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PM-0218-58480, Revision 0, Shutdown Capability of the NuScale Power Module.
	ML18037A673
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Site:	NuScale
Issue date:	02/08/2018
From:	Botha D, Gardner A; NuScale
To:	; Office of New Reactors
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	PM-0218-58480, Rev 0
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NuScale Nonproprietary Shutdown Capability of the NuScale Power Module Derick Botha Darrell Gardner February 8, 2018 PM-0218-58480

*; NUSCALE Revision
- O Copyright 2018 by NuScale Power, LLC. *.:. .~ POWER-Template #: 0000-21727-F01 R2

Acknowledgement & Disclaimer This material is based upon work supported by the Department of Energy under Award Number DE-NE0000633.

This report was prepared as an account of work sponsored by an agency of the United States (U.S.) Government. Neither the U.S.

Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S.

Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S.

Government or any agency thereof.

Outline

GDC 27 Exemption

NuScale Reactivity Control Systems

Exemption Review Criteria

Design Evaluation

Summary 3

PM-0218-58480 Revision : O Copyright 20 18 by NuScale Power, LLC.

GDC 27 Exemption

NuScale has identified the possibility of a return to power condition under very limited conditions and assumptions

- Passive cooldown to low RCS temperatures is unique to NuScale design

This condition was evaluated against the General Design Criterion NuScale did not identify a need for an exemption and believes current design satisfies the GDC

Draft GDCs explicitly required systems to make core "subcritical "

Final GDCs revised by the Commission to address "controlling reactivity changes" to assure acceptable radiological consequences

NuScale design approach is consistent with literal language and intent of final GDCs

- NuScale submitted a white paper on reactivity control (L0-1116-51829 , Nov 2016) and addressed compliance with GDC 26 and 27 reactivity control functions

protection function: rapid power reduction to protect fuel, assuming WRSO , to protect fuel (AOOs under GDC 26) or to maintain core cooling capability to mitigate the consequences of accidents (DBAs under GDC 27)

shutdown function : capability to hold the core subcritical under cold conditions 4

GDC 27 Exemption

NRC staff position (ML16116A083, Sep 2016) is that an exemption from GDC 27 is required

- design departs from precedent (i .e., long-term shutdown with WRSO)

NuScale complied with staff position and applied for exemption to GDC 27

- whether or not an exemption is required, NuScale believes the design solution and safety demonstration are unchanged

Exemption and FSAR establish PDC 27

- Regulations require NuScale to define the PDCs for the design , and relation of the design bases to the PDCs

- PDC 27 addresses precedent by explicitly defining requirement for long-term shutdown following postulated accident: NuScale design assures long-term shutdown with all rods in , but recriticality with WRSO would not exceed CHF :

The reactivity control systems shall be designed to have a combined capability of reliably controlling reactivity changes to assure that under postulated accident conditions and with appropriate margin for stuck rods the capability to cool the core is maintained.

Following a postulated accident, the control rods shall be capable of holding the reactor core subcritical under cold conditions, without margin for stuck rods, provided the specified acceptable fuel design limits for critical heat flux would not be exceeded by the return to power.

POWER-Template # : 0000-2 1727-F0 1 R2

NuScale Reactivity Control Systems

NuScale uses two primary reactivity control systems

- safety-related control rods and nonsafety-related chemical volume and control system (CVCS)

The selected reactivity control systems are consistent with NuScale design objectives for passive safety and simplicity

- design does not use ECCS makeup with boron - typically the only safety-related boron injection for PWRs

- Following transients, RCS passively cools down to low temperatures - not just to "hot shutdown"

- Control rods alone maintain shutdown through entire RCS temperature range

Assuming certain low probability conditions, there is a possibility of a return to power after a trip concurrent with a control rod malfunction (stuck rod) late in core life (low boron concentration) and at low decay heat (core has cooled significantly),

the small core with high control rod worth could experience a return to power if the highest worth rod is stuck out and AC power is not available to operate the active CVCS system

in all cases , reactor immediately shuts down after a trip using only control rods , even with WRSO

- the reactor remains shut down under cold conditions with reliance only on control rods

indefinitely when all control rods are inserted , or

indefinitely with WRSO during first 70 percent of equilibrium fuel cycle, or

for 30 days (typical) assuming WRSO while decay heat remains above 100 kW* (negative reactivity feedback from voiding in the core lim its return to power) 6 PM-0218-58480 --.-** NUSCALE Revision : O Copyright 20 18 by NuScale Power, LLC . ,: POWER-Template # : 0000-21727-F 01 R2

Exemption Review Criteria

NuScale believes maintaining core cooling is the design objective of reactivity control systems in GDC 27 (postulated accidents)

- the "safety concern " for a return to power event is that it could challenge heat removal system capability such that the core is insufficiently cooled resulting in core damage

- maintaining peak cladding temperature limits is considered sufficient to maintain core cool ing

- core cooling is conservatively demonstrated by maintaining CHF limits

NRC's proposed criteria for an exemption to GDC 27 is conservative

- maintain AOO acceptance criteria (CHF) and restrict frequency to less than that of an AOO (not expected in the life of a module) 7 PM-0218-58480 '; NUSCALE Revision : O Copyright 2018 by NuScale Power, LLC. ,: POWER-Template # : 0000-21727-FO1 R2

Design Evaluation Deterministic Evaluation Probabilistic Evaluation (Chapter 15)

Purpose Evaluate safety Evaluate shutdown reliability Conservative 1. WRSO with limiting shutdown margin 1. eves failure on demand vs . less likely extended assumptions 2. MTC unavailability of eves and CFDS

3. Cooldown rate 2 . Occurs throughout cycle vs . latter 30% of cycle

4. Xenon and boron concentration 3. No decay heat after restart vs . more likely decay

5. No credit for non-safety systems heat levels to prevent return to power Probability =1 under Ch 15 assumptions <1 E-6 per reactor module year

=O that all assumptions will actually occur Event 1. Return to power at 2+ hrs with DHRS 1. Return to power during DHRS cooldown is progression cooldown prevented

2. ECCS actuates resulting in subcriticality 2 . Without AC power, ECCS actuates after 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months

- in less than 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months if AC and DC power is 3. Remains shut down until decay heat reduces to <

lost, or 100 kW

- after 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months with DC power available 4. Sufficient time to restore function to eves or

3. Limiting condition for ECCS heat removal and CFDS to prevent a return to power CHF is subcriticality with maximum decay heat Criteria CHF limit not exceeded Not expected to occur during the life of a module 8

Design Evaluation

The capacity of the NuScale passive heat removal systems (DHRS, ECCS) are sufficiently sized to ensure the core remains cooled, irrespective of control rod performance

- core is protected after a return to power with a WRSO, or even after a failure to trip the reactor (ATWS)

NuScale safely controls reactivity through natural, predictable, and reliable phenomena (negative void and reactivity feedback)

- using additional systems to increase shutdown reliability will increase design complexity, reduce overall reliability and likely safety

licensed designs had to ensure subcriticality, using deterministic assumptions including a WRSO , to maintain core cooling (limit heat production within the capacity of decay heat removal systems) .

- DHRS heat removal characteristic in combination with negative moderator coefficient leads to self-limiting condition

higher power-> higher moderator temperature-> negative moderator feedback

- ECCS heat removal characteristic in combination with moderator density decrease due to voiding leads to self-limiting condition

higher power-> lower moderator density due to voiding-> negative density feedback 9

PM-0218-58480

,___,... N USCALE Revision : 0 Copyri ght 2018 by NuScale Power, LLC . ,__..,,: POWER-Temp late # : 0000-21727-F0 1 R2

Summary

NuScale is pursuing an exemption from GDC 27 consistent with NRC staff position

Reactivity control systems are consistent with design objectives for simplicity and passive safety and provide

- rapid shutdown to protect fuel

- reliable capability to maintain subcriticality under cold conditions

- passive heat removal that protects against control rod malfunctions

- alignment with the NRC's advanced reactor policy statement (73 FR 26349 ; October 14, 2008) for an advanced reactor design

A return to power with a WRSO is a benign low probability event with no radiological consequences 10 PM-02 18-58480 :* ";* NUSCALE Revision : O Copyright 2018 by NuScale Power, LLC. **.:. : *POWER-Template # : 0000-21727-F0 1 R2

Backup Slides Reactor Power (Peak Power Case, EDSS Available) 14 12 10

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420 0 400 0 2000 4000 6000 8000 10000 12000 14000 Time (sec) E

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Backup Slides Simplified eves Diagram X

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Template#: 0000-2 1727-F0 1 R2

Design Overview: Passive Decay Heat Removal System

Main steam and main feedwater isolated

Decay heat removal (OHR) valves opened

Decay heat passively removed via the steam generators and OHR heat condensers to the reactor pool

OHR system is composed of two independent and redundant trains (1 of 2 trains needed) NOTTO SCALE 13 PM-0218-58480

- ';* NUSCALE Revision: 0 Copyrig ht 20 18 by NuScale Power, LLC. *.:. ,;* POWER-Template # : 0000-21727-F01 R2

Design Overview: ECCS and Containment Heat Removal

Adequate core cooling is

+ t t +

provided without the need for safety-related injection

Reactor vent valves and feactor recirculation valves --r--;-~ --+--l-+- containment vessel open on emergency core reactor pool cooling system (ECCS) actuation signal

Decay heat removed

- condensing steam on inside surface of containment vessel reactor recirculation-+-+-- ~ reactor recirculation convection to the pool fluid valves valves on outside vessel wall NOTTO SCALE 14 PM-0218-58480 ';* NUSCALE Revisio n: 0 Copyright 2018 by NuScale Power, LLC. ,:. POWER~

Tem plate #: 0000-21727-FO 1 R2

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+44 (0) 2079 321700 http://www. nuscalepower.com WTwitter: @NuScale_Power 15 PM-0218-58480

NUSCALE Revision : 0 Copyright 2018 by NuScale Power, LLC. ,' POWERN Template # : 0000-21727-FO 1 R2

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