BWR SAMG EVALUATION

IAEA TECHNICAL MEETING ON THE VERIFICATION AND VALIDATION OF SEVERE ACCIDENT MANAGEMENT

GUIDELINES

JEFF GABOR JENSEN HUGHES

RANDY GAUNTT

SANDIA NATIONAL LABORATORIES

12-14 DECEMBER 2016

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PROJECT PARTICIPATION/SPONSORSHIP

Technical Input and Review provided by BWROG

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SAMG HISTORY IN THE US

Generic Letter 88-20 (1988) required Individual Plant Examinations (IPE) to be performed, however, there was no formal requirement for SAMG development

US Industry proposed voluntary effort to develop SAMGs

• NEI 91-04, Severe Accident Issue Closure Guidelines, December 1994

• All plants agreed to SAMG implementation by the end of 1998

• Owners Groups began developing vendor-specific SAMGs

• Technical Basis provided by EPRI

Post Fukushima activities

• Update of EPRI Technical Basis Report (2012)

• Accident reconstruction efforts at DOE and EPRI

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SAMG HISTORY IN THE US (CONT.)

US NRC Commission endorsed combining all remaining post-Fukushima activities into a single rulemaking – Mitigation of Beyond-Design-Basis Events (Jan 2014)

• The proposed rulemaking does not include an NRC requirement for SAMGs

• NEI Guidelines for development of SAMGs

− NEI 14-01 – Emergency Response Procedures and Guidelines for Extreme Events and Severe Accidents

− NEI 13-06 – Enhancements to Emergency Response Capabilities for Beyond Design Basis Accidents and Events

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SAMG HISTORY IN THE US (CONT.)

Both BWR and PWR Owners Groups have released updated SAMGs for implementation.

• BWR Emergency Procedures Guidelines (EPG) and Severe Accident Guidelines (SAG) Revision 3 – January 2013

• PWROG developed consolidated guidelines for all US PWRs

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BACKGROUND

Context for severe accident management

• Set of actions to limit effects of an accident that results in significant damage to fuel

• Focused on stopping progression of fuel damage and limiting releases to the environment

Nature of severe accident management guidelines

• Delineate strategies for response to symptoms of a severe accident

• Traditionally, rely on use of

− Existing equipment

− Existing instrumentation – with alternatives or compensatory measures as necessary

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SAMGS - BACKGROUND

Nature of SAMGs (cont.)

• Emphasis is on use of Technical Support Center to advise control room staff

• Guidelines, rather than step-by-step procedures, to provide flexibility to address a broad range of possible conditions

Origin of the SAMGs

• Originally developed as part of long-term response to TMI-2 accident

• Overall process:

Technical Basis

Report (EPRI)

Generic SAMGs

by Plant Type

(Owners Groups)

Plant-specific

SAMGs

(Utilities)

Technical foundation for severe accident management

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WHERE DO SAMGS FIT RELATIVE TO PLANT PROCEDURES?

Support

Procedures

Severe Accident

Management

Guidelines

Emergency

Operating

Procedures

Abnormal

Operating

Procedures

Alarm Response

Procedures

Normal Operating

Procedures

Flex

Support

Guidelines

EDMGs*

* Extensive Damage Mitigation Guidelines, per 10CFR50.54(hh)(2)

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BWR SAMGS FOCUS ON PRIORITIZATION

Based on plant conditions, set priorities for actions

• Water level in core region

• Core melt is ex-vessel

Actions include:

• RPV pressure control

• RPV injection recovery

• Containment pressure control

• Containment Venting

• Containment Sprays

• Severe Accident Water Addition (SAWA)

• Severe Accident Water Management (SAWM)

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BWR SAMG DEVELOPMENT AND MAAP

Developed by Emergency Procedures Committee

Consensus process

Targeted technical input

MAAP runs typically performed on a plant specific basis

Confirm that actions produce the expected results

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SAMG ANALYSIS GOALS

1. Use severe accident analysis accident signatures to probe SAMG actions

2. Address a wide-range of accident signatures and accident mitigation strategies.

3. Use existing severe accident guidance as part of the simulation

4. Assemble a expert panel from industry a. Identify the key scenarios

5. Will take into account : a. Potential failures of operator actions

b. Inaccurate diagnosis of plant conditions

c. Uncertainties in the severe accident simulation

6. Confirm that the SAMGs are robust considering uncertainties

7. Investigate the value of accurately identifying specific plant conditions

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SEVERE ACCIDENT UNCERTAINTIES

In-vessel hydrogen generation

Melt release characteristics at vessel breach

• Release rate

• Temperature

Ex-vessel debris coolability

Use both MAAP and MELCOR to bound uncertainties

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STEPS FOR EPRI/DOE PROJECT

Define accident scenario

• Base case assumptions

• Operator actions and diagnosis to represent

Perform analysis in parallel using MAAP and MELCOR

Combine results to provide an assessment of uncertainties and insights on SAMG training

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SANDIA’S EXPERTISE - MELCOR

Integrated, State of the Art, Severe Accident Analysis • SOARCA

− Peach Bottom (BWR), Surry (PWR – Large Dry Containment)

• SOARCA Uncertainty Analysis − Peach Bottom (BWR) , Surry (PWR), Sequoya (PWR – Ice

Condenser)

Dynamic PRA • Dynamic Event Trees (via the ADAPT code)

− Explores accident progression in a computationally efficient process when compared to simple Monte Carlo

− Evaluates human actions and provides context based human failure probability

» This capability may be critical to take credit for B.5.b. equipment

• SMART-SAMG efforts to automate the identification and response to low probability accident sequences

− Integral Pressurized Water Reactors – Limited Work

− Sodium Fast Reactors – Demonstrated diagnostics of accident sequences

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JENSEN HUGHES EXPERTISE - MAAP

Containment Protection and Release Reduction Rulemaking - Technical Basis

• In-depth evaluation of a variety of release mitigation strategies

− Severe Accident Water Addition – first step for any strategy

− Severe Accident Water Management – preserve scrubbing via Wetwell vent

Fukushima Technical Evaluation

• Accident reconstruction

• Uncertainty analysis

• Numerous applications and development

Uncertainty Analysis

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SCENARIO DEFINITION (EXAMPLE)

Step Description Options to be

modeled

Dependency

on previous action

Comments Notes

Initial Conditions

1. Loss of all onsite and offsite AC power

2. Turb Trip 3. MSIVs close 4. Scram

5. Loss of all injection except RCIC

6. Assume total RCP seal leakage of 36 gpm at normal operating conditions (e.g liq break of 5.7E-4 ft2)

None 1

RCIC Injection RCIC auto-start on low RPV

water level Assume suction immediately auto-switched to supp pool

1. Success

2. Failure

None 2

Defeat RCIC interlocks

Defeat RCIC trip logic for low RPV pressure and high turbine

exhaust

1. Success 2. Failure

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RPV Pressure Control 1

At 10 minutes: using 1 SRV, control pressure in 800-1000 psig range

3. Success 4. Failure

Prevent auto SRV cycling

RPV Pressure Control 2

At 1 hr: using 1 SRV, control pressure in 400-600 psig range

Only assume success if

previous pressure control 1 succeeds

Depress at approx. 80F/hr

RPV Pressure

Control 3

At 2 hr: using 1 SRV, control

pressure in 200-400 psig range

Only assume

success if previous

pressure control 1 succeeds

Hold above 200 psig to maintain

RCIC

Primary Containment

Control 1

Vent containment to maintain adequate core cooling (i.e.

RCIC)

1. Success 2. Failure

Only assume success if initial

RCIC Injection success

Assume vent pressure of 15 psig. We could consider different pressures

in future analysis, but again, focus is on SAG, not EOP.

RPV Level

Control 1

RPV water level drops below min

steam cooling water level limit –

1. Success

2. Failure

None 3

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CORE DAMAGE EVENT TREE (EXAMPLE)

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POSSIBLE RESULTS DISPLAY (EXAMPLE)

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CONCLUSIONS

Joint EPRI/DOE effort to systematically investigate BWR SAMG accident progression using MAAP and MELCOR

Based on Crosswalk findings, these codes should help to address uncertainties in severe accident phenomenology

Event Tree structure, similar to EPRI CPRR, to be used to account for success/failure of specific actions and decision points

Outcome to demonstrate robustness of SAMGs and provide insights for future consideration

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QUESTIONS?