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LLC Submittal of Presentation Materials Entitled ACRS Full Committee Presentation: NuScale Topic - Boron Redistribution and Associated Design and DCA Changes, PM-0620-70244, Revision 0
	ML20150C881
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Site:	NuScale
Issue date:	05/29/2020
From:	Rad Z; NuScale
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
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	LO-0620-70348 PM-0620-70244, Rev 0
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LO-0620-70348 May 29, 2020 Docket No.52-048 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738

SUBJECT:

NuScale Power, LLC Submittal of Presentation Materials Entitled ACRS Full Committee Presentation: NuScale Topic - Boron Redistribution and Associated Design and DCA Changes, PM-0620-70244, Revision 0 The purpose of this submittal is to provide presentation materials to the NRC for use during the upcoming Advisory Committee on Reactor Safeguards (ACRS) NuScale Full Committee Meeting on June 3, 2020. The materials support NuScales presentation of boron redistribution and associated design changes in the NuScale Design Certification Application.

The enclosure to this letter is the nonproprietary presentation entitled ACRS Full Committee Presentation: NuScale Topic - Boron Redistribution and Associated Design and DCA Changes, PM-0620-70244, Revision 0.

This letter makes no regulatory commitments and no revisions to any existing regulatory commitments.

If you have any questions, please contact Matthew Presson at 541-452-7531 or at mpresson@nuscalepower.com.

Sincerely, Zackary W. Rad Director, Regulatory Affairs NuScale Power, LLC Distribution: Robert Taylor, NRC Michael Snodderly, NRC Christopher Brown, NRC Gregory Cranston, NRC Prosanta Chowdhury, NRC Michael Dudek, NRC

Enclosure:

ACRS Full Committee Presentation: NuScale Topic - Boron Redistribution and Associated Design and DCA, PM-0620-70244, Revision 0 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO-0620-70348

Enclosure:

ACRS Full Committee Presentation: NuScale Topic - Boron Redistribution and Associated Design and DCA Changes, PM-0620-70244, Revision 0 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

NuScale Nonproprietary ACRS Full Committee Presentation NuScale Topic Boron Redistribution and Associated Design and DCA Changes June 3, 2020 1 Copyright 2020 by NuScale Power, LLC PM-0620-70244 Revision:0 Template #: 0000-21727-F01 R6

Presenters Matthew Presson Licensing Project Manager Paul Infanger Licensing Specialist Ben Bristol Supervisor, System Thermal-Hydraulics Hannah Rooks Supervisor, NSSS Systems Analysis Sarah Bristol Supervisor, Probabilistic Risk Assessment 2

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Agenda

Boron Redistribution Background

Boron Dilution Analysis

Event Scenarios

Design Changes

Safety Analysis

Mechanical Analysis

Probabilistic Risk Assessment

DCA and Conforming Changes

Conclusions 3

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Boron Redistribution Background

During ACRS subcommittee meetings in February 2020, scenarios were discussed that could lead to dilution of fluid in reactor pressure vessel (RPV) downcomer

Under certain conditions, ECCS actuation or restoration of natural circulation flow could transport diluted coolant to the reactor core

Postulated reactivity event could be outside those previously evaluated

- Return to power (FSAR 15.0.6)

- Boron dilution (FSAR 15.4.6)

- Boron redistribution (RAI 8930)

NuScale initiated an evaluation of the postulated scenarios in accordance with Corrective Action Program 4

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Boron Dilution Analysis Existing methods used bounding assumptions or assumed perfect mixing

Large analysis conservatisms lead to unrealistic results

>> Large boron loss term for fluid in containment (CNV) below reactor recirculation valves (RRVs)

>> Boron lost in vapor through reactor vent valves (RVVs) due to volatility

>> No mechanism for boron recovery

No methodology approved by the NRC exists for detailed boron mixing in downcomer or core

Determined that the postulated boron redistribution scenarios could be eliminated through several minor design changes

NRC initiated an audit (ML20059N687) to review the resolution of the postulated boron redistribution scenarios and associated changes to the NuScale Design Certification Application (DCA)

>> Daily interactions to support NRC staff review 5

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Event Scenarios

Potential events that required further evaluation for boron dilution

- LOCAs

Condensation of water in CNV from RVVs

Condensation of unborated water in downcomer

ECCS actuation can transport unborated water to core

- DHRS operation can shrink RCS inventory below top of riser

Condensation of unborated water in downcomer

Unborated water can enter core when restoring natural circulation or after ECCS actuation (24-hour timer on loss of AC power)

- Anticipated Transient Without Scram (ATWS)

Condensation of unborated water in downcomer and CNV after RCS fluid lost through reactor safety valves (RSVs)

Unborated water can enter core when restoring natural circulation or opening ECCS valves 6

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Design Changes

Design Considerations

- Ensure early actuation of ECCS for postulated LOCA events

Actuates ECCS while pressure and level in RPV are higher than CNV establishes flow out of RPV when ECCS valves open

After outflow, flow stagnates and slowly reverses through RRVs so that no large influx of unborated water can occur

- Provide mechanism for boron mixing in the unlikely event that RCS level drops below riser

Eliminates potential dilution from restored natural circulation following analyzed DHRS cooldown scenarios or ATWS events

Eliminates potential dilution from operator action to restore natural circulation or manual ECCS actuation 7

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Design Changes (continued)

Addition of ECCS actuation signal on low RCS pressure (less than 800 psia +/- 100 psi)

- ECCS actuation prevented by either of following interlocks

CNV pressure less than 1 psia

RCS Thot less than 475 ºF

Interlocks prevent ECCS actuation for Non-LOCA events

- RCS is subcooled in DHRS cooldown events (lower than 475 ºF) and have no increase in CNV pressure

- Main steam line breaks and feedwater line breaks

RCS pressure remains above ECCS actuation pressure range during initial transient response

RCS is subcooled below 475 ºF Thot interlock before RCS depressurizes to ECCS actuation range 8

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Design Changes (continued)

Lowered ECCS CNV level actuation setpoint

- 252 inches (from 282 inches)

- Level setpoint range remains above RRV

- Actuates ECCS while RCS level and pressure greater than CNV level and pressure

- Ensures initial flow out of RPV into CNV

- Establishes boron mixing before ECCS flow into RPV 9

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Design Changes (continued)

Diverse flowpaths added to RPV riser

- Provide a flow path from riser through the downcomer to the core to ensure boron mixing even if level drops below top of riser

- Four 3/4 inch holes located at midpoint of SG

- Impact to normal operation is minimal

Flow through holes is small (2 kg/s) compared to minimum 100% flow (535 kg/s)

- Holes located below RCS contracted level from extended DHRS cooldown

- Holes are above equilibrium riser level after ECCS actuation (no impact on LOCA cooling)

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Safety Analysis

Return to Power (FSAR 15.0.6)

- End of cycle conditions remain limiting

- No impact to the postulated limiting return to power scenario

- Flow through riser holes maintains downcomer greater than critical boron concentration for events earlier in cycle

Boron redistribution (RAI 8930)

- Modifications maintain assumptions in redistribution analysis

LOCA (FSAR 15.6.5)

- Some postulated events would actuate ECCS on low RCS pressure (most steam space LOCAs and some liquid space LOCAs)

- No change to limiting event MCHFR or collapsed liquid level

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Safety Analysis (continued)

Inadvertent RPV valve opening (FSAR 15.6.6)

- No impact to limiting events (Loss of DC power events actuate ECCS on IAB threshold)

- RVV opening events with power available, remaining ECCS valves open on RCS pressure or low valve P

- RRV events not affected (remaining valves open on CNV level)

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Mechanical Analysis

Mechanical design and analysis impacts of the riser holes

- Negligible impact on full power RCS flow rate ( approximately 0.1 kg/s)

- Does not introduce structural integrity concerns due to flow-induced vibration

- No update needed to CVAP Technical Report

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Mechanical Analysis (Continued)

Flow Induced Vibration (FIV) Evaluation

- The riser holes have been evaluated for FIV mechanisms

turbulent buffeting (TB)

>> Holes are small (volume reduction of upper riser approximately 1/26,000) -

no impact to riser structural properties and TB evaluations not affected

vortex shedding (VS)

>> Riser holes not expected to generate vortices due to continuous flow through the holes

>> If vortices did form, would not be coincident with riser acoustic modes

Reviewed Operating Experience for fuel pin vibration due to baffle gap cross-flow

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Probabilistic Risk Assessment

PRA Evaluation

- Thermal hydraulic evaluation

- PRA models

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Beyond Design Basis Thermal Hydraulic Impacts

Design changes incorporated into best estimate NRELAP5 PRA thermal hydraulics model

- riser holes credited for preventing significant boron redistribution (per Chapter 15 analyses)

- new ECCS actuation logic modeled

Impact to postulated event progression

- cycling RSV sequences now reach the new low RCS pressure ECCS actuation

- DHRS credit to reduce rate of coolant loss of unisolated injection line pipe breaks outside containment mitigated with core flood and drain system (CFDS)

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PRA Model Impacts

Negligible effect on ECCS fault tree model

- fault tree does not credit all sensors/signals

Negligible effect on ECCS initiating events

- requires three sensors to spuriously actuate

Success criteria

- RSV cycling

- DHRS credited to mitigate unisolated injection line pipe break

Negligible effect on operator actions

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PRA Results

Internal events

- negligible change in the core damage frequency (CDF) and large release frequency (LRF)

- negligible change in the conditional containment failure probability (CCFP)

- no change in candidate risk-significant structures, systems, or components (SSCs)

- no change in candidate risk-significant initiating events or human actions

- no change in the significant core damage cutsets (FSAR Table 19.1-18)

- no change in the significant large release cutsets (FSAR Table 19.1-26)

External events

- negligible effect

Low power and shutdown

- negligible effect

- CVCS remains available during the plant cooldown to respond to RCS volume changes, as well as maintain adequate shutdown margin

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PRA Summary

Implementing the design changes to ECCS actuation, and the addition of riser bypass flow holes have a negligible effect on PRA results and insights

- some PRA sequences that involve cycling the RSV will now result in ECCS actuation

- riser holes credited to prevent significant boron redistribution

Event tree logic has been updated to reflect the new event progressions for affected sequences.

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DCA and Conforming Changes

DCA Revision 4, Errata 2, submitted May 20, 2020

Tier 1 Chapter 2

- add new ECCS signal on low RCS Pressure

FSAR Chapters

- riser holes and structural evaluations (Sections 3.9, 5.4)

- new ECCS actuation, interlocks and setpoint changes, passive ECCS valve opening, RCS wide range pressure instrumentation (Sections 3.2. 6.2, 6.3, 7.1, 7.2, 15.0, 15.6, 17.4)

- downcomer boron dilution mechanism (Sections 4.3, 15.0)

- LOCA analysis changes, sequence of events (Section 15.6.5)

- PRA event sequences for new ECCS actuation and riser holes (Section 19.1)

Technical Specifications and Bases

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DCA and Conforming Changes (continued)

Technical Reports (submitted May 20, 2020)

- Instrument Setpoint Methodology (TR-0616-49121) - added ECCS Setpoint and Actuation range design changes

- NSSS Advanced Sensor (TR-0316-22048) - added ECCS Setpoint and Actuation range design changes

- Containment Response (TR-0516-49084) - added ECCS Setpoint and Actuation range design changes

- Long-Term Cooling Methodology (TR-0916-51299) - added ECCS Setpoint and Actuation range design changes

- Technical Specification Regulatory Conformance and Development (TR-1116-52011) - incorporated associated changes in instrumentation and bases

GDC 33 Exemption - Possible revision to exemption basis being discussed with NRC staff. Not included in DCA Revision 4, Errata 2.

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DCA Conforming Changes

Topical Reports (submitted May 27, 2020)

- LOCA Methodology (TR-0516-49422) - added ECCS Setpoint and Actuation range design changes. Added the Riser Hole design change.

No change to limiting cases for MCHFR and collapsed liquid level above TAF

Non-limiting cases remain non-limiting

- Non-LOCA Methodology (TR-0516-49416) - added ECCS Setpoint and Actuation range and the Riser Hole design changes

Results for RCS pressure, core inlet temperature, RCS flow, and steam generator pressure confirmed to have negligible differences under DHRS cooling 22 PM-0620-70244 Revision:0 Copyright 2020 by NuScale Power, LLC.

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Summary and Conclusions

Design changes preclude boron redistribution for postulated design basis and beyond design basis events

- ECCS actuation on low RCS pressure or high CNV level

Assures initial flow out of RPV to preclude influx of unborated water from CNV or downcomer

- Riser holes assure boron mixing in downcomer and core

when DHRS cools and shrinks RCS level below the riser

smaller LOCAs/RCS leaks while RCS level is above holes

Analyses demonstrate no significant changes to results and that acceptance criteria continue to be met

The evaluation and resolution of this topic involved the evaluation of a broad scope of postulated scenarios, extensive calculations, analyses and related document changes to develop and implement the design changes 23 PM-0620-70244 Revision:0 Copyright 2020 by NuScale Power, LLC.

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Acronyms AC - alternating current MCHFR - minimum critical heat flux ratio ACRS - Advisory Committee on Reactor Safeguards NPM - NuScale Power Module AOO - Anticipated Operational Occurrences P - differential pressure ATWS - anticipated transient without scram Psia - pounds per square inch absolute CCFP - conditional containment failure probability RAI - Request for Additional Information CDF - core damage frequency RCS - reactor coolant system CFDS - core flood and drain system RPV - reactor pressure vessel CHF - Critical Heat Flux RRV - reactor recirculation valve CNV - containment vessel RSV - reactor safety valve COL - Combined License RTP - return to power CVAP - Comprehensive Vibration Assessment Program RVV - reactor vent valve CVCS - chemical and volume control system /s - per second DCA - Design Certification Application SAFDL - specified acceptable fuel design limits DHRS - decay heat removal system SDM - shutdown margin ECCS - emergency core cooling system SG - steam generator EOC - end of cycle TB - turbulent buffering

ºF - degree Fahrenheit scale TAF - top of active fuel FIV - flow induced vibration VS - vortex shedding FSAR - Final Safety Analysis Report GDC - General Design Criteria IAB - inadvertent actuation block kg - kilogram LCO - Limiting Condition for Operation LOCA - loss-of-coolant accident LRF - large release frequency 24 PM-0620-70244 Revision:0 Copyright 2020 by NuScale Power, LLC.

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