Pressurizer Control Systems - Pressure Control System Feedback (L, H)

Control systems are only active for the beginning of period 1, and they are inactive for all other periods. As dilution will not occur in period 1, the panel agreed that control system feedback is of low importance for this period. The panel agreed the knowledge level for this phenomenon is high.

8.5.2 Primary and Secondary Control Systems

Primary and Secondary Control Systems - Signal Delay (L, H)

Control systems are only active for the beginning of period 1, and they are inactive for all other periods. As dilution will not occur in period 1, the panel agreed that control system feedback is of low importance for this period. The panel agreed the knowledge level for this phenomenon is high.

8.5.3 Reactor Protection

Reactor Protection - Detector Response (Including Instrumentation Response to Local Variations in Flow Fields) (L, M)

Signal delays or premature signals due to small variations in the local flow field will only cause a small variation in reactor trip times compared to the time scale of boron dilution. Therefore, this phenomenon will have a small impact on the FOMs and is rated wi th low importance. The staff considered the knowledge level and agreed on a ranking of medium.

8.5.4 Secondary Side Control System Feedback

Secondary Side Control System Feedback - Pressure Control System Feedback (L, M)

Control systems are only active for the beginning of period 1, and they are inactive for all other periods. As dilution will not occur in period 1, the panel agreed that control system feedback is

42 of low importance for this period. The panel agreed the knowledge level for this phenomenon is medium.

Secondary Side Control System Feedback - Resonant Interaction (L, M)

Control systems are only active for the beginning of period 1, and they are inactive for all other periods. As dilution will not occur in period 1, the panel agreed that control system feedback is of low importance for this period. The panel agreed the knowledge level for this phenomenon is medium.

Secondary Side Control System Feedback - Superheat Control System Feedback (L, M)

Control systems are only active for the beginning of period 1, and they are inactive for all other periods. As dilution will not occur in period 1, the panel agreed that control system feedback is of low importance for this period. The panel agreed the knowledge level for this phenomenon is medium.

8.6 Primary 8.6.1 All Components

Baffle Plate - Pressure Wave Propagation (L, H)

The panel agreed that this phenomenon is of relatively low importance for all periods. The dynamic scale of pressure wave propagation, which is related to the speed of sound, is much quicker than the dynamic scale of boric acid concentration changes. Pressure changes could be treated as occurring instantaneously and it would not noticeably affect the evolution of the boron content. The panel agreed that the knowledge level of pressure wave propagation is high.

8.6.2 Baffle Plate

Baffle Plate - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods in the baffle plate. The baffle plate will be in the vapor space for all but the beginning of the first period. In addition, density wave oscillations are unlikely in the first period. If they were to occur in the first period, they would be unlikely to affect dilution. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

Baffle Plate - Entrance Effects / Developing Length (L, M)

The fluid surrounding the baffle plate will primarily be vapor in all but the beginning of the first period. With this in mind, the ba ffle plate then only has two key influences on the transient progression. First, the baffle plate will cause a pressure los s in the steam flowing from the core to the RVVs. Second, the baffle plate will reduce entrainment in steam from the core by acting like a steam separator. These two factors are captured in later phenomena. Therefore, entrance effects/developing length are considered by the panel to be of low importance. The panel agreed the knowledge level for this phenomenon is medium.

43 Baffle Plate - Pressure Drop / Local Losses (M, H)

This phenomenon is expected to be of low importance during all periods of the LOOP scenario because the flow through the baffle plate is expected to remain small in all periods; in particular, in period 3, the differential pressure between the CNV and RPV will be small, resulting in small flow rates. During period 1 and 2 of the SBLOCA and inadvertent RRV scenarios, flow through the baffle plate is expected to be small because the RVVs will be closed. The panel agreed that this phenomenon is of low importance during the first two periods. During period 3 (blowdown), the baffle plate will cause pressure losses that may modestly affect RPV blowdown rates. As a result, the panel agreed that this phenomenon is of medium importance during period

3. During periods 4 and 5, steam pressure losses over the baffle plate are expected to be small and have a low importance with respect to dilution. The panel considered the knowledge level for this phenomenon and agreed it is high

Baffle Plate - Stagnation and Momentum Change (M, M)

For the purposes of the PIRT panel, the pressurizer surge line (below) refers to the series of holes in the baffle plate between the pressurizer fluid space and the upper plenum. These flow holes serve in the same capacity as the surge line in a conventional pressurized water reactor of allowing hydraulic communication between the pressurizer and the reactor coolant system. The baffle plate otherwise forms a barrier between the bulk natural circulation flow in the primary circuit and the pressurizer. Flow exiting the riser is directed from its upward direction to a radial outward direction by interaction with the baffle plate - herein this is process of the change in the flow direction is referred to as stagnation and momentum change by the PIRT panel.

When considering the possibility of entrained droplets from the riser section exiting the RPV through the RVVs, the steam flow must carry these droplets through a relatively tortuous path.

First, the bulk vapor flow changes direction above the riser at the baffle plate, reorienting from an upward to a radial direction. Second, the flow changes direction again above the downcomer as flow goes through the surge line, i.e., the baffle plate holes. The total flow path between the riser and the RVVs, therefore, involves multiple changes in bul k flow orientation that contribute to de-entrainment of most droplets in the vapor core.

When considering the importance of this phenomenon, it is important to recognize that the pressurizer will drain in period 1 and the RVVs will not open until period 3. Therefore, flashing of inventory in the pressurizer resulting in entrainment through the RVVs will not occur.

This phenomenon is of relatively low importance in periods 1 and 2 because the RVVs are not open, and flow will be small. The panel agreed that this pheno menon is of medium importance in periods 3 through 5 because stagnation and momentum change in the baffle plate will lead to steam separation and reduction of entrainment through the RVVs. Entrainment is significant because entrainment of borated water through the RVVs may result in boron plate out or boron transport to the CNV. The panel considered the state of knowledge of this phenomenon and agreed it is medium.

44 8.6.3 Break

Boron Dissolving (L, L)

During the first three periods for the SBLOCA transient, flashing and plate out will dominate, and dissolving is unlikely. Therefore, this phenomenon is of low importance during the first three periods. During periods 4 and 5, RPV and CNV pressure will equalize and there will not be liquid flow through the break. As a result, it will not be possible for boron to re-dissolve in the fluid and this phenomenon is inactive during these periods. The panel agreed that the knowledge level for this phenomenon is low given the limited modeling experience and limited data on boron re-dissolving following plate out.

Break - Boron Plate Out (L, L)

The break component is only in the SBLOCA transient. Boron plate out is likely to occur in the break when there is flashing in the break, and flashing is expected to occur in during periods 1 through 3. However, most of the plate out is expected to happen in the vicinity of the break, which is captured in a different phenomenon, instead of within the break. Boron plate out is expected to be limited within the break because the large break flow rate will discourage boron plating. As a result, the panel agreed that boron plate out is of low importance during these periods. The panel agreed that boron plate out will be minor d uring periods 4 and 5 because flashing over the break will be minimal or not occur. The pane l agreed that the importance ranking in periods 4 and 5 is low. The panel also agreed that the knowledge level for boron plate out is low based on the limited experimental database and modeling experience.

Break - Critical / Choked Flow (H, H)

There is only a break for the SBLOCA scenario. Therefore, this phenomenon is inactive for the inadvertent RRV opening and LOOP scenarios. Critical/choked flow will only occur in the break during the first three periods because there is a large differential pressure between the RPV and CNV. The panel agreed that critical/choked flow is of high importance during these periods because it will have a large impact on inventory (and therefore boron) transfer between the RPV and CNV and scenario progression. The panel discussed the state of knowledge of choked/critical flow and agreed it is high because there is sufficient data to calculate or bound this phenomenon in the ranges of interest.

Break - Inlet Losses / Orificing (M, H)

There may be some flow through t he break over all periods of the SBLOCA transient (the other scenarios do not have a break). Inlet/orifice pressure losses will have a modest effect on the rate of inventory transfer between the RPV to CNV, which will affect the evolution of the boron redistribution. The panel agreed that the importance ranking is medium over all periods of the SBLOCA transient. The panel also agreed that the knowledge level for this phenomenon is high based on the assumption that a spectrum of breaks will be evaluated.

Break - Single-phase Pressure Drop (M, H)

There will be flow through the break over the first three periods of the SBLOCA transient (the other scenarios do not have a break). Single phase pressure losses will have a modest effect on the rate of inventory transfer between the RPV to CNV, which will affect the evolution of the boron redistribution. The panel agreed that the importance ranking is medium over these periods

45 of the SBLOCA scenario. During periods 4 and 5, single phase flow will mostly be limited steam flow, and the pressure drop will be small. Therefore, the panel agreed this phenomenon is of low importance during these periods. The panel also agreed that the knowledge level for this phenomenon is high.

Break - Two-Phase Pressure Drop (M, H)

There will be flow through the break over the first three periods of the SBLOCA scenario (the other scenarios do not have a break). Two phase pressure losses will have a modest effect on the rate of inventory transfer between the RPV to CNV, which will affect the evolution of the boron redistribution. The panel agreed that the importance ranking is medium over these periods of the SBLOCA transient. During periods 4 and 5, two phase flow will include water entrained in steam flow, which will be insignificant and the pressure drop w ill be small. Therefore, the panel agreed this phenomenon is of low importance during these periods. The panel also agreed that the knowledge level for this phenomenon is high.

8.6.4 Downcomer

Downcomer - Boron Plate Out (M, L)

The panel agreed that this phenomenon is of medium importance i n period 5 and low importance in periods 1 through 4 for all three scenarios. The reasoning for these rankings is similar to the rankings for boron plate out in the riser (below). Compared to the hot leg riser, the downcomer will likely have subcooled conditions (i.e., a smaller solubili ty limit than the core/riser region) but a smaller concentration of boric acid. These competing effects may result in similar plate out behavior as discussed for the riser section. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level is low. This knowledge level was selected because (1) the state of practice for modeling boron plate out is immature and (2) the body of literate surrounding boron plate out for SMR applications is not large.

Downcomer - Boron Precipitation (M, M)

The panel agreed that this phenomenon is of medium importance i n period 5 and low importance in periods 1 through 4 for all three scenarios. The reasoning for these rankings is similar to the rankings for boron precipitation in the subchannel. Compared to the subchannel, the downcomer will likely have subcooled conditions (i.e., a smaller solubili ty limit than the core region) but a smaller concentration of boric acid. These competing effects may result in similar boron precipitation behavior as discussed for the subchannel. The panel agreed that the knowledge level for this phenomenon is medium for the same reasoning as boron precipitation in the subchannel.

Downcomer - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and

46 agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

Downcomer - Entrance Effects / Developing Length (L, H)

The panel agreed that entrance effects in the downcomer will have a relatively small impact on mixing and bulk flow rates. Therefore, this phenomenon was ranked with low importance for all periods. The panel agreed the knowledge level for this phenomenon is high.

Downcomer - Shrinkage (H, H)

During period 1, shrinkage of the primary fluid will significantly affect riser uncovery timing and is therefore of high importance (in the LOOP scenario, for example, riser uncovery would not happen without shrinkage of the primary loop fluid). During period 2, shrinkage has two key contributions to the transient progression. First, shrinkage will affect the timing of riser hole uncovery. Second, the downcomer is expected to be colder than the riser, which will result in different liquid levels due to shrinkage. The difference in riser and downcomer level will provide driving head for flow through the riser holes. As a result, the panel agreed shrinkage is of high importance during period 2. During the blowdown period, the panel ranked shrinkage with low importance because blowdown phenomena (e.g., choked/critical flow) will dominate level and flow behavior; there will still be shrinkage due to the rapid decrease in pressure (and therefore saturation temperature), but inventory loss will domi nate. In periods 4 and 5, shrinkage will affect the transients in the same ways as it did in period 2, but to a lesser extent. RPV and CNV pressures are expected to be approximately equilibrated at low pressure and temperature at the start of period 4. The pressure and temperatures will decr ease slowly as the decay heat decreases and stored energy is transferred to the reactor pool, but shrinkage will generally occur to a lesser extent in these periods. As a result, the panel ranked this phenomenon with low importance in periods 4 and 5. The panel agreed that shrinkage is well understood and the knowledge level is high.

Downcomer - Species Stratification (H, M)

During period 1, the nominal primary flow loop will be intact; therefore, stratification is of low importance. During period 3, blowdown forces will dominate; th erefore, stratification is of low importance. During periods 2 and 4 of the SBLOCA and LOOP scenarios, the bulk flow between the downcomer/lower plenum and core/riser may be low. In addition, the flow through the riser holes will be relatively borated (especially in the l ater part of period 2), and species stratification becomes more likely. While some mixing will cer tainly occur, the panel considered that the negatively buoyant forces due to differences in the boron concentration coupled with the relatively low flow may cause some stratification during period 2 and 4 of the SBLOCA and LOOP scenarios. Therefore, this phenomenon is ranked with medium importance during these periods. Due to the additional RRV flow blowdown in the downcomer in period 2 of the RRV opening scenario, the panel agreed that this phenomenon is of relatively low importance. Period 4 of the RRV scenario will be similar to the other scenarios, and therefore this phenomenon is ranked with medium importance. During perio d 5, stratification may be more likely because flow rates will further decrease as decay heat decreases and CNV-RPV pressure equalizes. Therefore, this phenomenon is ranked with high importance. The panel agreed that the knowledge level for species stratification is medium because the interplay

47 between stratification and natural convection forces are not fully validated for use in production evaluation models.

Downcomer - Thermal Stratification (H, M)

During period 1, the nominal primary flow loop will be intact; therefore, stratification is of low importance. During period 3, blowdown forces will dominate; th erefore, stratification is of low importance. During periods 2 and 4 of the SBLOCA and LOOP scenarios, the bulk flow between the downcomer/lower plenum and core/riser may be low. In addition, convection from the CNV-side of the RPV will occur preferentially towards the bottom of the downcomer and in the lower plenum where it is below CNV level, which may increase the likelihood of thermal gradients in the vertical direction. While some mixing will ce rtainly occur, the panel considered that the negatively buoyant forces due to differences in the temperature coupled with the relatively low flow may cause some stratification during period 2 and 4 of the SBLOCA and LOOP scenarios. Therefore, this phenomenon is ranked with medium importance during these periods. Due to the additional RRV flow blowdown in the downcomer in period 2 of the RRV opening scenario, the panel agreed that this phenomenon is of relatively low importance. Period 4 of the RRV scenario will be similar to the other scenarios, a nd therefore this phenomenon is ranked with medium importance. During period 5, stratification may be more likely because flow rates will further decrease as decay heat decreases and CNV-RPV pressure equalizes.

Therefore, this phenomenon is ranked with high importance. The panel agreed that the knowledge level for thermal stratification is medium because th e interplay between stratification and natural convection forces are not fully validated for use in production evaluation models.

Downcomer - Vertical / Radial Natural Circulation (H, M)

During period 1, the nominal primary flow loop will be intact; therefore, bulk flow behavior will dominate local vertical/radial natural circulation. During period 3, blowdown forces will dominate local vertical/radial natural circulation. Therefore, this phenomenon is ranked with low importance in periods 1 and 3. During periods 2, 4, and 5, temperature gradients between the riser wall (which is heated from the core/riser flow) and the RPV wall (which is cooled by CNV inventory) will cause local natural circulation patterns. These circulation patterns will compete with stratification phenomena to govern local boron concentrations in the downcomer and lower plenum. The panel believes that the natural circulation forces will be slightly more significant than stratification phenomena, and therefore ranks them with high importance in periods 2, 4, and 5. The panel agreed that the knowledge level for vertical/radial natural circulation is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models.

8.6.5 Hot Leg Riser

Hot Leg Riser - Boric Acid Volatilization (M, L)

During periods 1 through 3, the panel agreed that significant boric acid volatilization was unlikely because RPV pressure is expected to be high, which will suppress volatilization. The panel agreed that the importance ranking is low for these periods. During periods 4 and 5, system pressure is expected to decrease significantly, and volatilization is more likely as a result.

The panel agreed that the importance ranking for these periods is medium. The panel consider the state of knowledge with respect to boric acid volatilization and agreed it is low.

48 Hot Leg Riser - Boron Plate Out (M, L)

Throughout each scenario, boron will concentrate in the core/riser due to the distillation process described above. The panel agreed that boron plate out is most likely to occur in period 5 because system pressure and temperature will be the lowest (i.e., the solubility limit will be smallest in period 5) and the boron concentration will be great est. However, the panel also agreed that there is significant uncertainty on whether there will be sufficient boron in the core/riser to cause boron plate out. The panel noted that the boron solubility limit increases dramatically as the water approaches its boiling point (Ref. 13), and saturated conditions are expected in the core. Therefore, the panel concluded that a significant amount of boron would need to be added to the system by the dissolver basket and transported to the core region to cause boron plate out, and that there is significant uncertainty in whether this will occur. If there is adequate boron to cause boron plate out, plate out will preferentially occur on the riser wall because it is relatively cold. Given that there is uncertainty in whether this phenomenon will occur but that it could potentially directly impact the amount of dissolved boron in the primary, the panel agreed to rank this phenomenon with medium importance in period 5 for all scenarios.

The panel agreed that this phenomenon is of low importance during all other periods. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level is low. This knowledge level was selected because (1) the state of practice for modeling boron plate out is immature and (2) the body of literate surrounding boron plate o ut for SMR applications is not large.

Hot Leg Riser - Conduction (M, H)

The primary flow loop will be intact during period 1. Therefor e, the heat transfer via conduction through the riser wall will have an insignificant impact on boron redistribution. The panel agreed to rank this phenomenon with low importance for period 1 for all three scenarios. The importance of this phenomenon is complicated during period 2. The conductivity of the riser can have a strong influence on the temperature difference between the riser and downcomer inventories under certain conditions. This temperature difference will cause different shrinkage rates, and therefore levels, between the downcomer and riser. The difference in level will provide driving head for riser hole flow rates. For the SBLOCA and inadvertent RRV opening scenarios, however, DHRS condensation heat transfer will dominate because more of the SG tubes are in the vapor space. Saturated conditions will prevail in the RPV due to the effective DHRS heat removal and depressurization at the break/RRV. For these transients, the panel agreed that conduction in the riser is of relatively low importance. In the LOOP scenario, the level and pressure will decrease at a slower rate, subcooled conditions are more likely, and conduction heat transfer becomes more significant. The panel agreed that this phenomenon is of medium importance during period 2 of the LOOP transient. For all three transients, the panel agreed that conduction heat transfer in the riser is of low importance during period 3 because blowdown phenomena (e.g., critical/choked flow) will dominate. For periods 4 and 5, conduction in the riser is of medium importance for two reasons. First, the conduction-convection pathway (defined above under other phenomena) is more important during these periods. Second, conduction heat transfer in the riser and RPV walls will establish the temperature boundary conditions in the downcomer and riser. Th is temperature difference will drive vertical/radial natural circulation, which is an important mixing phenomenon. The panel agreed that the knowledge level for conduction is high.

49 Hot Leg Riser - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

Hot Leg Riser - Entrainment (M, M)

Before blowdown, the void fraction in the core/riser should remain relatively low. Therefore, the rate of boric acid entrainment should be relatively small. The panel agreed that the importance of entrainment in the riser prior to blowdown is low. During blowdown, flashing in the core will enhance entrainment and this phenomenon is more important. In periods 4 and 5, system pressures will reduce resulting in increased voiding and more potential for entrainment. The panel agreed that entrainment was of medium importance in perio ds 3 through 5 instead of high importance because entrainment will be mitigated by the torturo us flow path through the baffle plate near the pressurizer. The baffle plate will act as a flow separator to reduce the amount of entrained fluid removed from the core/riser region over time. The panel considered the knowledge level of entrainment and agreed that there is some phenomenological uncertainty.

The panel agreed on a knowledge level of medium.

Hot Leg Riser - Entrance Effects / Developing Length (L, M)

The panel agreed that entrance effects in the riser will have a relatively small impact on mixing and bulk flow rates compared to other phenomena (e.g., natural convection). Therefore, this phenomenon was ranked with low importance for all periods. The panel agreed the knowledge level for this phenomenon is medium.

Hot Leg Riser - Ledinegg Instability (L, M)

Ledinegg instabilities are considered unlikely in the core/riser because there is good radial flow communication between subchannels and in the riser. In addition, the void concentration is not expected to vary significantly (e.g., from the subcooled to dryout regimes) in the core riser, which should decrease the likelihood of these instabilities. Regardless, if these instabilities were to occur, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. As a result, the panel agreed that this phenomenon is of low importance for all periods. The panel discussed the state of knowledge for Ledinegg Instability and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities.

Hot Leg Riser - Riser Hole Flow (H, M)

During period 1, the primary flow loop is intact. Therefore, riser hole flow will have an insignificant impact on boron redistribution. During periods 2 through 5, riser hole flow is

50 expected to occur. This phenomenon will directly contribute to transfer of boron from the riser/core to the downcomer, and therefore have a direct impact on the figures of merit. As a result, this phenomenon is ranked with high importance during periods 2 through 5. The panel agreed that the knowledge level for this phenomenon is medium.

Hot Leg Riser - Riser Hole Orificing / Pressure Drop (H, M)

This phenomenon is tightly coupled with the riser hole flow phenomenon, and therefore has the same ranking and knowledge level.

Hot Leg Riser - Shrinkage (H, H)

During period 1, shrinkage of the primary fluid will significantly affect riser uncovery timing and is therefore of high importance (in the LOOP scenario, for example, riser uncovery would not happen without shrinkage of the primary loop fluid). During period 2, shrinkage has two key contributions to the transient progression. First, shrinkage will affect the timing of riser hole uncovery. Second, the downcomer is expected to be colder than the riser, which will result in different liquid levels due to shrinkage. The difference in riser and downcomer level will provide driving head for flow through the riser holes. As a result, the panel agreed shrinkage is of medium importance during period 2. During the blowdown period, the panel ranked shrinkage with low importance because blowdown phenomena (e.g., choked/critical flow) will dominate level and flow behavior; there will still be shrinkage in the liquid inventory due to the decrease in saturation temperature (which is caused by the rapid decrease in pressure and liquid flashing), but inventory loss will dominate. In periods 4 and 5, shrinkage will affect the scenarios in the same ways as it did in period 2, but to a lesser extent. RPV and CNV pressures are expected to be approximately equilibrated at low pressure a nd temperature at the start of period 4. The pressure and temperatures will decrease slowly as the decay heat decreases and stored energy is transferred to the reactor pool, but shrinkage will generally occur to a lesser extent in these periods. As a result, the panel ranked this phenomenon with low importance in periods 4 and 5. The panel agreed that shrinkage is well understood, and the knowledge level is high.

Hot Leg Riser - Subcooled Boiling (L, H)

During the first period, some subcooled boiling is likely to occur in the core, but it will not have a significant impact on dilution rates because the primary flow loop will be intact. Subcooled boiling may occur in the core during period 2 of the LOOP transient (see the ranking justification for conduction in the riser for more detail), but this is expected to have a minor impact on dilution rates. Therefore, this phenomenon is ranked with low importance. During all other periods for all three transients, saturated conditions are expected to prevail; therefore, this phenomenon will be inactive.

Hot Leg Riser - Turbulent Mixing (e.g., Hot Streaking) (M, M)

Turbulence in the riser is expected to occur in all periods due primarily to voiding. The panel agreed that turbulence is of low importance in the first period because it cannot contribute to changes in the boron concentration until after riser uncovery. For periods 2 through 5, steam flow from the core/riser will be replaced with relatively dilute coolant from the downcomer.

Turbulence is important during these periods because turbulence (and other mixing phenomena) will mix with the relatively dilute flow the relatively highly borated coolant and prevent

51 stratification. The panel agreed that turbulence in the riser is of medium importance for periods 2, through 5. Turbulence in the riser for these periods is slightly less than in the subchannel due to the absence of structures that might agitate the flow and cause additional turbulence. The panel considered the state of knowledge of this phenomenon and agreed the knowledge level is medium. Note that the knowledge level for this phenomenon is greater the knowledge level for turbulence in the subchannel because the geometry in the riser is significantly simpler.

Hot Leg Riser - Two-phase Level Swell (H, M)

Since the nominal primary flow path will be intact in period 1, two phase level swell will be minor and will have a negligible impact on FOMs. During period s 2, 4, and 5 of the SBLOCA and inadvertent RRV opening transients and periods 4 and 5 of the LOOP transient, two-phase level swell will create a driving head for flow through the riser holes. As a result, the panel agreed that this phenomenon is of medium importance during these periods. During the blowdown period of all scenarios, two-phase level swell and flush following ECCS actuation may cause significant mixing between the downcomer/lower plenum and core/riser inventories.

As a result, this phenomenon is of high importance in period 3. During period 2 for the LOOP scenario, subcooled conditions may prevail, and this phenomenon is of less important. The panel agreed that this phenomenon is of low importance in period 2. The panel considered the state of knowledge of this phenomenon and agreed the knowledge level is medium.

Hot Leg Riser - Vertical / Radial Natural Circulation (H, M)

The basis for the importance rankings for this phenomenon is similar to the basis for vertical/radial natural circulation in the subchannel. Note that this component includes the region above the core and below the narrowed section of the riser. The panel considered the knowledge level of vertical/radial mixing and agreed it is medium.

8.6.6 Lower Plenum

Lower Plenum - Boron Plate Out (M, L)

The panel agreed that this phenomenon is of medium importance i n period 5 and low importance in periods 1 through 4 for all three scenarios. The reasoning for these rankings is similar to the rankings for boron plate out in the riser (below) and downcomer. Compared to the hot leg riser, the lower plenum will likely have subcooled conditions (i.e., a smaller solubility limit than the core/riser region) but a smaller concentration of boric acid. These competing effects may result in similar plate out behavior as discussed for the riser section. The panel discussed the state of knowledge of boron plate out and agreed the knowledge level is low. This knowledge level was selected because (1) the state of practice for modeling boron plate out is immature and (2) the body of literate surrounding boron plate out for SMR applications is not large.

Lower Plenum - Boron Precipitation (M, M)

The panel agreed that this phenomenon is of medium importance i n period 5 and low importance in periods 1 through 4 for all three scenarios. The reasoning for these rankings is similar to the rankings for boron precipitation in the subchannel and downcomer. Compared to the subchannel, the lower plenum will likely have subcooled conditi ons (i.e., a smaller solubility limit than the core region) but a smaller concentration of boric acid. These competing effects may result in similar boron precipitation behavior as discussed for the subchannel. The panel

52 agreed that the knowledge level for this phenomenon is medium for the same reasoning as boron precipitation in the subchannel.

Lower Plenum - Dissolving (Boron) (M, M)

The panel agreed that this phenomenon is of medium importance i n period 5 and low importance in periods 1 through 4 for all three scenarios. As discussed under boron precipitation in the subchannel, it is possible for boron precipitate formed in the core to sink to the lower plenum region. This is considered most likely in period 5 for the reasoning discussed above. If the lower plenum is below the solubility limit, the boron may redissolve into solution and increase the average dissolved boron concentration in the lower plenum. Given the uncertainty in whether boron precipitation will occur but its potential impact on figures of merit, the panel agreed that this phenomenon is of medium importance in period 5 and low importance for all other periods. The panel considered the knowledge level and agreed it is medium for similar reason as the boron precipitation phenomenon in the subchannel.

Lower Plenum - Species Stratification (H, M)

During period 1, the nominal primary flow loop will be intact; therefore, stratification is of low importance. During period 3, blowdown forces will dominate; th erefore, stratification is of low importance. During period 2 of the SBLOCA and LOOP scenarios, the bulk flow between the downcomer/lower plenum and core riser may be low. While some mixing will certainly occur, the panel considered that the negatively buoyant forces due to differences in the boron concentration coupled with the relatively low flow may cause some stratification during period 2 of the SBLOCA and LOOP scenarios. Therefore, this phenomenon is ranked with medium importance. Due to the additional RRV blowdown flow in the downcomer in period 2 of the inadvertent RRV opening scenario, the panel agreed that this phenomenon is of relatively low importance. During periods 4 and 5, there will likely be less vertical/radial circulation in the lower plenum than the downcomer because the bottom of the RPV i s cooled by the inventory in the CNV. As a result, species stratification is more likely in the lower plenum, and this phenomenon is ranked with high importance in these periods. The panel agreed that the knowledge level for species stratification is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models. In particular, modeling stratification in the lower plenum may be challenging in periods 4 and 5, for example, because there will likely be an area at the top of the lower plenum where there is bulk flow from the downcomer to the core and an area at the bottom of the lower plenum where stratification will dominate. It is possible that modeling practices for modeling boron transport in the lower plenum of Boiling Water Reactors may provide usefu l insights for modeling boron transport in the lower plenum for the present application.

Lower Plenum - Thermal Stratification (H, M)

During period 1, the nominal primary flow loop will be intact; therefore, stratification is of low importance. During period 3, blowdown forces will dominate; th erefore, stratification is of low importance. During period 2 of the SBLOCA and LOOP scenarios, the bulk flow between the downcomer/lower plenum and core riser may be low. While some mixing will certainly occur to some extent, the panel considered that negatively buoyant forces due limited heat transfer to the lower plenum coupled with the relatively low flow may cause some stratification during period 2 of the SBLOCA and LOOP scenarios. Therefore, this phenomenon is ranked with medium

53 importance. Due to the additional RRV blowdown flow in the downcomer in period 2 of the RRV opening scenario, the panel agreed that this phenomenon is of relatively low importance.

During periods 4 and 5, there will likely be less vertical/radial circulation in the lower plenum than the downcomer because the bottom of the RPV is cooled by t he inventory in the CNV. As a result, thermal stratification is more likely in the lower plenum. Therefore, this phenomenon is ranked with high importance in these periods. The panel agreed that the knowledge level for thermal stratification is medium because the interplay between stratification and natural convection forces are not fully validated in modeling space. Some additional relevant discussion relating to the knowledge level is provided for species stratification in the lower plenum.

Lower Plenum - Vertical / Radial Natural Circulation (H, M)

During period 1, the nominal primary flow loop will be intact. Therefore, bulk flow behavior will dominate local vertical/radial natural circulation. During period 3, blowdown forces will dominate local vertical/radial natural circulation. Therefore, this phenomenon is ranked with low importance in periods 1 and 3. During periods 2, 4, and 5, natural circulation in the lower plenum may be caused by bulk flow or natural convection from the core/riser entraining or mixing flow in the lower plenum. This mixing is important for periods 2, 4, and 5 of the SBLOCA and LOOP scenarios and periods 4 and 5 for the RRV opening scenario because it may mix boron in the lower plenum with the core/riser inventories. As a result, this phenomenon is ranked with high importance during these periods. This phenomenon is ranked with medium importance in period 2 of the RRV opening scenario because RRV flow is expected to be more important than natural convection-induced flow. The panel agreed that the knowledge level for vertical/radial natural circulation is medium because the interplay between stratification and natural convection forces are not fully validated for use in production evaluation models.

8.6.7 Pressurizer

Pressurizer - Surge Line Flow (M, H)

For the purposes of the PIRT panel, the pressurizer surge line refers to the series of holes in the baffle plate between the pressurizer fluid space and the upper plenum. These flow holes serve in the same capacity as the surge line in a conventional pressurized water reactor of allowing hydraulic communication between the pressurizer and the reactor coolant system. The baffle plate otherwise forms a barrier between the bulk natural circulation flow in the primary circuit and the pressurizer. Flow exiting the riser is directed from its upward direction to a radial outward direction by interaction with the baffle plate - herein this is process of the change in the flow direction is referred to as stagnation and momentum change by the PIRT panel.

When considering the possibility of entrained droplets from the riser section exiting the RPV through the RVVs, the steam flow must carry these droplets through a relatively tortuous path.

First, the bulk vapor flow changes direction above the riser at the baffle plate, reorienting from an upward to a radial direction. Second, the flow changes direction again above the downcomer as flow goes through the surge line, i.e., the baffle plate holes. The total flow path between the riser and the RVVs, therefore, involves multiple changes in bul k flow direction that contribute to deentrainment of most droplets in the vapor core.

54 When considering the importance of this phenomenon, it is importance to recognize that the pressurizer will drain in period 1 and the RVVs will not open until period 3. Therefore, flashing of inventory in the pressurizer resulting in entrainment through the RVVs will not occur. Also, in the LOOP scenario, the RVVs open at low pressure, and the pressure difference between the CNV and RPV is expected to be small. Therefore, the surge line flow is expected to be small.

This phenomenon is of relatively low importance in periods 1 and 2 because the RVVs are not open, and flow will be small. The panel agreed that this pheno menon is of medium importance in periods 3 through 5 because the tortuous flow path in the baffle plate (i.e., the surge line) will lead to steam separation and reduction of entrainment through the RVVs. Entrainment is significant because entrainment of borated water through the RV Vs may result in boron plate out or boron transport to the CNV. The panel considered the state of knowledge of this phenomenon and agreed it is high.

Pressurizer - Heater (L, H)

Pressurizer control systems are expected to be become inactive early in period 1 for all transients. Therefore, this phenomenon is of low importance for period 1 and inactive for periods 2 through 5. The panel agreed the knowledge level is high.

Pressurizer - Letdown Rate / Timing (L, H)

CVCS will isolate early in period 1 for all three transients. Therefore, this phenomenon is of low importance for period 1 and inactive for all other periods. The panel agreed that the knowledge level for this phenomenon is high.

Pressurizer - Makeup Borated Water State and Injection Rate Timing (L, H)

CVCS will isolate early in period 1 for all three transients. Therefore, this phenomenon is of low importance for period 1 and inactive for all other periods. The panel agreed that the knowledge level for this phenomenon is high.

Pressurizer - Nitrogen Overpressure (I, H)

According to section 5.4.4 of the NuScale Design Certification (Ref. 10), the NuScale NPM will be purged of non-condensable gases during start up and continually purged, as necessary, during operation. Therefore, the panel agreed that this phenomenon is inactive during all periods. The panel also agreed the knowledge level for this phenomenon is high.

Pressurizer - Spray (L, H)

Pressurizer control systems are expected to be inactive early i n period 1 due to turbine trip or LOOP for all transients. Therefore, this phenomenon is of low importance for period 1 and inactive for all other periods. The panel agreed the knowledge level is high.

Pressurizer - Vertical / Radial Natural Circulation (L, M)

The pressurizer will remain in the vapor space during periods 2, 4 and 5. During period 3, blowdown forces will dominate local natural convection. During period 1, the riser will be covered, anddilution will generally not occur. Therefore, local natural convection will have a minimal impact on heat transfer or boron mixing in all periods, and the panel agreed this

55 phenomenon is of low importance. The panel agreed that the knowledge level for this phenomenon is medium.

Pressurizer - Water Thermal Expansion (L, H)

The pressurizer will remain in the vapor space during periods 2, 4 and 5. During period 3, liquid inventory may briefly flow through the pressurizer, but water thermal expansion will be negligible. During period 1, the riser will be covered, and dilution will generally not occur.

Therefore, water thermal expansion will have a minimal impact on heat transfer or boron mixing in all periods, and the panel agreed this phenomenon is of low importance. The panel agreed that the knowledge level for this phenomenon is high.

8.6.8 Riser Holes

Riser Holes - Flow Blockage from Debris (L, L)

The panel agreed that it is unlikely that there will be enough debris in the system to cause significant flow blockages in the riser holes. Therefore, the panel ranked this phenomenon with low importance. The panel agreed that the knowledge level for this phenomenon is low.

Riser Holes - Flow Blockage from Plate Out (L, L)

In general, plate out on the riser holes would only be expected to occur if significant flashing occurred at the riser hole location (for more detail, see the discussions for the plate out phenomena for other components). Flashing may occur on the riser holes during blowdown but is otherwise not expected. During blowdown, riser hole blockage due to plate out is considered unlikely. Furthermore, flashing will preferentially occur at the higher riser hole elevations due to reduced hydrostatic head. These riser holes will likely be uncovered following blowdown, and therefore their blockage would likely be inconsequential. If blockage were to occur below the liquid level in period 4, they would be significant due to their influence on riser hole flow.

However, this is considered unlikely. Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level for this phenomenon is low.

Riser Holes - Flow Regime on Core / Riser Side and Downcomer Side (H, M)

The flow regime on either side of the riser will be significant in determining the relative pressure difference over the riser holes. For example, significant voiding on the core side of the riser will increase level compared to the downcomer and drive riser flow. Flow through the riser will transport boron to the downcomer, which will have a direct impact on figures of merit. The panel agreed that this phenomenon is of medium importance for periods 2 through 4. During period 5, the integral of riser hole flow will be greater because the period is significantly longer than the other periods. Therefore, this phenomenon is ranked with high importance in period 5.

During period 1, dilution will not occur; therefore, this phenomenon is of low importance. The panel agreed that the knowledge level for this phenomenon is medium.

Riser Holes - Gravity Head due to Inclination Angle (M, M)

The inclination angle on the riser holes is expected to contribute only a small difference in hydrostatic head. As the first three periods are relatively short and other phenomena will dominate, the integral effect of this phenomenon is expected to be relatively small. As a result, 56 this phenomenon is ranked with low importance during periods 1 through 3. Periods 4 and 5 are longer, and the flow is expected to be calmer, and this phenomenon is more significant.

Therefore, the panel agreed this phenomenon is of medium importance for periods 4 and 5. The panel agreed that the knowledge level for this phenomenon is medium due to design uncertainty.

Riser Holes - Irreversible Pressure Losses (M, M)

The irreversible pressure losses in the riser holes are expected to be small. As the first three periods are relatively short and other phenomena will dominate, the integral effect of this phenomenon is expected to be relatively small. As a result, this phenomenon is ranked with low importance during periods 1 through 3. Periods 4 and 5 are longer, and the flow is expected to be calmer, and this phenomenon is more significant. Therefore, the panel agreed this phenomenon is of medium importance for periods 4 and 5. The pa nel agreed that the knowledge level for this phenomenon is medium due to design uncertainty.

Riser Holes (Downcomer-side) - Vortexing (M, M)

The panel agreed that vortexing is of low importance in periods 1 and 3 because bulk flow and blowdown phenomena would dominate. In periods 2, 4, and 5, the panel agreed to rank vortexing in the downcomer with medium importance. In general, the panel agreed that other mixing phenomena may dominate vortexing, but the panel agreed with a medium importance during these periods as a conservative assessment because vorte xing may affect boron mixing.

Note that the flow direction is expected to generally be from the riser to the downcomer, but it may reverse flow direction under certain conditions. The panel agreed that the knowledge level is medium because single phase CFD modeling is mature and can b e expected to model this phenomenon in the downcomer region.

Riser Holes (Riser-side) - Vortexing (L, L)

The panel agreed that vortexing may occur on the riser-side of the riser holes, but its effect is relatively small compared to bulk flow and natural circulation in the riser, core, or upper plenum region. Therefore, the panel agreed the importance ranking is low for all periods. The mixing behavior is localized and easily affected by the azimuthal flows in the domain, thus it requires higher resolution in modeling. The knowledge level is ranked low since two phase CFD modeling is not mature.

8.6.9 Steam Generator (SG) Annulus

SG Annulus - Condensation on Uncovered SG Tubes (H, M)

Condensation on the primary side of the SG tubes, which is part of the DHRS heat transfer pathway, is one of the primary dilution mechanisms. Therefore, it is important to capture the relative thermal resistance and heat removal capacity of the DHRS heat transfer pathway to accurately capture the amount and location of dilution compared to the other heat transfer pathways. For period 1, the primary flow loop will be intact, and dilution will not occur. As a result, the panel agreed that this phenomenon is of relatively low importance in period 1 due to its low direct impact on dilution. Dilution will begin in period 2, and the DHRS is expected to be the primary heat removal pathway (at least for the SBLOCA and LOOP scenarios; RRV flow will be competitive in the inadvertent RRV opening scenario). For this reason, the panel agreed that this phenomenon is of high importance in period 2. During periods 3 through 5, DHRS

57 cooling is expected to be active but secondary to other heat transfer pathways. One could speculate that the DHRS heat transfer rate will be quite small during these periods due to the low RPV pressure; however, as a conservatism, the panel agreed that this phenomenon is of medium importance during periods 3 through 5 due to its potential impact on dilution in the downcomer.

The panel discussed the state of knowledge of condensation on the SG tubes and agreed that it is medium due to the complicated geometry and uncertainty due to surface effects (e.g., fouling).

SG Annulus - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

SG Annulus - Entrance Effects / Developing Length (L, M)

In period 1, nominal bulk flow will occur because the riser will be covered. Therefore, dilution will not occur, and these effects are of low importance. Following riser uncovery, the single-phase flow rate through the SG annulus will be small because bulk flow is interrupted. These effects will have a small impact on boron dilution. In additio n to single phase entrance effects, however, condensate will drip from the SG tubes into the liquid in the downcomer. These entrance effects will create some mixing, but their importance is expected to be relatively small.

Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level for this phenomenon is medium.

SG Annulus - Non-condensable Gas Effects (L, M)

Non-condensable gases may form in the primary due to radiolysis. However, non-condensable gases will likely accumulate in the pressurizer, resulting in o nly small concentrations of non-condensable gases in the SG annulus. Therefore, the panel agreed that non-condensable gas effects in the SG annulus will have a low importance for all periods because they will not have a significant impact on condensation rates on the SG tubes. The panel agreed that the knowledge level for non-condensable gas effects on condensation is medium.

SG Annulus - Vertical / Radial Natural Circulation (M, M)

In period 1, the riser will be covered, primary flow will be intact, and dilution will be minimal.

Therefore this phenomenon is ranked with low importance in period 1. During period 2, this component will have both liquid and vapor inventory. Mixing in the liquid space is importance because it effects how the relatively pure condensate will mix with the rest of the downcomer inventory. The panel agreed that this phenomenon is of medium importance in period 2 for this reason. During period 3, blowdown forces will dominate, and this phenomenon is of low importance. The SG annulus will be filled with vapor in period s 4 and 5. While natural convection of the steam may have some small effect on condensation heat transfer, this effect is generally expected to be small and captured by the condensation heat transfer correlations.

58 Therefore, this phenomenon is ranked with low importance in periods 4 and 5. The panel agreed that the knowledge level for this phenomenon is medium.

8.6.10 Upper Plenum

Upper Plenum - Density Wave Propagation (L, M)

The panel agreed that density wave driven oscillations are of low importance for all periods.

Density wave driven oscillations may occur in the RPV, but their period will be short and tend to cancel out over long periods important to the evolution of the boron concentration, which evolves slowly. Since these oscillations will not affect time-average flow quantities, they should not cause bulk fluid motion that will affect figures of merit. While the oscillations may affect mixing, this phenomenon is considered separately under the Flow Oscillations and Reversal phenomenon. The panel discussed the state of knowledge for density wave oscillations and agreed that the state of knowledge is medium based on the current state of practice for modeling these instabilities in cases where the natural circulation flow loop is broken.

Upper Plenum - Downcomer Bypass Flow (L, H)

This phenomenon captures bypass flow paths around the outside of the riser wall. The panel has not rigorously defined all potential bypass flows in the system for all periods but expects that the flow in these paths to be small, especially during periods 2 through 5. Therefore, the panel agreed that this phenomenon will be a low importance during all periods due to its minimal impact on dilution rates and inventory mixing. The panel agreed that the knowledge level for this phenomenon is medium.

Upper Plenum - Entrance Effects / Developing Length (L, M)

Following period 1, the upper plenum will be in the vapor space, and this phenomenon will have a minimal impact on flow and heat transfer. During period 1, entrance effects are expected to have a minimal impact on FOMs because the riser will be covered, and dilution will generally not occur. As a result, this phenomenon was ranked with low importance for all periods. The panel agreed the knowledge level is medium.

Upper Plenum - Flooding / Countercurrent Flow Limitation (CCFL) (L, M)

CCFL would require significant void generation in the core coincident with significant liquid inventory in the riser/upper plenum section. These conditions are unlikely prior to blowdown, so this phenomenon is ranked with low importance in periods 1 and 2. During periods 3, 4, and 5, it is possible for increased voiding in the core to create the conditions of interest. However, the panel generally agreed that this phenomenon is relatively unlikely, and that the importance ranking is low for all periods. The panel agreed that the knowledge level is medium.

8.7 Reactor Pool 8.7.1 Reactor Pool

Reactor Pool - Critical Heat Flux (L, M)

The panel agreed that critical heat flux (CHF) in the reactor pool is unlikely. CHF on the pool-side of the CNV wall is unlikely because the CNVs large surface area will result in relatively

59 low heat fluxes. CHF is more likely during period one on the outside of the DHRS tubes, but the panel agreed this is unlikely because DHRS heat removal is expe cted to be within design limits, and design limits will likely preclude CHF. The panel agreed t hat the importance ranking for this phenomenon is low in all periods. The panel agreed the knowledge level is medium.

8.8 Secondary 8.8.1 SG Tubes

Secondary - Asymmetric Loading (L, M)

Asymmetric loading in the SG tubes will have an insignificant impact on DHRS heat removal and is therefore of low importance for all periods. The panel agreed that the knowledge level for asymmetric loading is medium.

Secondary - Conduction through the Tube Wall (H, H)

The basis for the importance rankings for this phenomenon is the same as the basis for the conduction through the tube wall in the DHRS tubes (and the condensation inside the DHRS tubes). The conduction through the SG tube wall is in series with the other thermal resistances that make up the DHRS heat removal pathway. The panel agreed that the knowledge level for conduction through a tube wall is high.

Secondary - Density Wave Propagation (L, M)

While some instabilities may occu r in the DHRS or SG tubes, they are not expected to affect the average heat removal capacity of the DHRS. Since the boron dilution phenomena that may be affected by DHRS cooling are relatively slow moving, only the time-averaged heat removal capacity of the DHRS is important. Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level for this phenomenon is medium.

Secondary - Feedwater Inlet Temperature / Feedwater Heating (L, H)

Feedwater will trip early in period 1; therefore, this phenomenon is inactive for periods 2 through

5. In period 1, dilution will not occur; therefore, this phenomenon is expected to be of low importance. The panel agreed the knowledge level for this phenomenon is high.

Secondary - Inlet Losses / Orificing (H, H)

Like the DHRS heat transfer phenomena (e.g., condensation heat transfer in DHRS tubes), the dominant pressure loss phenomena in the DHRS are important because they will affect the DHRS thermal resistance and heat removal capacity. The pressure loss phenomena affect the heat transfer properties by affecting the loop flow rate. Inlet/orifice losses for the SG tubes are expected to be a significant contributor to DHRS loop pressure drop. During period 1, the DHRS system cannot meaningfully affect boron dilution concentrations because the riser is still covered. Therefore, the importance ranking for this phenomenon is low. During period 2, DHRS heat transfer is dominant, and loop pressure drop is therefore important for determining dilution rates via the DHRS. The panel agreed that this phenomenon is of high importance for period 2. For periods 3 through 5, DHRS heat transfer is expected to be active but secondary to

60 other heat transfer pathways. As a result, this phenomenon is ranked with medium importance.

The panel considered the state of knowledge of this phenomenon and agreed it is high.

Secondary - Ledinegg Instability (L, M)

While some instabilities may occu r in the DHRS or SG tubes, they are not expected to affect the average heat removal capacity of the DHRS. Since the boron dilution phenomena that may be affected by DHRS cooling are relatively slow moving, only the time-averaged heat removal capacity of the DHRS is important. Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level for this phenomenon is medium.

Secondary - Minimum Stable Film Boiling (L, M)

Critical heat flux and transition boiling are not expected to occur or to only occur for very short periods in the steam generator tubes because heat fluxes are expected to be relatively low.

Therefore, the panel agreed that this phenomenon is of low importance for all periods. The panel agreed that the knowledge level is medium.

Secondary - Parallel Channel Effects (L, M)

Parallel channel effects will have an insignificant impact on DHRS heat removal, and is therefore of low importance for all periods. The panel agreed that the knowledge level for parallel channel effects is medium. Note that the knowledge level for parallel channel effects in the SG tubes (M) is less than the knowledge level in the DHRS tubes (H) because (1) there is less reliable experimental data for the pressure losses through the helical geometry of the SG tubes and (2) the pressure losses in the SG tubes is expected to vary from tube to tube due to differences in coil diameter (and therefore length) of the SG tubes.

Secondary - Transition Boiling (L, M)

Transition boiling is not expected to occur in the steam genera tor tubes because heat fluxes are expected to be relatively low and nucleate boiling will likely occur; however, if it were to occur, it is of low importance because it will not persist due to the unstable nature of the phenomenon.

The panel agreed that this phenomenon is of low importance for all periods and the knowledge level is medium.

61 Knowledge Level

P5: LTC

P4. CNV. Condensation

P3: Blowdown

P2: SG Condensation

P1: Draw Down / Shrinkage

P5: LTC

P4. CNV. Condensation

P3: Blowdown

P2: SG Condensation

P1: Draw Down / Shrinkage

P5: LTC

P4. CNV. Condensation

P3: Blowdown

P2: SG Condensation

P1: Draw Down / Shrinkage

9 Phenomena Importance / Knowledge Issues

One of the key outcomes of the PIRT is the identification of phenomena that have a knowledge level less than their importance. Phenomena with a high importance ranking for at least one period but a low knowledge level are provided in Table 5. Phenomena with a high importance ranking for a least one period but a medium knowledge level are provided in Table 6.

Phenomena with at most medium importance for at least one perio d but a low knowledge level are provided in Table 7. Note that the phenomena in the tables below are hyperlinked to their discussion in the text above for convenience.

These phenomena will be evaluated in a follow-on task to this P IRT to determine the applicability of TRACE to evaluate boron dilution transients for the NuScale SDA design. In the follow-on task, similar to what has been done previously in Ref. 6, the phenomena will be evaluated based on the following criteria:

Category A - TRACE is considered fully applicable and can be expected to accurately simulate associated processes and phenomena.

Category B - TRACE is expected to be applicable and should be capable of simulating the associated processes and phenomena with only limited code development or additional assessment.

Category C - TRACE models/correlations are not designed to simulate these phenomena.

However, the effects of the phenomena can be bounded using existing TRACE capabilities (i.e., code work-around) and the appropriate well scaled integral effects data representing the SMR geometry.

Category D - Additional TRACE model development and assessment is considered necessary to demonstrate TRACE applicability for the phenomena in this category. Also, the use of well-scaled integral and/or separate effects data representing the SMR geometry is required.

Table 5: Phenomena with high importance ranking for at least one period, but a low knowledge level.

System Component - Phenomenon CNV CNV - Species Stratification CNV Dissolver Basket - Boron CNV RCS Pressure Boundary Surfaces - Boron Plate Out ECCS RRVs - Boron Dissolving

73 Table 6: Phenomena with high importance ranking for at least one period, but a medium knowledge level.

System Component - Phenomenon

CNV CNV - Internal Natural Circulation (RPV-CNV wall driven)

CNV CNV - Thermal Stratification CNV CNV - Vertical / Radial Natural Circulation (Liquid)

Core Subchannel - Boron Precipitation Core Subchannel - Vertical / Radial Natural Circulation DHRS DHRS Tubes - Heat Transfer (Reactor Pool Side)

DHRS DHRS Tubes - Condensation Heat Transfer (Inside DHRS Tubes)

DHRS DHRS Tubes - Natural Circulation DHRS DHRS Tubes - Two-phase Pressure Drop ECCS RPV - Flow Oscillation and Reversal Primary Downcomer - Species Stratification Primary Downcomer - Thermal Stratification Primary Downcomer - Vertical / Radial Natural Circulation Primary Hot Leg Riser - Riser Hole Flow Primary Hot Leg Riser - Riser Hole Orificing / Pressure Drop Primary Hot Leg Riser - Two-phase Level Swell Primary Hot Leg Riser - Vertical / Radial Natural Circulation Primary Lower Plenum - Species Stratification Primary Lower Plenum - Thermal Stratification Primary Lower Plenum - Vertical / Radial Natural Circulation

Primary Riser Holes - Flow Regime on Core / Riser Side and Downcomer Side

Primary SG Annulus - Condensation on Uncovered SG Tubes

Table 7: Phenomena with at most medium importance ranking for at least one period, but a low knowledge level.

System Component - Phenomenon

CNV CNV - Boron Dissolving CNV RCS Pressure Boundary Surfaces - Boron Dissolving Core Subchannel - Boric Acid Volatilization Core Subchannel - Turbulent Mixing Primary Downcomer - Boron Plate Out (M, L)

Primary Hot Leg Riser - Boric Acid Volatilization Primary Hot Leg Riser - Boron Plate Out (M, L)

Primary Lower Plenum - Boron Plate Out (M, L)

74 10 Summary and Conclusions

The Office of Nuclear Reactor Regulation requested (Ref. 2) that the Office of Nuclear Regulatory Research staff convene a panel to identify the importance of the phenomena affecting core integrity and coolability during postulated design-basis events with significant boron dilution and redistribution in the NuScale Power Module (NPM) in accordance with the process described by Regulatory Guide 1.203 (Ref. 3). The panels findings, which built upon previous work (Ref. 4, Ref. 5, Ref. 6), are documented in this report as a phenomena identification and ranking table (PIRT).

The panel considered three design-basis transients that may be limiting for boron dilution in the NuScale SDA design: (1) a small break LOCA (SBLOCA) in the liquid space of the CVCS line, (2) inadvertent actuation of the RRV, and (3) and loss of offsite power (LOOP). The panel also determined that the figures of merit for these transients are t he average boron concentration in the downcomer and the average boron concentration in the lower plenum.

The panel convened several meetings to reach consensus on the identification of phenomena, the importance ranking for those phenomena, and the ranking of the associated knowledge level for each phenomenon. At the conclusion of these meetings, the panel compiled the PIRT. In total, the panel considered nearly 300 phenomena occurring in various components through the primary, secondary, Decay Heat Removal System (DHRS), Emergency Core Cooling System (ECCS), and containment (CNV) systems of the NuScale Power Module. The importance and knowledge level rankings for all the phenomena are summarized in Table 4 of this report.

After reviewing the final consensus rankings, the panel identified four phenomena with high importance rankings but low knowledge levels. They are:

Species stratification in the CNV,

Boron dissolving in the dissolver basket located in the CNV,

Boron plate out on the Reactor Coolant System (RCS) pressure bo undary surfaces, and

Boron dissolving in the RRVs,

The panel identified 22 phenomena with high importance rankings but medium knowledge levels:

Internal natural circulation in the CNV that is driven by the temperature difference between the Reactor Pressure Vessel (RPV) and CNV bulk fluid temperatures,

Thermal stratification in the CNV,

Vertical/radial natural circulation in the liquid in the CNV,

Boron precipitation in the core/subchannel,

Vertical/radial natural circulation in the core/subchannel,

Heat transfer on the pool-side of the DHRS condenser tubes,

Condensation heat transfer on the inside of the DHRS condenser tubes,

Natural circulation within the DHRS flow loop,

Two-phase pressure drop in the DHRS tubes,

Flow oscillation and reversal in the RPV,

Species stratification in the downcomer,

75

Thermal stratification in the downcomer,

Vertical/radial natural circulation in the downcomer,

Riser hole flow,

Orificing/pressure drop in the riser holes,

Two-phase level swell in the hot leg riser,

The flow regime on the core/riser-and downcomer-sides of the riser holes,

Vertical/radial natural circulation in the hot leg riser,

Species stratification in the lower plenum,

Thermal stratification in the lower plenum,

Vertical/radial natural circulation in the lower plenum, and

Condensation heat transfer on the uncovered steam generator (SG) tubes.

The panel identified eight phenomena with at most medium importance ranking but a low knowledge level:

Boron dissolving in the CNV,

Boron dissolving on the RCS pressure boundary surfaces,

Boric acid volatilization in the subchannel,

Turbulent mixing in the subchannel,

Boron plate out in the downcomer,

Boric acid volatilization in the hot leg riser,

Boron plate out in the hot leg riser, and

Boron plate out in the lower plenum.

These phenomena will be evaluated in a follow-on task to this P IRT to determine the applicability of TRACE to evaluate boron dilution transients for the NuScale SDA design. In the follow-on task, similar to what has been done previously in Ref. 6, the phenomena will be evaluated based on the following criteria:

Category A - TRACE is considered fully applicable and can be expected to accurately simulate associated processes and phenomena.

Category B - TRACE is expected to be applicable and should be capable of simulating the associated processes and phenomena with only limited code development or additional assessment.

Category C - TRACE models/correlations are not designed to simulate these phenomena. However, the effects of the phenomena can be bounded using existing TRACE capabilities (i.e., code work-around) and the appropriate well scaled integral effects data representing the SMR geometry.

Category D - Additional TRACE model development and assessment is considered necessary to demonstrate TRACE applicability for the phenomena in this category.

Also, the use of well-scaled integral and/or separate effects data representing the SMR geometry is required.

In addition, under the follow-on task (Ref. 2), RES staff will perform a detailed literature review and develop a TRACE Assessment Matrix and Gap Report based on the results of this PIRT. The TRACE Assessment Matrix will establish a set of assessment or validation activities needed to confirm that TRACE can reliably model the key phenomena identified in this PIRT to

76 perform the subject boron redistribution analyses. The assessment matrix will likely rely on a combination of separate effect tests (SETs) and integral effects tests (IETs). The Gap Report will identify assessment requirements that are not currently co vered by the existing TRACE assessment experience and that need to be performed to determine the applicability of TRACE to perform the subject analysis. It is possible that the gap report may include a recommendation to gather data that is not currently available in the open literature. In such an instance, the gap report will include recommendations for an experimental program (SETs and/or IETs, as appropriate) to gather the necessary data or other work-around approaches if possible. If the RES staff identifies such a gap, then RES and NRR will coordinate next steps, as necessary.

77 11 References

1. Research Work Request Form NRR-2021-019 (ADAMS Accession No. ML21207A220).

2. Patton, R., email to Armstrong, K., Hoxie, C., Request: Issuance of NuScale Boron Redistribution Tasks, April 28, 2022 (ADAMS ML22129A009).

3. RG 1.203, Transient and Accident Analysis Methods, December 2005.

4. Pre-Application Phenomena Identification and Ranking Table (PIRT) Report for NuScale-like Integral Pressurized Water Reactors (iPWRs), June 2011 (ADAMS Accession No. ML111600574).

5. ERI/NRC 12-203, APPLICABILITY OF THE TRACE AND PARCS COMPUTER CODES TO INTEGRAL PRESSURIZED WATER REACTORS, May 2012 (ADAMS Accession NO. ML120610573).

6. PIRT Report for NuScale Stability, January 2017 (ADAMS Accession No. ML17033A144).

7. Yarsky, P., Applicability of TRACE to Transient and Accident Analysis for the NuScale Power Module, May 2019 (ADAMS Accession No. ML19253A036).

8. Yarsky, P., NuScale Core Reactivity Consequences of Boron Dilution and Incursion during Postulated SBLOCA with Coincident Rod Insertion Failure Events, July 2020 (ADAMS Accession No. ML20191A069).

9. Yarsky, P., TER on Boron Redistribution Issues Raised by the ACRS - Rev. 1, July 2020 (ADAMS Accession No. ML20204A979).

10. NuScale Standard Plant Design Certification Application, Chapter. 5, Reactor Coolant System and Connection Systems, Part 2 - Tier 2 Rev. 2, NuScale Power, LLC, Agencywide Documents Access and Management System (ADAMS) Acces sion No.

ML18310A326 (Oct. 2018).

11. Lin, Chien C.,The Radiolytic Gas Production Rate in Boiling Water Reactor, Nuclear Science & Engineering, 99, pp. 390-393 (1988).

12. S. Lu, C. Thurston, S. I. Haider, A. Barrett, J. Staudenmeier, P. Yarsky, S. Bajorek, Long Term Effects and Numerical Simulation of Radiolytic Gas, Non-Condensable Gas and Boron Transport for Small Modular Light Water Reactors INTERNATIONAL CONFERENCE ON TOPICAL ISSUES IN NUCLEAR INSTALLATION SAFETY:

STRENGTHENING SAFETY OF EVOLUTIONARY AND INNOVATIVE REACTOR DESIGNS (TIC2022) 18-21 October 2022, Vienna.

13. USNRC, TRACE V5.0 Assessment Manual, 2007 (ADAMS Accession No. ML062720581, ML062720582, ML062720583, and ML071200473).

78 12 Appendix A: Reference LOCA PIRT Rankings (Ref. 4)

System Component Process/Phenomena BLD RVV LTC KL P

CORE FUEL RODS Decay Heat H H H H CORE FUEL RODS Fission Power L L L H CORE FUEL RODS Stored Energy L L L M CORE FUEL RODS Gap Conductance L L L M CORE FUEL RODS Fuel Conductivity L L L M CORE FUEL RODS Initial Gap Pressure L L L M CORE FUEL RODS Cladding Conductivity L L L M CORE FUEL RODS Cladding Oxidation (CORRELATIONS) I I I H CORE FUEL RODS Total Peaking Factor M L L H CORE FUEL RODS Burnup Distribution L L L H CORE FUEL RODS Boron Precipitation L L L M System Component Process/Phenomena BLD RVV LTC KL P

CORE SUBCHANNEL Single Phase Pressure Drop L L L H CORE SUBCHANNEL Two Phase Pressure Drop M L L M CORE* SUBCHANNEL Flashing L L L H CORE* SUBCHANNEL Natural Circulation M M L M CORE SUBCHANNEL Interfacial Drag L M M M CORE SUBCHANNEL Single Phase Convection L I I H CORE SUBCHANNEL Two Phase Convection L L L H CORE SUBCHANNEL CHF (correlations) M L L L CORE SUBCHANNEL Flow regime transition M L L M CORE SUBCHANNEL Grid spacer effects (entrainment/deentrainment) I I I M CORE SUBCHANNEL Grid spacer effects (heat transfer) L L L M CORE* SUBCHANNEL Cross flow / mixing M L L M CORE SUBCHANNEL Clad ballooning L L L M CORE SUBCHANNEL Void distribution M L L M CORE* SUBCHANNEL Turbulent mixing L L L L CORE SUBCHANNEL Boron blockage in subchannels L L L M System Component Process/Phenomena BLD RVV LTC KL P

CORE Barrel/Baffle Stored energy L L L H CORE Barrel/Baffle Bypass flow L L L H System Component Process/Phenomena BLD RVV LTC KL P

CORE Control rods/GT Effects on flow L L L M CORE Control rods/GT Effects on heat transfer L L L H System Component Process/Phenomena BLD RVV LTC KL P

CORE COREWIDE FLOW Stability L L L M System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY Hot leg riser Flashing H H L H PRIMARY* Hot leg riser Two-phase level swell L M M M PRIMARY Hot leg riser Two-phase pressure drop L L L M PRIMARY Hot leg riser Primary natural circulation flow/bulk flow M L L M PRIMARY Hot leg riser Interfacial drag/relative motion of phases L M M M PRIMARY* Hot leg riser Vertical/radial natural circulation L L L L PRIMARY Hot leg riser Inlet flow/temp distribution L L L M PRIMARY Hot leg riser Convection heat transfer to shroud/riser LLLH PRIMARY Hot leg riser Stored energy release/conduction of shroud/riser L L L H PRIMARY Hot leg riser Radiation heat transfer from shroud/riser L L L H PRIMARY Hot leg riser Control rod drives/supports structures affect on flow L L L H PRIMARY Hot leg riser Riser Bypass Flow L L L H PRIMARY* Hot leg riser Mixing L L L H System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY Upper plenum Flashing H L L H PRIMARY Upper plenum Two-phase level swell L L L M PRIMARY Upper plenum Two-phase pressure drop L L L M PRIMARY Upper plenum Single-phase pressure drop L L L H PRIMARY Upper plenum Primary natural circulation flow/bulk flow M L L M PRIMARY Upper plenum Interfacial drag/relative motion of phases L L L M PRIMARY Upper plenum Vertical/radial natural circulation L L L M PRIMARY Upper plenum Convection heat transfer to reactor vessel L L L H PRIMARY Upper plenum Stored energy release/conduction of vessel wall L L L H 79 PRIMARY Upper plenum Radiation heat transfer from reactor vessel to containment vessel L L L H System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY PRESSURIZER Flashing H M L H PRIMARY PRESSURIZER Phase separation M M L M PRIMARY PRESSURIZER Flooding at baffle plate L H L M PRIMARY PRESSURIZER Two-phase pressure drop L M L M PRIMARY PRESSURIZER Single-phase pressure drop L L L H PRIMARY PRESSURIZER Interfacial drag/relative motion of phases MMLM PRIMARY PRESSURIZER Primary natural circulation flow/bulk flow LLLH PRIMARY PRESSURIZER Convection heat transfer to vessel L L L H System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY* Reactor vent valves Two-phase pressure drop I M L M PRIMARY* Reactor vent valves Single-phase pressure drop I H M M PRIMARY* Reactor vent valves Choked flow I H L M System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY SG ANNULUS Flashing M L L H PRIMARY SG ANNULUS Two-phase level swell L L L M PRIMARY SG ANNULUS Two-phase pressure drop M L L M PRIMARY SG ANNULUS Single-phase pressure drop L L L H PRIMARY SG ANNULUS Primary natural circulation flow/ bulk flow MLLM PRIMARY SG ANNULUS Interfacial drag/relative motion of phases L L L M PRIMARY* SG ANNULUS Vertical/radial natural circulation L L L L PRIMARY SG ANNULUS Convection heat transfer to SG tubes M L L M PRIMARY SG ANNULUS Convection heat transfer to vessel L L L H PRIMARY SG ANNULUS Convection heat transfer to riser L L L H PRIMARY SG ANNULUS Stored energy in the steam generator tubes & fluid L L L M PRIMARY SG ANNULUS Stored energy release/conduction of vessel wall L L L H PRIMARY SG ANNULUS Stored energy release/conduction of riser L L L H PRIMARY SG ANNULUS Radiation heat transfer from vessel L L L H PRIMARY SG ANNULUS Radiation heat transfer from shroud/riser L L L H PRIMARY SG ANNULUS Tube bypass flow M L L M PRIMARY SG ANNULUS Feed header effect on flow M L L M PRIMARY SG ANNULUS Feed header stored energy M L L M PRIMARY SG ANNULUS Steam header effect on flow M L L M PRIMARY SG ANNULUS Steam header stored energy M L L M System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY DOWNCOMER Flashing M L L H PRIMARY DOWNCOMER Two-phase level swell M M L M PRIMARY DOWNCOMER Two-phase pressure drop L L L M PRIMARY DOWNCOMER Single-phase pressure drop L L L H PRIMARY DOWNCOMER Primary natural circulation flow/ bulk flow M L L M PRIMARY DOWNCOMER Interfacial drag/relative motion of phases M M L M PRIMARY* DOWNCOMER Vertical/radial natural circulation L L L L PRIMARY DOWNCOMER Convection heat transfer to vessel L L L H PRIMARY DOWNCOMER Convection heat transfer to shroud/riser L L L H PRIMARY DOWNCOMER Stored energy release/conduction of vessel wall L M M H PRIMARY DOWNCOMER Stored energy release/conduction of riser L L L H PRIMARY DOWNCOMER Radiation heat transfer from vessel L L L H PRIMARY DOWNCOMER Radiation heat transfer from shroud/riser L L L H System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY Reactor Recirc. Valves Single-phase pressure drop H L M H PRIMARY Reactor Recirc. Valves Choked flow H H L M PRIMARY Reactor Recirc. Valves Two-phase pressure drop H H L M System Component Process/Phenomena BLD RVV LTC KL P

PRIMARY Lower plenum Flashing M L L H PRIMARY Lower plenum Two-phase level swell L L L M PRIMARY Lower plenum Two-phase pressure drop L L L M PRIMARY Lower plenum Single-phase pressure drop M L L H PRIMARY Lower plenum Primary natural circulation flow/bulk flow M L L H PRIMARY Lower plenum Interfacial drag/relative motion of phases L L L M PRIMARY* Lower plenum Vertical/radial natural circulation L L L L PRIMARY Lower plenum Convection heat transfer to vessel L L L H PRIMARY Lower plenum Stored energy release/conduction of vessel L M M H System Component Process/Phenomena BLD RVV LTC KL P

80 SECONDARY SG tubes Void distribution L L L L SECONDARY SG tubes Single-phase convection heat transfer M L L M SECONDARY SG tubes Two-phase convection heat transfer M L L M SECONDARY SG tubes Flashing M L L H SECONDARY SG tubes Stored energy of tubes L L L H System Component Process/Phenomena BLD RVV LTC KL P

CHRS Containment vessel Condensation heat transfer H H H M CHRS Containment vessel Vertical/radial natural circulation flow (gas space) H H H M CHRS Containment vessel Single-phase convection heat transfer L L L H CHRS Containment vessel Conduction heat transfer (vessel and containment) M M M H CHRS Containment vessel Effect of noncondensables H H H L CHRS Containment vessel Fluid properties for pressures < 0.10132 MPa M I I H CHRS* Containment vessel Thermal stratification L L M L CHRS* Containment vessel Vertical/radial natural circulation flow (liquid space) L L M L CHRS Containment vessel Interfacial heat / mass transfer (pool to gas) L L M L System Component Process/Phenomena BLD RVV LTC KL P

Containment cooling CHRS pool Nucleate boiling L L M H Containment cooling CHRS pool Single-phase convection heat transfer L L M H Containment cooling CHRS pool Vertical/radial natural circulation L L M L Containment cooling CHRS pool Thermal stratification L L M L

Phenomena highlighted in dark grey were re-ranked for the present study.

81 13 Appendix B: List of Acronyms

Acronym Definition CFD Computation Fluid Dynamics CNV Containment Vessel CVCS Chemical and Volume Control System DCA Design Certification Application DHRS Decay Heat Removal System ECCS Emergency Core Cooling System EMDAP Evaluation Model Development and Assessment Process FOM Figure of Merit IET Integral Effects Test LOCA Loss of Coolant Accident LOOP Loss of Offsite Power NPM NuScale Power Module NRR Office of Nuclear Reactor Regulation PIRT Phenomenon Identification and Ranking Table RCS Reactor Coolant System RES Office of Nuclear Regulatory Research RPV Reactor Pressure Vessel RRV Reactor Recirculation Valve RVV Reactor Vent Valve SBLOCA Small Break LOCA SDA Standard Design Approval SET Separate Effects Test SG Steam Generator SMR Small Modular Reactor

82

ML24064A042

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