Th SOARCA P j t dTh SOARCA P j t dThe SOARCA Project and The SOARCA Project and Analysis of Heat Transfer/TwoAnalysis of Heat Transfer/Two--yyPhase Flow during Postulated Phase Flow during Postulated

Severe AccidentsSevere AccidentsSevere AccidentsSevere Accidents

Karen VierowDept. of Nuclear EngineeringTexas A&M University, College Station, TX USA

12012 Japan-US Seminar on Two-Phase Flow DynamicsJune 11, 2012

OverviewOverviewOverviewOverview• Brief Introduction to the SOARCA Project• Selected Heat Transfer/Multiphase Flow Topics

– Issues Arising from Fukushima – Hydrogen Buildup in Containmenty g p– Drywell Mixing– Conservatisms in SOARCA

• ConclusionsConclusions

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction• Goals

Incorporate into anal sis models– Incorporate into analysis models:• state-of-the-art integrated modeling of severe accident

behavior • significant plant improvements and updates not reflected in

earlier assessments – Perform realistic offsite consequence analyses for commercial

light water reactors– Assess potential public health consequences from a hypothetical

nuclear power plant accident in which release of radioactive material to the environment occurs

Risk = (probability of occurrence) * (consequences)

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Risk (probability of occurrence) (consequences)SOARCA = State-of-the-Art Reactor Consequence Analysis

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction

• Motivation– Past consequence analyses contained excessive conservatism– Our understanding of severe accidents has increased

dramatically since health consequences were previouslydramatically since health consequences were previously evaluated.

– Plant safety has increased since the last estimates

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction

Peer Review• An independent set of experts from the industry, consulting,

laboratories and academia performed an in-depth review of the SOARCA effort.– Commenced activity in July 2009– Held five in-person meetings with the SOARCA team– Have submitted comments from each reviewer to the SOARCA teamHave submitted comments from each reviewer to the SOARCA team

throughout the peer review effort– Completed a peer review report in January 2012

• I was the chair of the Peer Review Committee

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction

MITIGATIVE

SOARCA MethodologyMITIGATIVEMEASURESANALYSES

STRUCTURALANALYSIS

INITIALSEQUENCESELECTION

MELCORANALYSIS

SOURCETERM

DETERMINECONTAINMENT

SYSTEMS STATES

SITE-SPECIFICINFORMATION MACCS2

ANALYSIS

METEOROLOGY

RESULTS

EMERGENCY PREPAREDNESS

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RESULTS

SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction

MELCOR Code• The severe accident code MELCOR was selected to model the

event progression out to the source term.– A fully integrated, engineering-level computer codeA fully integrated, engineering level computer code – Models the progression of severe accidents in light-water reactor

nuclear power plants. – Developed at Sandia National Laboratories for the U.S. Nuclear p

Regulatory Commission

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction• In the current analysis, two representative reactors were

l t devaluated– Surry Power Station (Westinghouse PWR)– Peach Bottom Atomic Power Station (GE BWR)( )

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SOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project IntroductionSOARCA Project Introduction• Peach Bottom scenarios

– Long term Station Blackout– Short term Station Blackout

• Surry scenariosSurry scenarios– Long term Station Blackout– Short term Station Blackout

Th ll i d d t t t b t– Thermally induced steam generator tube rupture– Interfacing systems LOCA in the Low-Head Safety Injection

System

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SOARCA Consideration of the SOARCA Consideration of the Fukushima DaiFukushima Dai--ichiichi AccidentAccidentFukushima DaiFukushima Dai ichiichi AccidentAccident

• The SOARCA team considered the knowledge from the Fukushima Dai-ichi accident on the SOARCA study for the following topics:– operation of the reactor core isolation cooling (RCIC) systemoperation of the reactor core isolation cooling (RCIC) system– hydrogen release and combustion– 48-hour truncation of releases in SOARCA

lti it i k– multiunit risk– spent fuel pool (SFP) risk

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SOARCA Consideration of the SOARCA Consideration of the Fukushima DaiFukushima Dai--ichiichi AccidentAccidentFukushima DaiFukushima Dai ichiichi AccidentAccident

• Operation of the RCIC system

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SOARCA Consideration of the SOARCA Consideration of the Fukushima DaiFukushima Dai--ichiichi AccidentAccidentFukushima DaiFukushima Dai ichiichi AccidentAccident

• Hydrogen release and combustion– “The precise pathway by which hydrogen was released from

the containment to the reactor building at Fukushima is uncertain.”

• “The Japanese Report to the IAEA suggests the pathway was leakage through the drywell head flange”.

• SOARCA modeled leakage through the drywell flange.g g y g– Only one scenario in SOARCA had release through the

flange.– All other containment failure predictions and the drywell liner

melting through first.

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SOARCA Consideration of the SOARCA Consideration of the Fukushima DaiFukushima Dai--ichiichi AccidentAccidentFukushima DaiFukushima Dai ichiichi AccidentAccident

• Multi-unit risk• The NRC recognized the need to evaluate multi-unit risk

– Beyond the resources available to the SOARCA team.

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Selected Heat Selected Heat Transfer/Multiphase Flow TopicsTransfer/Multiphase Flow TopicsTransfer/Multiphase Flow Topics Transfer/Multiphase Flow Topics Hydrogen Buildup in Containment• Scenario: mitigated short-term station blackout for

Surry• Uncertainty: ignition locationUncertainty: ignition location• Potential ignition sources

– Debris when the vessel failsHot gasses exiting the failed hot leg– Hot gasses exiting the failed hot leg

• Locations examined– Containment cavity– Containment dome

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Hydrogen Buildup in Hydrogen Buildup in ContainmentContainmentContainmentContainment

2,859 GRADUATE

MELCOR Containment

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Containment Nodalization

Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentContainment Cavity• Following hot leg failure, H2-rich gas was released to the

cavity.– Gas temperature exceeded the auto-ignition temperaturep g p– Ignition was predicted in the base calculation.

• Gas flow stopped shortly afterwards, due to accumulator injection and depressurization

• Steam from boiling accumulator water soon inerted the cavity

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Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentContainment Cavity• After vessel failure, hot debris falling into the cavity

presented another ignition source.– Debris fell into water pool on cavity floor, producing a steam p y , p g

source.– Containment was inerted again by steam

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Hydrogen Buildup in Hydrogen Buildup in ContainmentContainmentContainmentContainment

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Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentContainment Dome• Following hot leg failure, H2-rich gas jet was released

to the dome.– Gas temperature exceeded the auto-ignition temperaturep g p– Ignition is likely within the gas jet.

• Gas flow stopped shortly afterwards, due to accumulator injection

• Steam from boiling accumulator water soon inerted the cavity

• After vessel failure, hot debris could entrain hot debris particles and small aerosols into dome.– H2 concentration was above ignition criterion – O2 concentration was below ignition criterion – Steam concentration was high enough to inert.

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g g

Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in Containment

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Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in Containment• Scenario: mitigated short-term station blackout for

Surry• Uncertainty: combustion timingUncertainty: combustion timing

– Spray operation could render hydrogen combustion a possibility

– A delayed burn with a larger amount of H2 accumulation wasA delayed burn with a larger amount of H2 accumulation was simulated.

• Conservatisms:– No combustion allowed until spray operation was completedNo combustion allowed until spray operation was completed

at about 15 hrs.– Ignition sources were assumed present at 15 hrs.– Ignition sources were activated simultaneously in containment.

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Ignition sources were activated simultaneously in containment.

Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in Containment

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Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in Containment• Uncertainty: combustion timing

– Sensitivity study on timing• Delayed burn until 9.5 hrs and until 11.5 hrs

– Examined the effect of pre-burning H2 at low concentration• Final case terminated spray at 13 hrs instead of 15 hrs.

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Hydrogen Buildup in Hydrogen Buildup in ContainmentContainmentContainmentContainment

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Hydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in ContainmentHydrogen Buildup in Containment• Uncertainty: combustion timing

– Results:• Increased containment leakage occurred only when combustion

was delayed to the end of spray operation.C• Cesium and iodine contributions to source term were evaluated as being very small out to 24 hours.

– Conclusions:C t i t ff ti t d i t d• Containment sprays were effective at reducing source term and containment pressure leakage.

• Negative effects of spray operation were offset by scrubbing of containment sprayscontainment sprays

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Conservatisms in SOARCAConservatisms in SOARCAConservatisms in SOARCAConservatisms in SOARCA• Conservatisms in the SOARCA

– Creep rupture of the Main Steam Line is assumed to be a “fully offset, guillotine break of one MSL”.

• Maximizes hydrodynamic load on the containmentF ilit t fi i d t t t f th t l t• Facilitates fission product transport from the reactor coolant system

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Conservatisms in SOARCAConservatisms in SOARCAConservatisms in SOARCAConservatisms in SOARCA• Conservatisms in the SOARCA

– Reactor Core Isolation Cooling system (RCIC) is conservatively modeled

• e.g., during some scenarios (Loss of Vital AC Bus E-12), RCIC ld t d id i i t i i t l lwould operate and aid in maintaining core water level.

• In this SOARCA analysis, RCIC flow is assumed to terminate when station batteries deplete at 4 hours.

– In a low pressure scenario such as this, the RCIC would a o p essu e sce a o suc as t s, t e C C ou dmore likely operate until RPV pressure increased above 400 psig.

• Uncertainty Analysis– Not yet available.

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ConclusionsConclusionsConclusionsConclusions

• The SOARCA project evaluated the risk posed by p j p ynuclear power plant operation

• SOARCA used state-of-the-art methods• Heat transfer and multiphase flow phenomena presented

analysis challenges that were addressed by assumptions and/or uncertainty analysis.and/or uncertainty analysis.

• Further work on these issues could allow for relaxation of conservatisms.

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2020

SOARCA Results Dissemination

• SOARCA documents are publicly available on the NRC website.– Summary Report– Volume 1 – Peach Bottom Integrated Analysis– Volume 2 – Surry Integrated Analysis– Public Brochure– MELCOR Best Modeling Practices