Sev Severe A Acci ccidents and and Opt ptions ons f for Proc - - PowerPoint PPT Presentation

Sev Severe A Acci ccidents and and Opt ptions

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for Proc

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eeding w i g w ith h Lev Level 3 3 PR PRA Acti ctivities

July 28, 2011 Office of Nuclear Regulatory Research

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Overvie view o w of Sever ere Ac Accide dent nts

Brian Sheron, Director Office of Nuclear Regulatory Research

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Accident ident T Terminology inology

• Design-basis accidents (DBAs)
• Beyond-design-basis accidents
• Core damage accidents
• Severe accidents

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Histor

• rical P

ical Perspect pectiv ive ( e (Pre-TM TMI)

• Focus on DBAs
• Defense-in-depth safety strategy
• Reactor Safety Study (WASH-1400)
• TMI-2 Accident

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Histor

• rical P

ical Perspect pectiv ive ( e (Post-TM TMI) I)

• Severe accident research program
• Severe accident policy statement
• Individual plant examination (IPE) and

IPE of external events (IPEEE) programs

• Chernobyl accident

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Sever ere A e Accident ent P Progres ession ion

Stat ation ion Blackout ckout (SBO) Exampl ple: e:

• Initiating event: Loss of offsite pow er
• Reactor trip
• Failures lead to unavailability of
• nsite emergency ac pow er sources
• Core injection and cooling capability

become inadequate

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SBO E Examp xample ( (con cont. t.)

• Coolant boiloff leads to sustained core

uncovery and heatup

• Oxidation of fuel cladding results in

hydrogen production and release of gaseous fission products to coolant

• Clad melting and fuel liquefaction

accelerate release to coolant

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SBO E Examp xample ( (con cont. t.)

• Flow of molten fuel into reactor

pressure vessel (RPV) low er plenum

• Failure of RPV and release into

containment

• Challenges to containment integrity
• Containment failure and release to the

environment

200 400 600 800 1000 1200 1400 2 4 6 8 10 12 14 16 18 20 22 24

Pressure [psia] time [hr]

RPV Pressure Batteries exhaust

• SRV recloses

Operator manually

• pens 1 SRV

SRV sticks open Lower head failure Large scale debris relocation into lower head

SBO E Examp xample ( (con cont. t.)

GE BWR RPV Pressure in a Long-Term SBO

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TMI MI-2 Core

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Relocat

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ion

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Propo

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d Level el 3 PRA RA Activi ivitie ies

Dan Hudson, Project Manager Office of Nuclear Regulatory Research

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PRA is a structured, analytical process that provides both qualitative insights and quantitative estimates of risk.

Risk Triplet Scenarios Likelihood Consequences

1) What can go w rong? 2) How likely is it? 3) What are the consequences?

Probabilis babilistic R ic Risk A Assessment ment

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PRA E End nd State tates

• Level 1 PRA: Onset of core damage or

achievement of a safe state

• Level 2 PRA: Release of radioactive

material to the environment

• Level 3 PRA: Offsite radiological

consequences

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Overall V all Vision

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To To e enha hance e reg egulat atory y decisi sion m mak aking ng by y extrac acting n new an and i improve ved d risk sk insi nsigh ghts by: s by:

• Incorporating technical advances
• Expanding analysis scope
• Achieving analytical consistency

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Commis mission T ion Tasking ing

St Staff requ quirements s memorandu dum M100 00218 8 (March 19, 9, 2010) 0) expressed ssed co conditional sup support and d directed t the st staff to:

• Continue internal coordination efforts and

engage external stakeholders

• Provide various options for proceeding w hich

include costs and perspectives on future uses for Level 3 PRAs

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Pote

• tential Future

Future U Uses es

• Assess NRC’s current use of PRA
• Verify/revise regulatory requirements &

guidance

• Support specific risk-informed regulatory

applications

• Prioritize generic safety issues & nuclear

safety research programs

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Pote

• tential Future

Future U Uses es (con cont. t.)

• Develop and pilot test PRA technology,

standards, guidance

• Support PRA know ledge management &

risk communication

• Support future risk-informed licensing of

new /advanced reactors

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Primar mary O Options

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for Proceeding eeding

• Option 1: Status Quo – Evolutionary

Development of PRA Technology

• Option 2: Focused Research to Address

Identified Gaps Before Performing a Full- Scope Comprehensive Site Level 3 PRA

• Option 3: Full-Scope Comprehensive Site

Level 3 PRA – Operating Plant

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Othe ther O r Opti ptions

• Limited-scope Level 3 PRA
• Full-scope Level 3 PRA for new or

advanced reactor design

• Licensee-developed Level 3 PRA

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Opti ption 1: 1: Status tatus Q Quo uo

Objectives

• Continue evolutionary development of PRA

technology on a resource-available basis

Scope

• Determined by program office user need

requests, Commission tasking, and the agency’s long-term research plan (LTRP)

• Level 2/3 PRA related projects in LTRP

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Opti ption 1: 1: Status tatus Q Quo uo

Advantages

• Consistent w ith the current fiscal climate

Disadvantages

• Insights from a new and more comprehensive

site Level 3 PRA w ould not be realized

• Can result in inconsistent and more costly

treatment of potential future issues

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Opti ption 2: 2: Focus Focused R Res esea earc rch

Objectives

• Address identified gaps in PRA technology

Scope (example research areas)

• Consequential multiple initiating event modeling
• Multi-unit modeling
• Post core damage and external events HRA
• Spent fuel PRA technology
• Level 2 and Level 3 PRA uncertainty analysis

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Opti ption 2: 2: Focus Focused R Res esea earc rch

Advantages

• Focuses limited available staff and contract

support resources on mission-critical w ork

• Focuses additional resources already requested

through budget process on needed research

• Enhances PRA capability in specific areas

Disadvantages

• Delays insights that could be gained

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Opti ption 3: 3: Site Lev te Level el 3 3 PRA

Objectives

• Extract new and improved risk insights
• Enhance PRA capability, expertise, and

documentation

• Demonstrate technical feasibility and evaluate

realistic cost of developing new Level 3 PRAs

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Opti ption 3: 3: Site Lev te Level el 3 3 PRA

Scope

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Opti ption 3: 3: Site Lev te Level el 3 3 PRA

Advantages

• Provides new and improved risk insights earlier
• Enhances PRA capability, expertise, and

documentation earlier

Disadvantages

• Resource-intensive
• Requires reallocation of qualified risk analysts

from other ongoing important activities

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Staff’s R Recommendat mmendation ion

• A new and more comprehensive site

Level 3 PRA w ould be beneficial.

• Obtaining additional resources to

support this initiative w ould be challenging.

• The staff recommends the Commission

approve Option 2.

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Co Conclus usion

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Acron cronyms

BWR boiling w ater reactor CSA core support assembly DBA design-basis accident GE General Electric HRA human reliability analysis IPE individual plant examination IPEEE individual plant examination of external events LTRP long-term research plan

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Acron cronyms ( (cont cont.)

NRC U.S. Nuclear Regulatory Commission PRA probabilistic risk assessment RES NRC Office of Nuclear Regulatory Research RPV reactor pressure vessel SBO station blackout SRV safety/relief valve TMI Three Mile Island