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LLC Submittal of Presentation Materials Entitled NRC Public Meeting Presentation: Boron Redistribution and Associated Design and DCA Changes, PM-0620-70336, Revision 0
	ML20150C517
Person / Time
Site:	NuScale
Issue date:	05/29/2020
From:	Rad Z; NuScale
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	LO-0620-70351, PM-0620-70336, Rev 0
	Download: ML20150C517 (29)
	v • d • e

LO-0620-70351 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 NRC Public Meeting Presentation: Boron Redistribution and Associated Design and DCA Changes, PM-0620-70336, Revision 0 NuScale Power, LLC (NuScale) has requested a meeting with the NRC technical staff on June 1, 2020, to discuss recent design changes and design certification application (DCA) changes associated with mitigation of postulated boron redistribution events.

The purpose of this submittal is to provide presentation materials to the NRC for use during this meeting.

The enclosure to this letter is the nonproprietary presentation entitled NRC Public Meeting Presentation: Boron Redistribution and Associated Design and DCA Changes, PM-0620-70336, Revision 0.

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

If you have any questions, please contact John Fields at 541-452-7425 or at JFields@nuscalepower.com.

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

Enclosure:

NRC Public Meeting Presentation: Boron Redistribution and Associated Design and DCA Changes, PM-0620-70336, 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-0620-70351

Enclosure:

NRC Public Meeting Presentation: Boron Redistribution and Associated Design and DCA Changes, PM-0620-70336, 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 NRC Public Meeting Presentation Boron Redistribution and Associated Design and DCA Changes June 1, 2020 1

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Presenters Matthew Presson Licensing Project Manager Paul Infanger Licensing Specialist John Fields Licensing Project Manager James Curry Licensing Specialist 2

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Agenda

Boron Redistribution Background

Boron Redistribution Analysis

Postulated Event Scenarios

Design Changes

Safety Analysis

Mechanical Analysis

Probabilistic Risk Assessment

DCA Revision Scope - Design Changes

DCA Revision Scope - Resulting Analysis Changes

Conclusions 3

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Boron Redistribution Background

In February 2020, scenarios were discussed that could lead to dilution of fluid in RPV downcomer

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

Resulting 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 the Corrective Action Program 4

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Boron Dilution Analysis NuScale methods use bounding assumptions or assumed perfect mixing

- Large analysis conservatisms lead to unrealistic, but conservative 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 for detailed boron mixing in downcomer or core NuScale determined that postulated boron redistribution scenarios could be eliminated through minor design changes NRC initiated an audit (ML20059N687) to review these postulated boron redistribution distribution scenarios and NuScale changes to the DCA 5

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Postulated Event Scenarios

Potential events that were evaluated for boron redistribution

- 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

Actuation of 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 redistribution from restored natural circulation following analyzed DHRS cooldown scenarios or ATWS events

Eliminates potential redistribution 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 WR 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

- Postulated MSLBs and FWLBs

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

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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)

- Design Changes 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 (TR-0918-60894)

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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)

>> No resonance due to flow through the holes

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

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Probabilistic Risk Assessment

Scope of PRA Evaluations performed

- Initiating Events

- System Impact

- Thermal Hydraulic Evaluation

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Thermal Hydraulic Analysis

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

Effects on 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) 16 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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PRA Model Assessment

Negligible effect on ECCS fault tree model

- fault tree does not credit all sensors/signals

Negligible effect on ECCS initiating event

- 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

core damage frequency (CDF) and large release frequency (LRF)

conditional containment failure probability (CCFP)

- no change in

candidate risk-significant structures, systems, or components (SSCs)

candidate risk-significant initiating events or human actions

significant core damage cutsets (FSAR Table 19.1-18)

significant large release cutsets (FSAR Table 19.1-26)

External events

- negligible effect

Low power and shutdown operations

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PRA Summary

Implementing the design changes had a negligible effect on PRA results and insights

- Sequences that involve a cycling 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 19 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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DCA Revision Scope - Design Changes

Tier 1 Changes

- Ch 2 - Technical changes to Tables 2.5-2 and 2.5-4 to incorporate ECCS actuation setpoints and ranges

Tier 2 (FSAR) Changes

- Ch 3 - Technical change to 3.9 to add the riser holes design change

- Ch 5 - Technical change to 5.4 to add the riser holes design change

- Ch 6 - Technical changes to 6.2 and 6.3 to add revised ECCS setpoints and actuation ranges

- Ch 7 - Technical changes throughout 7.1 to add revised ECCS setpoints and actuation ranges, including associated instrumentation and logic

- Sect 15.0 - Added ECCS setpoints and actuation range changes to Table 15.0-7

- Ch 19 - Modified ECCS setpoints and actuation ranges in accident sequences and mitigating systems in 19.1, Table 19.1-8, 19.1-24 and associated event trees, riser hole changes added in Table 19.1-2 20 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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DCA Revision Scope - Design Changes

TS and TS Bases Sections 3.3.1, 3.5.1 - added new operating conditions, required actions and completion times for new instrumentation signals and interlocks

Technical Reports

- Instrument Setpoint Methodology (TR-0616-49121) - Added ECCS setpoints and actuation ranges design change to Tables 5-1, 5-2, 6-2, 6-17, 6-23, 6-24 (new), and 7-1

- NSSS Advanced Sensor (TR-0316-22048) - Added ECCS setpoints and actuation ranges design change to Tables 4-1 and 5-1, and text in Sections 5.2 and 5.4

- Containment Response (TR-0516-49084) - Added ECCS setpoints and actuation ranges design change to text in Executive Summary, Section 3.3, 5.1, and 6.0; added notes to section 3.2, Tables 3-6, and 3-8

- Long-Term Cooling Methodology (TR-0916-51299) - Added ECCS setpoints and actuation ranges design change to text in Sections 5.3 and 5.6; added the riser holes design change to text in Section 5.6

- TS Regulatory Conformance and Development (TR-1116-52011) - Added technical changes to Tables 3-5 and A-1 to incorporate associated TS changes in instrumentation 21 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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DCA Revision Scope - Design Changes

Topical Reports

- LOCA Methodology (TR-0516-49422)

Added ECCS setpoints and actuation ranges design change to text in Section 3.3, Tables 5-3, 5-4, 5-5, A-3, and B-6

Added the riser holes design change to text in Section 5.1.

- Non-LOCA Methodology (TR-0516-49416)

Discussed the riser holes design change in Section 1.2 22 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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DCA Revision Scope - Resulting Analysis Changes

Tier 2 (FSAR) Changes

- Ch 3 - Technical changes to text in 3.9.5 to add structural integrity assessment of riser holes - FIV and acoustic noise

- Ch 4 - Technical change to text in 4.3 to add boron distribution in LOCA and Non-LOCA events

- Ch 6 - Material changes to Table 6.2-2 to modify results of limiting LOCA event on containment response

- Sect 15.0 - Added technical description of boron redistribution during DHRS and LOCA events in Sections 15.0.4, and 15.0.5, and Table 15.0-7

- Sect 15.6 - Modified sequence of events and analysis results for limiting steam space break, limiting IORV and LOCA events in sections 15.6.5, 15.6.6, and Tables 15.6-12, 15.6-14, 15.6-19, 15.6-20, 15.6-21, 15.6-22, 15.6-23, 15.6-24

- Ch 16 - Reference change only

- Ch 17 - Material changes to D-RAP Functions and Categorization in Table 17.4-1

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DCA Revision Scope - Resulting Analysis Changes

Technical Reports

- Long-Term Cooling Methodology (TR-0916-51299) - Added statement in Section 5.6 that there is no impact to this analysis from design changes

Topical Reports

- LOCA Methodology (TR-0516-49422) - Material text revisions in sections 3.3, 5.1, and B.8, and Tables 5-3, 5-4, 5-5, 9-1, 9-2, A-3, B-6, table notes indicate that there is no impact to this analysis from design changes

- Non-LOCA Methodology (TR-0516-49416) - paragraph added in Section 1.2 concludes that there is no impact to this analysis from design changes 24 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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Summary and Conclusions

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

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

- ECCS actuation on low RCS pressure or high CNV level assures initial flow out of RPV precludes influx of unborated water from CNV or downcomer

- Riser holes assure boron mixing in downcomer and core when:

DHRS cools and contracts RCS level below the top of riser

for smaller LOCAs/RCS leaks while RCS level is above riser holes

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

NuScale and the NRC have engaged through numerous technical calls, audit questions, and audit discussions to support NRC SER completion by the end of June 2020 to support ACRS meetings 25 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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Acronyms AC - alternating current kg/s - kilograms per second ACRS - Advisory Committee on Reactor Safeguards LCO - Limiting Condition for Operation AOO - Anticipated Operational Occurrences LOCA - loss-of-coolant accident ATWS - anticipated transient without scram MCHFR - minimum critical heat flux ratio CCFP - conditional containment failure probability MSLB - Main Steam Line Break CDF - core damage frequency NPM - NuScale Power Module CFDS - core flood and drain system OCRP - overcooling return to power CHF - Critical Heat Flux P - differential pressure CNV - containment vessel psi -pounds per square inch COL - Combined License psia - pounds per square inch absolute CVAP - Comprehensive Vibration Assessment Program RAI - Request for Additional Information CVCS - chemical and volume control system RCS - reactor coolant system D-RAP - Design Reliability Assurance Program RPV - reactor pressure vessel DCA - Design Certification Application RRV - reactor recirculation valve DHRS - decay heat removal system RSV - reactor safety valve ECCS - emergency core cooling system RVV - reactor vent valve EOC - end of cycle SAFDL - specified acceptable fuel design limits

ºF - degree Fahrenheit scale SDM - shutdown margin FIV - flow induced vibration SER - Safety Evaluation Report FSAR - Final Safety Analysis Report SG - steam generator FWLB - Feedwater Line Break TB - turbulent buffering GDC - General Design Criteria TAF - top of active fuel IAB - inadvertent actuation block TS - Technical Specifications IORV - Inadvertent operation of and RPV valve VS - vortex shedding kg - kilogram WR - wide range 26 PM-0620-70336 Revision: 0 Copyright 2020 by NuScale Power, LLC.

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