ML23279A051

Yarsky, P., Pirts for Hcsg Dwo and Dhrs Loop Instability, October 2023

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Issue date:	10/27/2023
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USNRC Oce of Nuclear Regulatory Research PIRTs for HCSG DWO and DHRS Loop Instability Phenomena Iden "ca on and Ranking Tables [PIRTs] for Helical Coil Steam Generator [HCSG] Density Wave Oscilla on [DWO]

This work supports User Need Requests NRR-2023-017 and NRR-2023-019 Peter Yarsky 10-4-2023 0

Executive Summary The Office of Nuclear Reactor Regulation (NRR) Division of Safety Systems (DSS) and Division of Engineering and External Hazards (DEX) have requested the Office of Nuclear Regulatory Research to conduct confirmatory analyses related to flow instability in the helical coil steam generator (HCSG) of the NuScale power module design (Ref. 1 and 2, respectively). Before the RES staff can embark on such confirmatory analyses, the applicability of TRACE to perform these calculations must be established. According to the Evaluation Model Development and Approval Process, code applicability cannot be determined until the code is assessed against relevant experimental validation data for the highly ranked phenomena (Ref. 4). A Phenomena Identification and Ranking Table (PIRT) documents the importance ranking for each phenomenon. This report documents the HCSG density wave oscillation (DWO) and decay heat removal system (DHRS) flow loop instability PIRTs. The PIRTs provide a list of the key phenomena, the importance of each according to the consensus of the RES subject matter experts, and a ranking of the general knowledge level for each phenomenon.

The staff has developed two PIRTs. The first is for the analysis of DWOs in the HCSG during normal operation (excluding upset conditions from anticipated transients). The figures of merit (FoMs) considered are the decay ratio and the tube wall temperature. The PIRT importance rankings are shown in Table 1. The staff also considered the knowledge level ranking for these phenomena, which are provided in Table 2. The following phenomena were identified as knowledge gaps: (1) centrifugal forces from coil geometry and (2) orifice pressure loss.

The second PIRT is for DHRS flow loop instability. The FoMs considered are the flow rate through the piping track between the DHRS heat exchanger (HX) outlet plenum and the HCSG inlet plenum as well as the aggregate DHRS heat removal rate. The PIRT importance rankings are shown in Table 3 and the knowledge level rankings are shown in Table 4. The following phenomena were identified as knowledge gaps: (1) HCSG Coils: Centrifugal Forces from Coil Geometry, (2) HCSG Coils: Shell Side Heat Transfer, (3) HCSG Coils: Density Wave Propagation, and (4) Reactor Pool: Heat Transfer.

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Table of Contents 1 Introduction ............................................................................................................................ 5 2 Helical Coil Steam Generator Tube Description .................................................................... 6 3 DHRS Loop Description ......................................................................................................... 9 4 HCSG DWO......................................................................................................................... 12 4.1 Scenario Description .................................................................................................... 12 4.2 Figures of Merit ............................................................................................................. 13 4.3 Phenomena Identification and Ranking ........................................................................ 13 4.3.1 Single -Phase Vapor Heat Transfer (Rank: M / KL: H) .......................................... 16 4.3.2 Plugging and Fouling (Rank: M / KL: M) ............................................................... 16 4.3.3 Centrifugal Forces (Rank: H / KL: M) .................................................................... 17 4.3.4 Shell Side Heat Transfer (Rank: H / KL: H) and Feedback: Void and Feedback:

Fuel Temperature ................................................................................................................. 17 4.3.5 Ledinegg Instability (Rank: H / KL: H) ................................................................... 18 4.3.6 Gravity Head (Rank: M / KL: H) ............................................................................. 19 4.3.7 Subcooled Boiling (Rank: M / KL: H) ..................................................................... 20 4.4 Knowledge Level and Knowledge Gaps ....................................................................... 20 5 DHRS Loop Instability .......................................................................................................... 24 5.1 Scenario Description .................................................................................................... 24 5.2 Figures of Merit ............................................................................................................. 24 5.3 Phenomena Identification and Ranking ........................................................................ 24 5.3.1 HCSG Inlet Orifice Critical Flow (Rank: I / KL: N/A) .............................................. 34 5.3.2 HCSG Coils Dryout (Rank: M / KL: H) ................................................................... 34 5.3.3 HCSG Coils Fouling (Rank: L / KL: L) ................................................................... 34 5.3.4 HCSG Coils Centrifugal Forces from Coil Geometry (Rank: H / KL: M) ................ 35 5.3.5 HCSG Coils Interfacial Shear (Rank: H / KL: H) ................................................... 35 5.3.6 HCSG Coils Density Wave Propagation (Rank: H / KL: M) ................................... 35 5.3.7 DHRS HX Tubes Density Wave Propagation (Rank: L / KL: M) ............................ 35 5.3.8 DHRS HX Tubes Entrance Effects / Developing Length (Rank: L / KL: M) ........... 35 5.3.9 DHRS HX Tubes Flashing (Rank: I / KL: N/A) ....................................................... 36 5.3.10 DHRS HX Tubes Interfacial Shear (Rank: M / KL: M) ........................................... 36 5.3.11 DHRS HX Tubes Pressure Wave Propagation (Rank: L / KL: H) .......................... 36 5.3.12 DHRS HX Tubes Heat Capacitance of the Tubes (Rank: L / KL: H) ..................... 36 5.3.13 DHRS HX Tubes Transition Boiling (Rank: I / KL: N/A) ......................................... 36 5.3.14 Ledinegg Instability (Rank: H / KL: H) ................................................................... 37 5.4 Knowledge Level and Knowledge Gaps ....................................................................... 37 5.4.1 HCSG Coils Fouling (Rank: L / KL: L) ................................................................... 46 2

5.4.2 HCSG Coils Shell Side Heat Transfer (Rank: H / KL: M) ...................................... 46 5.4.3 HCSG Coils Centrifugal Forces from Coil Geometry (Rank: H / KL: M) ................ 46 5.4.4 HCSG Coils Ledinegg Instability (Rank: H / KL: H) and DHRS HX Tubes Ledinegg Instability (Rank: H / KL: H) ................................................................................................. 47 5.4.5 HCSG Coils Density Wave Propagation (Rank: H / KL: M) ................................... 47 5.4.6 DHRS HX Tubes Density Wave Propagation (Rank: L / KL: M) ............................ 47 5.4.7 DHRS HX Tubes Interfacial Shear (Rank: M / KL: M) ........................................... 47 5.4.8 Reactor Pool Thermal Stratification (Rank: H / KL: M) .......................................... 47 5.4.9 Knowledge Gaps ................................................................................................... 48 6 Conclusions ......................................................................................................................... 49 7 References .......................................................................................................................... 50 3

Table of Figures Figure 1: Helical Coils in SIET Experimental Facility (Ref. 6) ....................................................... 6 Figure 2: Location of Helical Coils in NuScale Power Module (Ref. 7) ......................................... 7 Figure 3: Schematic of Primary and Secondary Side Flows in Helical Coils (Ref. 8) ................... 8 Figure 4: NuScale Helical Coil Steam Generator Bundle (Ref. 9) ................................................ 9 Figure 5: Schematic (not-to-scale) NuScale Decay Heat Removal System (Ref. 7). ................. 10 Figure 6: DHRS Heat Exchanger Relative to Containment Vessel (Ref. 9) ................................ 11 Figure 7: Boiler/Condenser Flow Circuit ..................................................................................... 12 Figure 8: Observed Superposition of Ledinegg Instability and Density Wave Oscillations (Ref.

13) ............................................................................................................................................... 19 Table of Tables Table 1: HCSG DWO PIRT ......................................................................................................... 15 Table 2: Knowledge Level Rankings for HCSG DWO Phenomena ............................................ 22 Table 3: DHRS Flow Instability PIRT........................................................................................... 27 Table 4: Knowledge Level Rankings for DHRS Flow Loop Instability Phenomena ..................... 39 4

1 Introduc on The Office of Nuclear Reactor Regulation (NRR) Division of Safety Systems (DSS) and Division of Engineering and External Hazards (DEX) have requested the Office of Nuclear Regulatory Research to conduct confirmatory analyses related to flow instability in the helical coil steam generator (HCSG) of the NuScale power module design (Ref. 1 and 2, respectively). Before the RES staff can embark on such confirmatory analyses, the applicability of TRACE to perform these calculations must be established. According to the Evaluation Model Development and Approval Process, code applicability cannot be determined until the code is assessed against relevant experimental validation data for the highly ranked phenomena (Ref. 4). A Phenomena Identification and Ranking Table (PIRT) documents the importance ranking for each phenomenon. This report documents the HCSG density wave oscillation (DWO) and decay heat removal system (DHRS) flow loop instability PIRTs. Before the PIRTs, Section 2 provides a description of the helical coils and the HCSG configuration, then Section 3 describes the DHRS loop with uses the HCSG to remove heat from the primary system. After these descriptions, Section 4 discusses the PIRT for HCSG DWO while Section 5 discusses the DHRS flow loop instability PIRT.

The PIRTs provide a list of the key phenomena, the importance of each according to the consensus of the RES subject matter experts, and a ranking of the general knowledge level for each phenomenon. Knowledge gaps are noted when the importance of a phenomenon exceeds the level of knowledge. The PIRTs documented in this report are used in the TRACE applicability determination process.

In conducting the PIRT, the RES staff assigned importance rankings to identified phenomena on a scenario- and component-specific basis. The importance rankings are: High (H), Medium (M),

Low (L), or Inactive (I) depending on the occurrence of the phenomenon and the effect of that phenomenon on the figure of merit for the specific scenario. Further, once the phenomena are ranked by their importance, the RES staff assigned knowledge level (KL) rankings of High (H),

Medium (M), Low (L), or Not-applicable (N/A) depending on whether the phenomenon is active and how well the phenomenon is understood. The KL rank of N/A is reserved for inactive phenomena.

Knowledge level gaps are identified when the KL is ranked below the importance. For example, a highly important phenomenon with a medium knowledge level is flagged in this report as a knowledge gap.

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2 Helical Coil Steam Generator Tube Descrip on A novel feature of the NuScale power module is the design of the steam generator, which is based on a collection of helical coils in a bundle comprising individual tubes arranged in rows and columns. Two helical coils are illustrated in Figure 1, which are from a test performed at Societa lnformazioni Esperienze Termoidrauliche S.p.A. (SIET) laboratory in collaboration with researchers from the Polytechnic University of Milan (Ref. 6). The figure shows how the coils have a circular arc with a given helix diameter in the lateral direction but ascend vertically with a given inclination, or helix, angle.

Figure 1: Helical Coils in SIET Experimental Facility (Ref. 6)

In the NuScale power module, the helical coils surround the riser section and are located within the annulus between the riser and reactor pressure vessel. This is illustrated in a simple diagram in Figure 2. During normal operation of the power module, primary fluid flows upward through the riser and then downward over the tubes in the annulus. The primary water on the shell side heats up the secondary water in the tubes. The secondary water is supplied by the 6

feedwater system (F/W). The feed enters an inlet plenum or feed header at the bottom of the coil bundle. The water is boiled in the steam generator tubes and is then directed via the outlet header (or outlet plenum) to the main steam system (M/S), which is illustrated in Figure 2.

Figure 2: Location of Helical Coils in NuScale Power Module (Ref. 7)

Figure 3 provides a schematic of an individual coil and shows the downward direction of the primary side water flow on the shell side and the upward direction of the secondary flow provided by the feedwater system. The secondary water flows through the coil, as shown in Figure 3, which loops around the riser.

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Figure 3: Schematic of Primary and Secondary Side Flows in Helical Coils (Ref. 8)

The NuScale power module has two HCSGs. The geometry of the HCSGs is complex, but Figure 4 illustrates how the bundle of coils is arranged. Individual coils with a given a helix diameter are arranged into columns (in essence, they are stacked). The columns are combined with three sisters at 90-degree azimuthal separation. The overall bundle is built up from several columns.

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Figure 4: NuScale Helical Coil Steam Generator Bundle (Ref. 9)

Feed flow is distributed to the tubes through a feedwater inlet header (or plenum). The steam exiting the tubes is collected in the outlet header (or plenum) and then directed to the steam line via piping.

3 DHRS Loop Descrip on During upset conditions, the decay heat removal system (DHRS) can remove the decay heat and cool the power module. The DHRS uses the HCSGs to remove heat from the primary system as part of a passive, natural circulation flow loop. Water in the HCSGs boils and the steam is directed to the DHRS heat exchanger (DHRS HX) which condenses the steam. The condensate can flow to the inlet of the HCSGs to complete the loop. This is illustrated by the schematic shown in Figure 5. The top of Figure 5 shows that the feedwater and main steam systems are isolated by the four closed valves.

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Figure 5: Schematic (not-to-scale) NuScale Decay Heat Removal System (Ref. 7).

Steam generated in the HCSGs is instead directed to the upper plenum of the DHRS HXs which are within the reactor pool. The HX shell side is cooled by the water in the reactor pool and the steam condenses on the inside of the HX tubes. The arrangement of the DHRS HX inlet piping and tubes is shown in Figure 6. The DHRS HX is a series of vertical tubes outside, but near the containment vessel.

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Figure 6: DHRS Heat Exchanger Relative to Containment Vessel (Ref. 9)

The combination of the HCSG and DHRS HX and the connecting piping creates a typical boiler/condenser natural circulation flow circuit. This flow circuit is illustrated in Figure 7. The density difference between the steam and condensate drives the flow around the path from the HCSG to the DHRS HX.

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Primary Coolant DHRS HX Containment Wall Reactor Pool Riser HCSG Wall Figure 7: Boiler/Condenser Flow Circuit In Figure 7, the red lines indicate the DHRS hot leg piping, which connects the HCSG outlet plenum to the DHRS HX inlet plenum. Red arrows are shown on the red lines to indicate the direction of the steam flow in the hot leg. The blue lines indicate the DHRS cold leg piping.

The cold leg piping connects the DHRS HX outlet plenum to the HCSG inlet plenum (or feed header). The condensate column and return line comprise the DHRS cold leg piping.

4 HCSG DWO 4.1 Scenario Descrip on The scenario considered by the staff is normal operation. When evaluating stability margin, or the propensity to instability, the initial condition is typically one of steady-state, normal operation.

When a time-domain code like TRACE is used for such an analysis, a perturbation is applied to a steady-state condition and the evolution of the response is used to determine whether the perturbation decays away or grows with time. In this report, normal operation considers primary and secondary side conditions covering the full range of shutdown, startup, steady power operation, and power maneuvering. This is somewhat different from the usual meaning of normal operation which would include anticipated operational occurrences. In the current ranking, the reactor coolant system is intact with normal flow for the given power levels and the balance of plant equipment (e.g., feedwater pumps, feedwater heaters, etc.) are assumed available and operating as expected.

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4.2 Figures of Merit The RES staff considered two figures or merit (FoMs). Many DWO analyses are aimed at demonstrating margin to instability. A typical FoM in such analysis is the decay ratio (DR) which is the ratio of subsequent peaks in an oscillation that occurs in response to a perturbation.

When the DR is less than one, the oscillations decay away, indicating the system is stable. A smaller DR indicates a larger margin to the instability threshold. The DR can be calculated by observing the transient response of the HCSG tube inlet mass flow rate in response to a perturbation (e.g., a steam line pressure perturbation). Alternatively, the DR can be calculated using noise analysis based on output signals from the TRACE calculation (e.g., tube mass flow rate and steam line pressure) with noisy boundary conditions.

Additionally, the RES staff anticipates analyses examining the consequences of instability in the HCSG tubes. If the flow oscillates in the HCSG this can lead to thermal stresses and/or other hydrodynamic loads on the tubes and associated structures. Therefore, the RES staff also considered the transient tube wall temperature as a FoM. This FoM is an output from the TRACE calculation that serves as a surrogate FoM related to the tube thermal stress. Unlike the DR, the tube wall temperature is the FoM considered for cases where the tube is unstable and subject to growing or limit-cycle DWOs.

To summarize, the FoMs are DR and tube wall temperature.

4.3 Phenomena Iden "ca on and Ranking In developing a PIRT for HCSG DWO, the staff began with a PIRT developed for boiling water reactor (BWR) coupled neutronic / thermal-hydraulic instability (Ref. 10) because the density wave propagation mechanism is the underlying cause of instability in both cases. The general approach was to list all the phenomena from the BWR PIRT and then determine what the analogous phenomenon or process would be for the HCSG. In assigning the analogous process, the staff did not merely consider processes that are necessarily physically similar in terms of their mechanism, but rather, the staff considered processes and phenomena that would yield a similar transfer function. The staffs default position was to retain the importance ranking from the BWR PIRT unless there was a specific justification for the revision. The results of this process are provided in Table 1.

In Table 1, some of the cells are shaded. If a cell is shaded in dark gray, this indicated that the phenomena does not have an equivalent or analogous process in the HCSG. An example would be Control Rod Pattern/Movement which is listed in the BWR PIRT. There is not an equivalent or analogous process for the HCSG. Therefore, this entry in the table has a gray shaded cell under the analogous phenomena/process header. Another example is single-phase: vapor heat transfer. The HCSG is designed to achieve superheat in the secondary side, therefore, there will be superheated vapor inside the tubes over an appreciable length of the tube. For BWR instability there will be a limited number of rods that experience post-critical heat flux heat transfer and so there will be primarily two-phase flow in the core region. So single-phase vapor heat transfer is added as a phenomenon in the analogous phenomenon/process column in Table 1.

Several cells are shaded in a blue color. This shading indicates that the analogous phenomena or process warrants some clarification in the documentation and a rationale for the importance ranking. These highlighted phenomena are discussed, in turn, in the sub-sections below.

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Where the cells are not shaded, the analogous phenomena or process was deemed to be essentially the same as the reference BWR PIRT and the reference ranking was preserved.

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Table 1: HCSG DWO PIRT HCSG BWR Phenomenon/Process Analogous Phenomenon/Process DWO Ranking Ranking Boiling: Film H Two-phase Heat Transfer H Boiling: Subcooled H Single-phase Liquid Heat Transfer H Single-phase Vapor Heat Transfer M Control Rod Pattern/Movement H N/A Dryout H Dryout H Feedback: Fuel Temperature H N/A Feedback: Void H N/A Flow: Coastdown H N/A Flow: Natural Circulation H N/A Flow: Multi-channel T/H Effect H Tube-to-Tube Flow Distribution H Plugging N/A Fuel: Burnup H Fouling M Fuel: Design/Type H Centrifugal Forces from Coil Geometry H Heat Conductance: Fuel-Clad Gap H Heat Conduction through Tube Wall H Interphase Shear H Interfacial Shear H Power Distribution: Axial H Shell-side Heat Transfer H Power Distribution: Radial H Orifice Pressure Loss H Exit Loss H Pressure Drop H Single-phase Liquid Pressure Drop H Single-phase Vapor Pressure Drop H Two-phase Pressure Drop H Ledinegg Instability H Reactivity: SCRAM L N/A Stability: Neutronic/Thermal-Hydraulic H Density Wave Propagation H Subcooling: Coolant H Subcooling H Void: Collapse H N/A Void: Distribution H Gravity Head M Void: Subcooled Boiling H Subcooled Boiling M 3D Kinetics Effects H N/A 15

4.3.1 Single -Phase Vapor Heat Transfer (Rank: M / KL: H)

In the BWR PIRT, only single-phase liquid and two-phase heat transfer were considered. While dryout was expected to occur on some fuel rods, it was not thought that the flow conditions in the core region would ever reach superheated steam conditions. This is due, in part, to the presence of the liquid bypass, which would keep the fuel channels near saturated conditions.

However, the HCSG is designed to produce superheated steam. Therefore, dryout will occur in many coils. This prompted the staff to include single-phase vapor heat transfer in the current PIRT. When compared to single-phase liquid and two-phase heat transfer, the single-phase vapor heat transfer coefficient is expected to be smaller and the temperature difference driving the heat transfer is expected to be smaller. Therefore, the staff expects the single-phase vapor heat transfer to be less significant than the single-phase liquid and two-phase heat transfer based on heat transferred per unit length of the coil. Furthermore, considering that the degree of super heat is expected to be smaller than the degree of subcooling, the absolute, total heat transfer in the single-phase liquid regime is expected to be more significant than the single-phase vapor regime. For these reasons, the single-phase vapor heat transfer was ranked as medium compared to the high rankings for the single-phase liquid and two-phase heat transfer phenomena.

4.3.2 Plugging and Fouling (Rank: M / KL: M)

Rather than fuel burnup - which was identified in the reference BWR PIRT - the staff identified plugging and fouling as analogous processes. The rationale is that the fuel burnup in the reference PIRT captures changes in the fuel over a long period of time, which can change the stability characteristics of the fuel by changing the strength of certain feedback mechanisms, such as the void reactivity coefficient. For the HCSG there is not a directly analogous phenomenon to fuel exposure, but the tubes will be subject to certain ageing effects. The staff identified tube fouling as one effect. Fouling refers to the accumulation of deposits on the tube inside wall due to the evaporation of water containing impurities. These deposits on the tube wall will likely increase with the service life of the tubes. The accumulation of deposits will affect the surface heat transfer coefficient and friction losses. Therefore, the staff expects that the fouling will have a modest effect over service life on some of the parameters important to the stability performance. Therefore, fouling was ranked as being of medium importance. Here it is noted that the fouling process is relatively slow and therefore, it is not necessary to account for the dynamic change in the fouling during a few oscillations of the flow. Rather, like burnup, it can be input as a static condition of the calculation as an input.

In addition to fouling, the tubes will likely suffer from stress corrosion cracking or other sources of cracks. While these cracks would not necessarily have an important effect on stability, when these cracks are detected during routine inspections this will lead to tube plugging. Tube plugging will essentially remove a tube from service. This will change to total number of active tubes in the HCSG and can change the power-to-flow ratio in the remaining in-service tubes.

Therefore, it is expected that plugging will affect the stability performance of the HCSG tube bundle. However, the plugging is not an active process during operation. Therefore, the staff has marked plugging as N/A because it is not an active process. Rather, the effect of plugging can be considered in the analysis by changing the calculation inputs to reflect a different number of active tubes in the HCSG.

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4.3.3 Centrifugal Forces (Rank: H / KL: M)

In the reference BWR PIRT, the fuel design/type was ranked as a highly important phenomenon. While the fuel design itself is not a process or a phenomenon, the fuel design or type affects many aspects of the thermal-hydraulic and neutronics performance of the fuel during postulated density wave oscillations. For example, the fuel design may or may not include part length fuel rods and water rods. The presence of these design features has a strong impact on the distribution of bundle pressure drop and the reactivity feedback coefficients.

The staff identified centrifugal forces from the coil geometry as being the analogous phenomena for the HCSG tubes. The helix diameter affects the degree to which the fluid in the tube experience centrifugal force due to the change in flow direction. This force has a significant impact on the flow that would, in turn, have a significant contribution to the stability characteristics of the coil. Depending on the flow average velocity and slip, the centrifugal force will affect the flow regime. Hypothetically, if the vapor velocity were to be very high, such that the vapor is carrying more momentum than the liquid phase, centrifugal forces will contribute to a stratification where the vapor is flung outward and the liquid inward relative to the helix diameter. For instances where more momentum is carried by the liquid, at lower velocities, the exact opposite trend in the stratification is expected.

The flow regime affects the heat transfer, dryout, interfacial shear, and wall shear. The coil also induces acceleration pressure drop from the change in flow direction which is a result of the centrifugal forces experienced by the fluid Most of these phenomena have a strong influence on the DR and transient wall temperature. Therefore, the centrifugal forces are ranked as a highly important phenomenon. However, the staff stresses that it may not be necessary to explicitly model the centrifugal forces if the effect of the coil geometry on heat transfer and pressure drop can be captured implicitly. For example, if the heat transfer and pressure drop can be characterized empirically this would inherently capture the impact of the centrifugal forces.

4.3.4 Shell Side Heat Transfer (Rank: H / KL: H) and Feedback: Void and Feedback: Fuel Temperature Rather than boiling in the fuel channels, the two-phase conditions in the HCSG tubes arise from heat transfer from the primary side flow on the outside (or shell side) of the tubes. Therefore, the staff has identified shell side heat transfer as being an analogous process or phenomenon to reactor power in the reference BWR PIRT. Since this is the primary mechanism for transferring the reactor power to the HCSG tube side fluid, the phenomenon is ranked as highly important. In the reference BWR PIRT, the importance of the power distribution was recognized. The wall heat flux distribution in the HCSG tubes will be a function of the shell and tube side heat transfer. This further supports the high ranking for the shell side heat transfer.

It is worth mentioning here that the staff considered the possibility of neutronic feedback in the core coupling with secondary side flow oscillations and performed an extensive analysis of this issue as part of a separate effort (Ref. 11). While the details are complex, the staff found that the neutronic feedback in the core would have a negligible effect on secondary side flow instability. The reader is directed to Ref. 11 for a more thorough discussion of the analysis, specifically the reader should refer to Section 3.1.1.4. To briefly synopsize here, the nuclear feedback mechanism in the core has the effect of filtering signals that occur with a higher 17

frequency than the natural frequency of the primary system. Since the transit time for fluid in the HCSG tubes is smaller than the loop transit time of the reactor coolant system, this generally means that the core will act to filter (i.e., apply a gain much smaller than one) to any oscillation occurring in the secondary side. As a result, there will be little influence of the primary side feedback on the secondary side flow oscillation, such that it could be effectively ignored for the purpose of analyzing DWO on the secondary side. For a quantitative analysis of the core feedback transfer function, the reader should look to Ref. 12 and for a more detailed explanation of the staffs analysis the reader should refer to Ref. 11.

4.3.5 Ledinegg Instability (Rank: H / KL: H)

Ledinegg instability was not considered in the reference BWR PIRT but has been added in the current PIRT based on the incidence of superposition of Ledinegg and DWO observed in tests performed by Papini, et al., at SIET laboratory (Ref. 13). These tests conducted by Papini, et al.

are hereafter referred to as the POLIMI tests in reference to the Polytechnic University of Milan.

A sample set of observation from POLIMI is shown in Figure 8. The Ledinegg mechanism occurs when two flow rates produce the same pressure loss in a channel where there are parallel channels. If two flow rates can produce the same pressure loss, the channel can flip between these flow rates at a fixed pressure drop. This type of instability is commonly referred to as a static instability. This flipping can be aperiodic. The Ledinegg mechanism can become coupled with the density wave mechanism if one of the available flow states is more adverse from a stability perspective. An example might be a case where the power-to-flow ratio in one Ledinegg flow state is high. When the channel flips to the high power-to-flow ratio state, it may be subject to unstable DWO.

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Figure 8: Observed Superposition of Ledinegg Instability and Density Wave Oscillations (Ref. 13)

Figure 8 illustrates how a static instability mode (i.e., Ledinegg) can couple with a dynamic mode (i.e., DWO). In panel (a) of the figure, the flows in Channel A and Channel B separate.

This flow separation is due the Ledinegg mechanism since the two channels share a common pressure drop but have a different mass flow rate. After the separation in the flow, the individual channel flows oscillate out-of-phase, which is indicative of the dynamic instability mode (DWO in this case). The result is a superposition of the Ledinegg and DWO instability mechanisms.

Particularly in light of the experimental evidence that the Ledinegg and DWO mechanisms can superpose, the staff has ranked the Ledinegg instability phenomenon as being highly important.

4.3.6 Gravity Head (Rank: M / KL: H)

In the reference BWR PIRT the void distribution was ranked as highly important. This is due, largely, to the interaction between the void fraction and the reactor power level in the BWR core.

The void fraction affects the strength of the void reactivity coefficient. As void fraction increases 19

the void reactivity coefficient first strengthens and then as the void fraction becomes quite high, the void reactivity coefficient weakens. The coupling between the void and the reactor power mediates one of the strongest feedback mechanisms in the BWR core and flow-loop system, and therefore, has a very strong influence on the stability performance.

By contrast, in the HCSG, the void fraction itself is not a major contributor to the feedback. In heated channels where there is no neutronic feedback, the density wave mechanism operates primarily by the interaction between the inlet flow to the channel and the channel pressure drop.

While the void fraction changes as the fluid is heated in the channel, and this affects different heat transfer and flow regimes, these effects are indirect and have already been captured in other phenomena listed in the table (i.e., single-phase liquid, two-phase, and single-phase vapor heat transfer and pressure drop). In terms of affecting the pressure drop, the void fraction distribution can directly affect this through the gravity head. Therefore, the staff has listed gravity head as being the analogous phenomenon for void distribution. It is noted that the more indirect effects of void fraction on heat transfer and pressure drop are already captured by other phenomena listed in Table 1.

Recognizing that the gravity head feedback would be less important than neutronic feedback, and further recognizing that the HCSG is subject to forced convection from the feed pumps, the gravity head phenomenon is ranked only as medium.

4.3.7 Subcooled Boiling (Rank: M / KL: H)

Subcooled boiling was retained from the reference BWR PIRT, but the ranking was reduced from high to medium. In a BWR, the subcooled boiling affects the void fraction, which, affects the void reactivity feedback. Since the HCSG tubes lack such a feedback mechanism, this warrants a reduction in the importance from high to medium.

4.4 Knowledge Level and Knowledge Gaps The staff reviewed the scientific literature (Ref. 6, 8, and 13) and submittals made by the applicant (Ref. 14, 15, and 16) to determine the knowledge level regarding the staff-identified phenomena in Table 1. Table 2 summarizes the knowledge level rankings and the rationale for the rankings. In general, the phenomena have been studied in representative separate effects and integral effects tests (SETs and IETs). These tests were primarily conducted at the SIET laboratory. See Figure 1 for a photograph of the POLIMI test apparatus used to investigate DWO in helical coils at SIET.

Therefore, the staff found that many of the phenomena were well understood owing to the extensive experimental database available to study the subject phenomena. In some instances, the phenomena were not specifically studied, but are well understood theoretically. An example is tube wall heat conduction. Phenomena that have been studied through experiments or are well understood theoretically have been assigned a high knowledge level. This leaves only four phenomena with a knowledge level below high. Two of these phenomena relate to ageing (plugging and fouling). The ageing related phenomena have been assigned a medium knowledge level because, while no operational history is available to shed light on these phenomena, it is straightforward how to model the effect these phenomena have on the stability characteristics of the tube (i.e., it is simple to adjust parameters to change the number of active tubes, surface roughness, or heat transfer resistance). Therefore, it would not be difficult to conduct sensitivity studies to explore the effect of these phenomena on the FoMs.

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This leaves two phenomena that are ranked as highly important but have a medium knowledge level. These two have been highlighted in orange in Table 2. One of these is the orifice pressure loss. The orifice pressure loss has an important effect on stability because it controls the single-phase pressure loss at the inlet of the tube. This was assigned a medium knowledge level ranking because the detailed pressure losses for the orifice have not been established by the applicant (Ref. 20). Therefore, the loss coefficient for the orifice is not well known.

However, much like plugging or fouling, the effect of the orifice on the calculations can be modeled in a straightforward manner and studied through sensitivity analysis. Any calculations can study variations in the orifice loss coefficient to see the effect on the FoMs. Therefore, the knowledge level was assigned a medium value.

The centrifugal force was listed as a highly important phenomena but as having only a medium knowledge level. This is because the centrifugal force is not directly measured. Rather, the effect of these forces on the resultant wall heat transfer, flow, and pressure drop is measured in the tests. These data are then commonly used to fit correlation parameters such as those for pressure loss factors or heat transfer coefficients. So long as the effect of the centrifugal forces on the heat transfer and pressure drop are captured in the analysis, then it should be capable of modeling DWOs. However, it will be important to qualify such analyses insofar as they may be limited in their application range depending on how centrifugal forces are captured in that analysis. Given the nature of the available data, and the current capabilities in TRACE, it is likely that an empirical approach will be necessary where measured data from helical coils is used to fit pressure loss and heat transfer coefficient correlation parameters.

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Table 2: Knowledge Level Rankings for HCSG DWO Phenomena HCSG Phenomenon/Process DWO KL KL Rationale Ranking Studied in representative SETs or IETs, process can be measured Two-phase Heat Transfer H H through wall thermocouples.

Studied in representative SETs or IETs, process can be measured Single-phase Liquid Heat Transfer H H through wall thermocouples.

Studied in representative SETs or IETs, process can be measured Single-phase Vapor Heat Transfer M H through wall thermocouples.

Studied in representative SETs or IETs, process can be measured Dryout H H through wall thermocouples.

Flow distribution is a function of the pressure drop, and can be Tube-to-Tube Flow Distribution H H calculated straightforwardly based on tube pressure losses.

No existing operational data to know about ageing effects, but can Plugging N/A M be easily studied by changing problem inputs.

No existing operational data to know about ageing effects, but can Fouling M M be easily studied by changing problem inputs.

Studied in representative SETs or IETs, but can only be measured Centrifugal Forces from Coil Geometry H M indirectly by observing the effect of these forces on flow rate, pressure drop, and/or wall temperature.

Heat Conduction through Tube Wall H H Simple process with well-established theoretical modeling.

Studied in representative SETs or IETs, but can only be measured Interfacial Shear H H indirectly by observing the effect of interfacial shear on flow rate, pressure drop, and/or wall temperature.

Studied in representative SETs or IETs, but the experimental bases Shell-side Heat Transfer H H do not consider uncover of the tubes in the RPV annulus region.

Loss factor has not been established by the applicant, but can be Orifice Pressure Loss H M easily studied by changing problem inputs.

Sudden expansion losses are well understood, it is a simple process Exit Loss H H with well-established theoretical modeling.

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HCSG Phenomenon/Process DWO KL KL Rationale Ranking Studied in representative SETs or IETs, process can be measured Single-phase Liquid Pressure Drop H H through differential pressure cells.

Studied in representative SETs or IETs, process can be measured Single-phase Vapor Pressure Drop H H through differential pressure cells.

Studied in representative SETs or IETs, process can be measured Two-phase Pressure Drop H H through differential pressure cells.

Ledinegg instability is a function of the pressure drop, and can be calculated straightforwardly based on tube pressure losses. Studied Ledinegg Instability H H in representative SETs or IETs, process can be measured through differential pressure cells.

Studied in representative SETs or IETs, process can be measured Density Wave Propagation H H through differential pressure cells.

Studied in representative SETs or IETs, process can be measured Subcooling H H through wall thermocouples.

Gravity head is well understood and is a simple process with well-established theoretical modeling. Studied in representative SETs or Gravity Head M H IETs, but can only be measured indirectly by observing the effect of void on pressure drop.

Studied in representative SETs or IETs, but can only be measured Subcooled Boiling M H indirectly by observing the effect of void on pressure drop.

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5 DHRS Loop Instability 5.1 Scenario Descrip on The DHRS provides heat removal during normal shutdown and upset conditions for the NuScale power module. In the current evaluation, the staff assumes that the DHRS is operating following a design basis event such as a loss-of-coolant-accident (LOCA) or an anticipated operational occurrence (AOO). Under these conditions, the primary side may remain at high pressure or may have depressurized. The DHRS trip signal will have isolated the mainsteam and feedwater systems. The DHRS flow loop will be established by boiling in the HCSG and condensation in the DHRS HX. The primary side level will remain above the HCSG for some time before dropping below the top of the riser and partially uncovering the HCSG tubes.

While the DHRS is operating, it may experience flow instability due to either one or a combination of various phenomena, which include:

1. Boiling-condensation flow loop instability

2. Density wave oscillations

3. Manometer level oscillations

4. Ledinegg instability

5. Breaking and re-establishing natural circulation in the DHRS loop 5.2 Figures of Merit The RES staff considered two FoMs. The first FoM is the DHRS cold leg mass flow rate.

Here the cold leg refers to the piping that connects the outlet or lower plenum of the DHRS HX with the inlet or lower plenum of the HCSG (see Section 3 and Figure 7). If the DHRS loop becomes unstable, this will lead to growing oscillations in the cold leg mass flow rate. Margin to instability can be examined by analyzing the response of the cold leg mass flow rate in response to a perturbation. However, the cold leg mass flow rate is not directly tied to safety margins or the DHRS safety function. Even if the cold leg mass flow rate oscillates, this may not have a substantial effect on the DHRS safety function. Therefore, in addition to the cold leg mass flow rate, the staff also considered the aggregate DHRS loop heat removal. This can be calculated by TRACE by integrating the heat transfer to and from the DHRS loop at the HCSG and DHRS HX. The aggregate heat removal FoM can be used directly to characterize DHRS performance during postulated transients and accidents.

To summarize the figures or merit are cold leg mass flow rate and aggregate DHRS heat removal.

5.3 Phenomena Iden "ca on and Ranking The RES staff assembled an expert panel for the phenomena identification and ranking, but in advance of the panel meeting, one member assembled a draft PIRT using the phenomena listed for the HCSG above and the DHRS phenomena listed in the boron dilution PIRT (Ref. 21 and 22). Components and phenomena were added to represent the piping connections between the 24

HCSG and DHRS HX. In a series of meetings, the panel members reached a consensus on the importance and knowledge level rankings, which are summarized in Table 3.

Rather than explain the rationales for each individual phenomenon, it is worthwhile to note some general observations about the flow loop and the predominant phenomena, as this will provide some economy in the explanation of the rankings for the phenomena.

Observation 1: The primary driving force for the flow in the DHRS loop is boiling in the HCSG and condensation in the DHRS HX.

This observation means that heat transfer mechanisms in the HCSG apart from boiling and in the DHRS HX apart from condensation are going to be secondary effects compared to this primary mechanism. Further, because the DHRS HX is condensing the steam, it is likely that the condensate will remain near the saturation temperature when it returns to the HCSG. This will limit the importance of single-phase liquid heat transfer in the HCSG because the boiling length with likely be short.

The primary heat removal pathway from the primary side is: (1) either convective or condensation heat transfer on the HCSG tubes shell-side, (2) conduction through the HCSG tube wall, (3) boiling in the HCSG tubes, (4) condensation of that steam in the DHRS HX, (5) conduction through the DHRS HX tube walls, and (6) pool heat transfer in the reactor pool.

These phenomena or related phenomena will have a direct impact on the heat removal FoM and are ranked as highly important.

Observation 2: The natural circulation driving head is provided by the density difference between the liquid column of condensate in the DHRS HX and cold leg and the steam in the HCSG tubes and hot leg.

Under natural circulation conditions, the flow rate in the loop will be small compared to the feedwater flow rate. This low flow rate implies a relatively low velocity, meaning that local irreversible and friction pressure losses will likely be small when compared to gravity head as these losses scale with the velocity squared (v2). Therefore, phenomena related to flow resistance or pressure losses where flow velocity is a determining factor are ranked commensurately.

On the other hand, phenomena that affect the liquid column height or the average liquid inventory in the HCSG will have a substantial effect on the loop flow. It is convenient in this instance to imagine the HCSG as having a short boiling boundary and then a two-phase level that defines the axial height where dryout occurs in the HCSG, on-average. The phenomena affecting the two-phase level will affect the gravity head and have a strong influence on the loop flow.

There will be a dryout location in the HCSG tubes whenever the primary side temperature exceeds the saturation temperature in the DHRS loop. During the transient it is possible and, indeed, likely that the primary side pressure will decrease and the DHRS loop pressure will increase, eventually coming into a balance with a small temperature difference between the two systems. Even if the DHRS approaches saturated conditions in the hot leg, and dryout is no longer occurring in the coils, it is still a useful exercise in analyzing the loop behavior to imagine a level in the HCSG tubes with the two-phase conditions covering all or most of the axial length.

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It is worthy to note that the inventory at the initiation of the DHRS has some variation owing to the specific two-phase flow conditions in the HCSG during design basis events when the DHRS trip actuates. The initial inventory can be considered, essentially, as an input to the analysis rather than a distinct and separate phenomenon affecting the level, but its mentioned here because any analysis would be sensitive to such an input.

Observation 3: The NuScale design is compact. The DHRS HX is located near the containment vessel.

Because the design is compact, there is only a limited length of piping connecting the HCSG and DHRS HX. Therefore, it is unlikely for significant heat transfer or pressure drops to occur in these components.

Observation 4: The HCSG and DHRS HX are comprised by many parallel flow channels.

The HCSG includes a bundle of many parallel helical coils and the DHRS HX includes several vertical pipes. It is possible for flow instabilities to occur within a single channel in either case.

Such single channel instabilities are unlikely to affect the aggregate behavior of the HCSG or DHRS HX because these components include many parallel channels. One channel experiencing an oscillation that does not affect the other channels will only have a small impact on the overall behavior. Therefore, single channel instabilities would not have a strong impact on the FoMs. Similarly, behaviors where parallel channels oscillate out-of-phase would be unlikely to affect the average behavior. This is because it would represent a small number of channels compared to the total number of channels, but also, because the parallel channels would oscillate out-of-phase, their oscillations would tend to cancel out in terms of their effect on the overall heat transfer of the entire component. While there are many mechanisms that can contribute to instability, those that would cause only a single channel, or perhaps a small subset of the channels to become unstable, will be unlikely to have a meaningful effect on the FoMs.

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Table 3: DHRS Flow Instability PIRT Component Phenomenon/Process Ranking Importance Rationale Flow velocity will be low, so v2 losses will be small HCSG: Inlet Local Pressure Loss M compared to gravity head, but this loss coefficient is quite Orifice large by design.

Critical flow is not expected to occur because a substantial pressure drop across the orifice is required to support HCSG: Inlet Critical Flow I critical flow, which cannot occur because the upstream and Orifice downstream areas are exposed to steam pressure from the same hot leg.

Two-phase Heat HCSG: Coils H Primary mechanism for removing primary side heat Transfer Single-phase Liquid Liquid will remain near saturated conditions, so will only be HCSG: Coils L Heat Transfer single phase liquid over a short distance in the coil.

Vapor temperature will remain near primary side Single-phase Vapor HCSG: Coils L temperature, so this heat removal should be small Heat Transfer compared to two-phase heat transfer Affects the two-phase "level" in the HCSG, but only at high HCSG: Coils Dryout M void fraction Tube-to-Tube Flow Parallel channel effects tend to self-cancel in terms of HCSG: Coils L Distribution impact on the loop dynamics This is not a process. Impact is captured in setting number HCSG: Coils Plugging N/A of available tubes in the analysis.

Flow velocity will be low, so v2 losses will be small HCSG: Coils Fouling L compared to gravity head. Heat flux should be low, but could affect tube wall temperature slightly 27

Component Phenomenon/Process Ranking Importance Rationale The centrifugal forces impact pressure losses and heat transfer and can have a substantial effect on both. Further, Centrifugal Forces from centrifugal forces can affect flow regime and flow HCSG: Coils H Coil Geometry stratification, which also has ramifications for the acceleration and friction pressure drops, interfacial shear, and surface heat transfer.

Heat Conduction HCSG: Coils H Primary mechanism for removing primary side heat through Tube Wall Affects the two-phase "level" in the HCSG, impacting gravity head driving force. This also affects flow regime, HCSG: Coils Interfacial Shear H and therefore, it affects the heat transfer from the wall to the secondary fluid.

Shell-side Heat HCSG: Coils H Primary mechanism for removing primary side heat Transfer Flow velocity will be low, so v2 losses will be small HCSG: Coils Exit Loss L compared to gravity head Single-phase Liquid Liquid will remain near saturated conditions, so will only be HCSG: Coils L Pressure Drop single phase liquid over a short distance in the coil.

Single-phase Vapor Affects the two-phase "level" in the HCSG, impacting HCSG: Coils H Pressure Drop gravity head driving force Two-phase Pressure Affects the two-phase "level" in the HCSG, impacting HCSG: Coils H Drop gravity head driving force Potential mechanism contributing to changes in two-phase HCSG: Coils Ledinegg Instability H "level" if this instability mode can occur Density Wave Dominant contributor to potential density wave oscillation HCSG: Coils H Propagation growth 28

Component Phenomenon/Process Ranking Importance Rationale HCSG: Coils Subcooling L Liquid will remain near saturated conditions HCSG: Coils Gravity Head H Dominant force in establishing natural circulation flow HCSG: Coils Subcooled Boiling M Liquid will remain near saturated conditions Flow velocity will be low, so v2 losses will be small HCSG: compared to gravity head in the loop. However, in the Steam Flow Resistance M steam region velocities will remain high compared to the Plenum liquid regions, so pressure drops might have a medium impact on the loop flow.

Heat transfer in the steam plenum should be small HCSG:

compared to heat transfer in the HCSG tubes because of Steam Heat Transfer L the much smaller surface area of the plenum compared to Plenum the tubes.

Hot Leg:

Steam Flow velocity will be low, so v2 losses will be small Connection compared to gravity head in the loop. However, in the Piping Flow Resistance M steam region velocities will remain high compared to the between liquid regions, so pressure drops might have a medium HCSG and impact on the loop flow.

DHRS HX Plena Hot Leg:

Steam Heat transfer in the piping track in the CNV should be small Connection owing to either vacuum conditions or a steam environment.

Piping Heat Transfer M Outside the CNV, in the Reactor Pool, heat transfer may be between more significant depending on the length of the piping and HCSG and the surface area available for heat transfer to the pool.

DHRS HX Plena 29

Component Phenomenon/Process Ranking Importance Rationale DHRS HX: Flow velocity will be low, so v2 losses will be small Flow Resistance L Inlet Plenum compared to gravity head Heat transfer in DHRS HX inlet plenum should be small DHRS HX: compared to heat transfer in the DHRS HX tubes because Heat Transfer L Inlet Plenum of the much smaller surface area of the plenum compared to the tubes.

DHRS HX: Parallel channel effects tend to self-cancel in terms of Asymmetric Loading L Tubes impact on the loop dynamics Condensation Heat Dominant heat transfer mechanism in the DHRS HX tubes, DHRS HX:

Transfer (Inside DHRS H primary source of liquid affecting liquid column height and, Tubes Tubes) therefore, gravity head DHRS HX: Conduction Through Primary mechanism for removing heat from the DHRS loop, H

Tubes the Tube Wall this represents the pathway to the ultimate heat sink Pressure losses dominated by gravity head, which will have DHRS HX: Density Wave L little lag, so density wave propagation should not Tubes Propagation meaningfully impact flow oscillation in the DHRS HX tubes DHRS HX: Entrance Effects / Expected to have only a small impact on pressure loss and L

Tubes Developing Length condensation heat transfer DHRS HX:

Flashing I Not expected to occur in the DHRS HX tubes Tubes DHRS HX: Expected to have only a small impact on pressure loss and Inlet Losses / Orificing L Tubes condensation heat transfer Affects flow characteristics in the DHRS HX tubes as DHRS HX: condensation begins on the tube inner walls. It will affect Interfacial Shear M Tubes interfacial area which could have a modest impact on condensation rate.

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Component Phenomenon/Process Ranking Importance Rationale Potential mechanism contributing to changes in liquid column height if this instability mode can occur. It might not DHRS HX: be possible but would require further study to rule out this Ledinegg Instability H Tubes phenomenon. Since the tubes might be at different temperatures this mechanism appears plausible without further detailed evaluation.

DHRS HX: Minimum Stable Film Condensation heat transfer should dominate the DHRS HX L

Tubes Boiling heat transfer DHRS HX: Natural circulation is the primary mode of the flow through Natural Circulation H Tubes the DHRS loop DHRS HX: Parallel Channel Parallel channel effects tend to self-cancel in terms of L

Tubes Effects impact on the loop dynamics DHRS HX: Pressure Wave Pressure waves will propagate very quickly compared to L

Tubes Propagation loop transit time DHRS HX: Single Phase Heat Condensation heat transfer should dominate the DHRS HX L

Tubes Transfer to Liquid heat transfer DHRS HX: Single Phase Heat Condensation heat transfer should dominate the DHRS HX L

Tubes Transfer to Vapor heat transfer DHRS HX: Single Phase Pressure Liquid column gravity head will be significant contributor to H

Tubes Drop flow loop driving force DHRS HX: Heat Capacitance of Heat capacity of the DHRS tubes will be quickly exhausted L

Tubes the Tubes after opening of the DHRS loop Condensation heat transfer should dominate the DHRS HX DHRS HX:

Transition Boiling I heat transfer. Transition boiling is not expected since the Tubes heat is transferred from the fluid to the wall.

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Component Phenomenon/Process Ranking Importance Rationale DHRS HX: Two Phase Convective Condensation heat transfer should dominate the DHRS HX L

Tubes Heat Transfer heat transfer DHRS HX: Two Phase Pressure Should have a small effect on driving head compared to the L

Tubes Drop liquid column DHRS HX: Will affect gravity head, but not as significantly as the liquid Void Distribution M Tubes column DHRS HX:

Flow velocity will be low, so v2 losses will be small Outlet Flow Resistance L compared to gravity head Plenum Heat transfer in DHRS HX outlet plenum should be small DHRS HX:

compared to heat transfer in the DHRS HX tubes because Outlet Heat Transfer L of the much smaller surface area of the plenum compared Plenum to the tubes.

Cold Leg:

Condensation Line Piping Flow velocity will be low, so v2 losses will be small between Flow Resistance L compared to gravity head DHRS HX and HCSG Plena Cold Leg:

Heat transfer in the piping track in the CNV should be small Condensation owing to either vacuum conditions or a steam environment.

Line Piping Outside the CNV, in the Reactor Pool, heat transfer may be between Heat Transfer L more significant, but the track of piping is expected to be DHRS HX small, meaning there is only a small surface area for any and HCSG potential heat transfer in the pool.

Plena HCSG: Inlet Flow velocity will be low, so v2 losses will be small Flow Resistance L Plenum compared to gravity head 32

Component Phenomenon/Process Ranking Importance Rationale Heat transfer in the inlet plenum should be small compared HCSG: Inlet Heat Transfer L to heat transfer in the HCSG tubes because of the much Plenum smaller surface area of the plenum compared to the tubes.

Heat transfer from the DHRS HX tubes to the reactor pool Reactor Pool Heat Transfer H is the primary mechanism of heat removal from the DHRS loop.

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Some of the phenomena are not covered by the rationale given in then general observations.

While Table 3 provides a brief description of rationale for the rankings for these phenomena, the following sections provide a lengthier discussion of the basis for the rankings.

5.3.1 HCSG Inlet Ori"ce Cri cal Flow (Rank: I / KL: N/A)

The panel considered whether there could be critical flow through the HCSG inlet orifices. In particular, if the tubes experience significant flow oscillations, due to density wave driven flow oscillation or another mechanism, there could be significant and rapid changes in the local void fraction, flow velocity, and pressure. In the forward direction, it seems impossible for a pressure drop to develop across the orifice that would be significant enough to enable critical flow. The HCSG tube outlet and the DHRS HX liquid column are both exposed to the same steam in the hot leg piping. As such, there is the same back pressure applied to the flow both upstream and downstream of the orifice.

In the reverse direction, perhaps a condition of relative vacuum could occur on the HCSG tube inlet. For this to occur a substantial oscillation would need to occur such that there is a backward flow of vapor from the HCSG tube into the HCSG plenum. If that reversed vapor flow were to condense and the void collapse, this could create a local vacuum condition. However, when the DHRS is operating, the liquid in the lower plenum will likely be close to the saturation temperature. As such, there is not a substantial temperature difference between the phases to drive rapid interfacial heat transfer sufficient for the localized void collapse near the tube inlet.

For these reasons, the panel concluded that critical flow in the orifices would not occur.

5.3.2 HCSG Coils Dryout (Rank: M / KL: H)

The location of dryout in the tubes affects the height of the two-phase level in the steam generator. However, if there were deviations in the dryout location, this would affect this two-phase level would not have a substantial impact on the equivalent, collapsed level. Therefore, while it can have an impact on the height of the two-phase level, it should not have a strong influence on the gravity head, or the flow rate. Dryout can affect the transition between the two-phase and single phase vapor flow and heat transfer regimes. The relative portion of the HCSG tubes experiencing two-phase as opposed to single-phase heat transfer can affect the heat removal. However, dryout has a narrow influence on this transition only at the highest void fraction and highest elevation of the two-phase region. Therefore, the effect on heat transfer should be relatively small as well. It was ranked as medium to reflect that it affects two important parameters, but the influence is not strong enough to warrant a high ranking.

5.3.3 HCSG Coils Fouling (Rank: L / KL: L)

Fouling has two effects on key parameters. First, it can affect the loop flow FoM. Fouling can increase the friction in the HCSG tubes and contribute to higher flow resistance. However, since the flow velocity will be low in the loop, the contribution of the fouling to surface roughness should only have a weak effect on the flow FoM. Surface fouling can also affect the conduction heat resistance between the primary and secondary side. This would impact the heat removal FoM. The staff, however, expects that routine inspection, maintenance, and feedwater chemical controls would limit surface fouling in practice, such that it would not significantly impact the thermal resistance. Further, changes in the thermal resistance are more likely to result in higher primary side temperatures than to lead to significant changes in the loop dynamics. Therefore, fouling was ranked as having a low importance.

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5.3.4 HCSG Coils Centrifugal Forces from Coil Geometry (Rank: H / KL: M)

Centrifugal forces were ranked as having a high importance because they affect flow resistance and heat transfer in the HCSG tubes. These affect both FoMs considered in the analysis.

Generally, the centrifugal forces are not measured directly, and the effect of these forces on the pressure loss and heat transfer are captured through empirically derived parameters.

Therefore, while the staff can make assertions about the relative importance of friction to the pressure drop - the effect of the centrifugal force on acceleration pressure drop is more difficult to ascertain without detailed analysis. To err on the conservative side, in light of these uncertainties, the panel has ranked the centrifugal forces as having a high importance.

5.3.5 HCSG Coils Interfacial Shear (Rank: H / KL: H)

The panel expects that the primary heat transfer pathway from the primary to secondary will be through two-phase heat transfer of the tube side of the helical coils. Interfacial shear will control the flow regime in the two-phase region, and, therefore, will have a significant influence on the two-phase heat transfer. Therefore, the panel ranks interfacial shear with high importance.

5.3.6 HCSG Coils Density Wave Propaga on (Rank: H / KL: M)

The HCSG may be subject to density wave oscillations. While these can be expected to occur in individual tubes, or in sets of tubes that oscillate out-of-phase, the staff has not been able to rule out the possibility of in-phase oscillations. In-phase, also known as coherent, density wave oscillations in the coils can result in changes in the HCSG pressure drop. This will have a first order effect on the loop flow rate. Certainly, as there are no external forces controlling the HCSG pressure drop, it is difficult to see how the oscillations may become coherent, but it cannot be ruled out. Complex and interacting phenomena within the loop may lead to fluctuations in the condensate line liquid level, which might have some feedback connection to the pressure losses in the coils. If this is the case, then, theoretically, the HCSG tubes may be subject to in-phase, or coherent, density wave oscillation.

5.3.7 DHRS HX Tubes Density Wave Propaga on (Rank: L / KL: M)

Unlike the HCSG, density wave oscillation is ranked as low in the DHRS HX. The flow will experience pressure losses within the DHRS HX tubes, but the impact on loop flow will be dwarfed by the gravity head and inertia of the liquid column. Since it is the integral of the density in the condensate line and the DHRS HX that contributes to the gravity head, density wave oscillations in the vapor and two-phase region of the DHRS HX tubes are unlikely to manifest in a change in the driving force since the peaks and troughs of the density wave oscillations will tend to cancel out when the integral along the height is considered. Therefore, in the DHRS HX, density wave propagation is ranked with low importance.

5.3.8 DHRS HX Tubes Entrance Eects / Developing Length (Rank: L / KL: M)

Whether the flow is fully developed or developing, in the DHRS HX tubes the heat transfer will be dictated by condensation. This is a stronger function of the temperature difference and the pressure than on the flow regime. Therefore, the panel considered this to have a minor influence on the heat removal and flow.

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5.3.9 DHRS HX Tubes Flashing (Rank: I / KL: N/A)

In the closed DHRS loop, the panel could not postulate a mechanism that would lead to a sudden decrease in pressure sufficient to cause flashing. Therefore, flashing was listed as being inactive.

5.3.10 DHRS HX Tubes Interfacial Shear (Rank: M / KL: M)

In the DHRS HX tubes there will be a two-phase region above the liquid column. The heat transfer from the DHRS loop to the reactor pool will be a function not only of the direct condensation on DHRS tube walls, but also of any condensation due to interfacial heat transfer between the colder liquid and the steam vapor. Interfacial shear will affect the interfacial area and will affect the condensation rate. However, this is an indirect effect on that heat transfer, therefore, interfacial shear was ranked with medium importance.

5.3.11 DHRS HX Tubes Pressure Wave Propaga on (Rank: L / KL: H)

In studying flow instability in the DHRS loop it will be important to capture the propagation of various effects, such as density waves and temperature fronts. However, pressure waves will travel through the system at the speed of sound. This speed is very rapid compared to the other dynamic processes at work. In fact, so fast that it would be a reasonable approximation to treat pressure waves as traveling across the loop within a single time step in any analysis.

Therefore, this phenomenon is not expected to meaningfully impact FoMs, which are driven by process with, relatively, long time constants. Therefore, the panel ranked this as having low importance.

5.3.12 DHRS HX Tubes Heat Capacitance of the Tubes (Rank: L / KL: H)

While the staff has called this phenomenon the stored energy in the tubes, it really refers to the heat capacitance of the tubes, and the process by which these tubes can accumulate and store heat. When the DHRS is first actuated, and the DHRS HX tubes are exposed to steam, the DHRS HX tubes will be cold, near the reactor pool temperature. Therefore, these tubes will absorb heat from the steam and warm up before they achieve a higher temperature to begin transferring heat to the reactor pool. The panel expects that the tubes will heat up rapidly after this initial startup of the loop. Further, this is expected to be a transient process of heating up the tubes which will essentially vanish as an effect during DHRS operation. The tubes themselves will have some thermal inertia that can affect the dynamical response, but such a lag from the inertia is expected to be small and not contribute to feedback phenomena occurring on the scale of the loop transit time. Therefore, the tube heat capacitance is not expected to have a strong influence on the heat transfer (apart from the initial transient period) or on the loop flow. The panel has ranked this phenomenon as having low importance.

5.3.13 DHRS HX Tubes Transi on Boiling (Rank: I / KL: N/A)

As there is no source from the DHRS HX tubes providing heat to the DHRS loop flow, transition boiling should not occur in the DHRS HX tubes. Therefore, this phenomenon was ranked as inactive.

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5.3.14 Ledinegg Instability (Rank: H / KL: H)

Ledinegg instability was ranked as highly important for both the HCSG and the DHRS HX. It may prove to be the case after further analysis that neither component is subject to Ledinegg instability, but the staff cannot rule out such a possibility currently. Whenever there is two-phase flow there is the possibility, theoretically, the changes in the flow regime or void distribution can lead to two or more conditions of flow that produce the same pressure drop. When such conditions are possible, the component or system may swap back and forth between these various states in an aperiodic manner. The Ledinegg mechanism may couple with another dynamic aspect of the loop to produce instability, or it may lead to flow conditions that are more susceptible to another instability mode. Therefore, the panel considers Ledinegg instability to be highly important. The staff, however, notes that if the behavior of pressure drop with flow can be shown to be unconditionally monotonic, then the components are not susceptible to this mechanism.

5.4 Knowledge Level and Knowledge Gaps The panel evaluated the knowledge level for each of the ranked phenomena. The knowledge level rankings and bases are summarized in Table 4. In instances where the phenomenon has a high importance, but the knowledge level is medium or low, the staff has highlighted the phenomenon in orange. In assigning these knowledge level rankings, the panel often cites two rationales in the table below, and it is worth describing these two common rationales in some more detail here.

Common Rationale 1: Studied in representative SETs or IETs.

When the panel cites this rationale, it is usually to provide a basis for ranking the knowledge level for a given phenomenon as high. Many of the phenomena listed by the panel have been studied through specific experimental campaigns to study system performance and to assess systems analysis codes. Therefore, the knowledge level is tied to applicable experimental data concerning the phenomenon. For clarity, when this rationale is cited, the staff has provided a reference to the experiment or experiments in-mind for the subject phenomenon. In several instances, the phenomenon is active in several experiments, and these are listed in Table 4.

Eight test facilities/test campaigns are referenced in Table 4 which are briefly described below.

KAIST High pressure steam condensation tests were performed by the Korea Advanced Institute of Science and Technology (KAIST) in vertical tubes (Ref. 27).

NIST-1 The NuScale Integral Systems Test 1 (NIST-1) facility is a multi-component flow loop operated by Oregon State University. In the NIST-1 campaign many tests were performed, but the staff examined results from quasi-steady pressurization tests, power maneuvering tests, LOCA from chemical and volume control system line break, inadvertent opening of the reactor vent value, and inadvertent opening of the reactor recirculation valve (Ref. 18, 22, and 28)

NIST-2 This designation reflects the NIST test facility after some modifications were performed to better represent the NuScale US460 design. Tests performed at NIST-2 are described in various licensing topical reports (Ref. 23, 24, and 25) covering LOCA, non-LOCA transients, and extended 37

passive cooling scenarios. In some of these tests, the DHRS loop is active.

PERSEO The in-Pool Energy Removal System for Emergency Operation (PERSEO) test was used to validate TRACE as part of an Organization of Economic Cooperation and Development (OECD) Nuclear Energy Agency (NEA) Committee of the Safety of Nuclear Installations (CSNI) cooperative research program. The tests studied transient steam condensation in straight tubes (Ref. 26).

POLIMI A series of tests were performed to study density wave oscillations in helical coils at the SIET laboratory. These tests are referred to the POLIMI tests in reference to the collaboration of the laboratory with scientists of the Polytechnic University of Milan. Two parallel helical coils were electrically heated and supplied with different mass fluxes to study the stability boundary (Ref. 13).

SIET-TF1 SIET-TF1 refers to a campaign of experiments performed on electrically heated helical coils at SIET laboratory. These tests measured the axial pressure drop distribution and wall temperature at various steady-state conditions (Ref. 17).

SIET-TF2a SIET-TF2a is an integral test facility where a bundle of helical coils are heated on the shell side (or primary side) by heated water that flows over the coils to provide convective heat transfer. These tubes are instrumented to measure the temperature, pressure, and heat removal of the coil bundle (Ref. 17).

SIET-TF2b Similar to the NIST-1 and NIST-2 distinction, SIET-TFb refers to the same facility as SIET-TF1, except the facility includes some modifications and upgrades. A second campaign of tests were performed by NuScale at SIET-TFb to study flow oscillations in the coils of the bundle (Ref. 15 and 16).

Common Rationale 2: Depends on final design information and testing.

When the panel cites this rationale, the panel is referring to a phenomenon that is generally well understood in practice, or is commonly treated in systems analysis, but has some empirical component. The empirical component relies on specific aspects of the final design of the component and is often established through empirical means, such as testing. A good example is a flow orifice. The purpose and impact of a flow orifice on a design is well understood in principle. The orifice constricts flow to create a localized pressure loss. However, the precise loss for a given flow rate, or the change in the flow with changing pressure, must be determined based on empirical measurements. These measurements are then translated to k-factors to characterize the losses. In several instances the knowledge level was ranked as medium because the method to treat the phenomenon in systems analysis is well understood in practice; however, final information regarding the design and/or testing is missing.

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Table 4: Knowledge Level Rankings for DHRS Flow Loop Instability Phenomena Component Phenomenon/Process Ranking KL KL Rationale Test Data Depends on final design HCSG: Inlet information and testing. Detailed Local Pressure Loss M M Orifice value has not been established (see Ref. 20).

HCSG: Inlet Critical Flow I N/A Orifice SIET-TF1, SIET-TF2a, SIET-Two-phase Heat Studied in representative SETs or HCSG: Coils H H TF2b, POLIMI, NIST-1, and Transfer IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Single-phase Liquid Studied in representative SETs or HCSG: Coils L H TF2b, POLIMI, NIST-1, and Heat Transfer IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Single-phase Vapor Studied in representative SETs or HCSG: Coils L H TF2b, POLIMI, NIST-1, and Heat Transfer IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Studied in representative SETs or HCSG: Coils Dryout M H TF2b, POLIMI, NIST-1, and IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Tube-to-Tube Flow Studied in representative SETs or HCSG: Coils L H TF2b, POLIMI, NIST-1, and Distribution IETs NIST-2 HCSG: Coils Plugging N/A N/A Depends on water purity in the HCSG: Coils Fouling L L secondary system Studied in representative SETs or Centrifugal Forces from IETs, but can only be measured SIET-TF1, SIET-TF2a, SIET-HCSG: Coils H M Coil Geometry indirectly by observing the effect TF2b, and POLIMI of these forces on flow rate, 39

Component Phenomenon/Process Ranking KL KL Rationale Test Data pressure drop, and/or wall temperature.

SIET-TF1, SIET-TF2a, SIET-Heat Conduction Studied in representative SETs or HCSG: Coils H H TF2b, POLIMI, NIST-1, and through Tube Wall IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Studied in representative SETs or HCSG: Coils Interfacial Shear H H TF2b, POLIMI, NIST-1, and IETs NIST-2 Studied in representative SETs or Shell-side Heat IETs. However, the tests may not SIET-TF2a, SIET-TF2b, NIST-1 HCSG: Coils H M Transfer be representative if the HCSG and NIST-2 uncovers.

Depends on final design HCSG: Coils Exit Loss L M information and testing SIET-TF1, SIET-TF2a, SIET-Single-phase Liquid Studied in representative SETs or HCSG: Coils L H TF2b, POLIMI, NIST-1, and Pressure Drop IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Single-phase Vapor Studied in representative SETs or HCSG: Coils H H TF2b, POLIMI, NIST-1, and Pressure Drop IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Two-phase Pressure Studied in representative SETs or HCSG: Coils H H TF2b, POLIMI, NIST-1, and Drop IETs NIST-2 The phenomenon is well understood theoretically and can be analyzed with thermal-HCSG: Coils Ledinegg Instability H H POLIMI hydraulic codes if those codes can predict the pressure drop accurately 40

Component Phenomenon/Process Ranking KL KL Rationale Test Data Studied in representative SETs or IETs. Testing generally performed under forced circulation conditions - during DHRS operation the flow will be in natural circulation and the flow velocities might be lower than the Density Wave HCSG: Coils H M experimental range. The friction SIET-TF2a, SIET-TF2b, POLIMI Propagation losses may not be as significant compared to other momentum flux terms and this introduces some uncertainty about density wave propagation, warranting a slightly lower knowledge level ranking.

SIET-TF1, SIET-TF2a, SIET-Studied in representative SETs or HCSG: Coils Subcooling L H TF2b, POLIMI, NIST-1, and IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Studied in representative SETs or HCSG: Coils Gravity Head H H TF2b, POLIMI, NIST-1, and IETs NIST-2 SIET-TF1, SIET-TF2a, SIET-Studied in representative SETs or HCSG: Coils Subcooled Boiling M H TF2b, POLIMI, NIST-1, and IETs NIST-2 HCSG:

Depends on final design Steam Flow Resistance M M information and testing Plenum HCSG:

Depends on final design Steam Heat Transfer L M information and testing Plenum 41

Component Phenomenon/Process Ranking KL KL Rationale Test Data Hot Leg:

Steam Connection Piping Depends on final design Flow Resistance M M between information and testing HCSG and DHRS HX Plena Hot Leg:

Steam Connection Piping Depends on final design Heat Transfer M M between information and testing HCSG and DHRS HX Plena DHRS HX: Depends on final design Flow Resistance L M Inlet Plenum information and testing DHRS HX: Depends on final design Heat Transfer L M Inlet Plenum information and testing DHRS HX: Depends on final design Asymmetric Loading L M Tubes information and testing Condensation Heat DHRS HX: Studied in representative SETs or KAIST, PERSEO, NIST-1, and Transfer (Inside DHRS H H Tubes IETs NIST-2 Tubes)

DHRS HX: Conduction Through Studied in representative SETs or KAIST, PERSEO, NIST-1, and H H Tubes the Tube Wall IETs NIST-2 Limited experimental data for DHRS HX: Density Wave L M inverse density waves in Tubes Propagation condensers 42

Component Phenomenon/Process Ranking KL KL Rationale Test Data DHRS HX: Entrance Effects / Depends on final design L M Tubes Developing Length information and testing DHRS HX:

Flashing I N/A Tubes DHRS HX: Depends on final design Inlet Losses / Orificing L M Tubes information and testing Studied in representative SETs or IETs, but there are interacting phenomena (i.e., condensation)

DHRS HX:

Interfacial Shear M M that can complicate the analysis NIST-1 and NIST-2 Tubes of the experimental results.

Therefore, the knowledge level is ranked as Medium The phenomenon is well understood theoretically and can DHRS HX: be analyzed with thermal-Ledinegg Instability H H Tubes hydraulic codes if those codes can predict the pressure drop accurately DHRS HX: Minimum Stable Film Studied in representative SETs or L H NIST-1, and NIST-2 Tubes Boiling IETs DHRS HX: Studied in representative SETs or Natural Circulation H H NIST-1 and NIST-2 Tubes IETs DHRS HX: Parallel Channel Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Effects IETs DHRS HX: Pressure Wave Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Propagation IETs 43

Component Phenomenon/Process Ranking KL KL Rationale Test Data DHRS HX: Single Phase Heat Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Transfer to Liquid IETs DHRS HX: Single Phase Heat Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Transfer to Vapor IETs DHRS HX: Single Phase Pressure Studied in representative SETs or H H NIST-1 and NIST-2 Tubes Drop IETs DHRS HX: Stored Energy of the Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Tubes IETs DHRS HX:

Transition Boiling I N/A Tubes DHRS HX: Two Phase Heat Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Transfer IETs DHRS HX: Two Phase Pressure Studied in representative SETs or L H NIST-1 and NIST-2 Tubes Drop IETs DHRS HX: Studied in representative SETs or Void Distribution M H NIST-1 and NIST-2 Tubes IETs DHRS HX:

Depends on final design Outlet Flow Resistance L M information and testing Plenum DHRS HX:

Depends on final design Outlet Heat Transfer L M information and testing Plenum Cold Leg:

Condensation Line Piping Depends on final design between Flow Resistance L M information and testing DHRS HX and HCSG Plena 44

Component Phenomenon/Process Ranking KL KL Rationale Test Data Cold Leg:

Condensation Line Piping Depends on final design between Heat Transfer L M information and testing DHRS HX and HCSG Plena HCSG: Inlet Depends on final design Flow Resistance L M Plenum information and testing HCSG: Inlet Depends on final design Heat Transfer L M Plenum information and testing Studied in representative IETs.

May be affected by thermal stratification in the pool, which Reactor Pool Heat Transfer H M NIST-2 depends on the final design, pool level, and could depend on the number of active units 45

5.4.1 HCSG Coils Fouling (Rank: L / KL: L)

The knowledge level for fouling was ranked as low. The propensity for and degree of fouling will be highly sensitive to the chemistry and purity controls on the balance of plant water. Since the staff does not have access to information regarding the detailed design of the balance of plant, the assumptions that can be made with respect to fouling are purely conjectural. Therefore, the phenomenon was ranked with low knowledge level.

5.4.2 HCSG Coils Shell Side Heat Transfer (Rank: H / KL: M)

The heat transfer of the shell side has been studied through testing programs such as SIET-TF2a, SIET-TF2b, NIST-1, and NIST-2. The staff reviewed the SIET-TF2a tests and found shortcomings in the test design and procedures that limited the viability of using these data for qualification of the TRACE shell side heat transfer models (Ref. 17). For NIST-1 the data are only available for the helical coils when feedwater is operating as the DHRS heat removal was not modeled in the NIST-1 tests. This means there is a limited number of data sets applicable to prototypical conditions during DHRS operation.

Since the staff found shortcomings in the SIET-TF2a experimental setup, the staff was unable to use data with tube bundles to validate the TRACE predictions of shell side convective heat transfer. While this may be resolved in the SIET-TF2b tests, there is uncertainty about the viability of using those data to assess tube bundle effects.

Furthermore, it is possible during LOCA and during certain AOOs for the liquid level in the RPV to drop below the top of the riser. When the level drops below the top of the riser, the natural circulation flow loop breaks and the HCSG may uncover. Under these conditions, condensation heat transfer on the shell side may become much more important than convective heat transfer (Ref. 22 and 29). However, in all these tests mentioned, the primary circuit flow is uninterrupted. Therefore, these tests do not provide information regarding condensation heat transfer on the shell side of the HCSG.

Therefore, while the staff has found there to be a large experimental database to support the convective shell side heat transfer, for transients where condensation heat transfer dominates convective heat transfer, there are no applicable integral effects tests. Therefore, the knowledge level was ranked as medium to reflect the gap in knowledge for certain postulated conditions.

5.4.3 HCSG Coils Centrifugal Forces from Coil Geometry (Rank: H / KL: M)

There are a variety of tests available to study to the effect of the helical coil geometry on the flow and heat transfer. However, these tests do not measure the centrifugal forces directly and can only measure the impact of these forces indirectly insofar as the forces affect the heat transfer and pressure drop. It is possible to capture these effects in an empirical approach, using heat transfer coefficients and pressure loss k-factors tuned to experimental data, but these techniques are not mechanistic and can limit the applicability of the model to different coil geometries. Therefore, this phenomenon was characterized as having a medium knowledge level.

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5.4.4 HCSG Coils Ledinegg Instability (Rank: H / KL: H) and DHRS HX Tubes Ledinegg Instability (Rank: H / KL: H)

Ledinegg instability is hypothesized to be possible for the HCSG and the DHRS HX. While the phenomenon could be definitively ruled in or ruled out through an analysis of the pressure drop as a function of flow over the full pressure range, the staff does not have such analyses available. Therefore, the staff, out of an abundance of caution, has ruled Ledinegg instability as being highly important. The knowledge level was also ranked as high because the Ledinegg mechanism is well understood theoretically, and the analysis necessary to determine if it can and would occur is a straightforward one.

5.4.5 HCSG Coils Density Wave Propaga on (Rank: H / KL: M)

Density wave propagation is well studied in a variety of tests conducted at SIET (i.e., SIET-TF2a, SIET-TFb, and POLIMI). However, the staff note that the focus of the testing was on normal operating conditions (including start-up) with some tests done at lower flow rates to initiative unstable oscillations in the flow. However, it is not clear if the low flow range of these tests covers the flows expected during DHRS operation. When the DHRS is in operation the loop flow will be substantially lower than the flow during normal power operations. For the meanwhile the panel considers the knowledge level to be medium as the DHRS conditions may not be enveloped by test conditions for the referenced experimental campaigns.

5.4.6 DHRS HX Tubes Density Wave Propaga on (Rank: L / KL: M)

Density wave phenomena have been the subject of several test campaigns in the helical coils, but not in the DHRS HX tubes. In these tubes density waves may occur in the downward direction (an inverted density wave) which are not as thoroughly studied. Inverted density waves are important in NuScale primary side stability and have been studied in NIST-1 (Ref.

30). These NIST-1 tests are not applicable to the DHRS HX so the phenomenon was considered to have a medium knowledge level. The process is well understood, but there are not applicable experimental data to observe the phenomenon in a prototypical or scaled configuration.

5.4.7 DHRS HX Tubes Interfacial Shear (Rank: M / KL: M)

Interfacial shear is not directly measured in applicable integral tests performed at NIST-1 and NIST-2. Therefore, the staff considers the knowledge level to be medium. Interfacial shear in inverted flows in tubes is well understood, but with condensation occurring the interaction between these phenomena can be complicated and it would not be straightforward to ascribe portions of the integral effects test results to one model or the other. Therefore, the panel considers the knowledge level to be medium.

5.4.8 Reactor Pool Thermal Stra "ca on (Rank: H / KL: M)

Pool heat transfer is well understood; however, the heat transfer will be sensitive to the degree of thermal stratification that occurs in the reactor pool. This will be sensitive to a variety of factors occurring in the reactor pool, such as the level, the distance between the DHRS HX and the containment vessel, and perhaps even the size and number of units in the reactor pool.

These myriad factors affecting the thermal stratification leads to considerable uncertainty in the 47

temperature boundary conditions for the heat transfer on the pool-side. Therefore, the panel ranked the knowledge level for the heat transfer with a medium rank to account for considerable uncertainty in the local pool water temperature due to thermal stratification.

5.4.9 Knowledge Gaps As listed in Table 4, four of the phenomena have a high importance with a knowledge level below high. In this case, the knowledge level ranking was medium for each instance. The nature and importance of these knowledge gaps is discussed below.

HCSG Coils Centrifugal Forces from Coil Geometry (Rank: H / KL: M)

Centrifugal forces were ranked as being highly import in the coils. This is because these forces affect heat transfer (one of the FoMs) and pressure drop (which can have a strong effect on density wave stability margin). The knowledge level was ranked as medium because the centrifugal force is not directly measured in the applicable experiments. Generally, it is possible to treat the impact of the centrifugal forces by deriving empirical parameters for the heat transfer coefficients and pressure loss factors based on test data. However, such factors are going to be geometry dependent and are unlikely to be applicable over a wide range of coil geometry.

This gap is likely surmountable in the subject analysis by adopting this approach of tuning the heat transfer and pressure loss factors to relevant experimental data. However, the range of application would be limited.

HCSG Coils Shell Side Heat Transfer (Rank: H / KL: M)

Shell side heat transfer is the primary heat removal mechanism and therefore, is highly important because it directly controls one of the FoMs. The knowledge level was ranked as medium because the current body of relevant experimental IETs does not include tests where the reactor water level uncovers the HCSG on the primary side. This condition can be expected for certain design basis scenarios, such as LOCA. When the HCSG uncovers, condensation can occur on the coils - which is a different heat transfer mechanism than the usual convective heat transfer. Also, when the HCSG uncovers, this interrupts the normal, primary flow circuit, which also impacts convection on the shell side.

HCSG Coils Density Wave Propagation (Rank: H / KL: M)

Density wave oscillations are possible in the coils and may contribute to DHRS loop instability, which is why the panel ranked this phenomenon as highly important. The knowledge level was ranked as medium because the existing experimental data may not consider a flow range that encompasses normal flows in the DHRS loop. Experiments have largely been designed to consider normal power operation and startup conditions. The DHRS loop flow may be lower than the tested flow range. A comparison must be made between the flow range tested and the flow range expected during DHRS operation.

Reactor Pool Heat Transfer (Rank: H / KL: M)

Reactor pool heat transfer is the primary heat removal mechanism (affecting a FoM directly) and is therefore highly important. Uncertainty about thermal stratification in the pool lead the panel to rank the knowledge level as being medium. One approach would be to conduct sensitivity 48

analysis with boundary assumptions regarding the pool temperature or the degree of stratification in the pool.

6 Conclusions The staff has developed two PIRTs. The first is for the analysis of DWOs in the HCSG during normal operation. The FoMs considered are the decay ratio and the tube wall temperature.

The PIRT importance rankings are shown in Table 1. The staff also considered the knowledge level ranking for these phenomena, which are provided in Table 2. The following phenomena were identified as knowledge gaps: (1) centrifugal forces from coil geometry and (2) orifice pressure loss.

The second PIRT is for DHRS flow loop instability. The FoMs considered are the DHRS cold leg flow rate and the aggregate DHRS heat removal rate. The PIRT importance rankings are shown in Table 3and the knowledge level rankings are shown in Table 4. The following phenomena were identified as knowledge gaps: (1) HCSG Coils: Centrifugal Forces from Coil Geometry, (2) HCSG Coils: Shell Side Heat Transfer, (3) HCSG Coils: Density Wave Propagation, and (4) Reactor Pool: Heat Transfer.

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7 References

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