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 ons f for Proc oceed eeding w i g w ith h Lev Level 3 3 PR PRA Acti ctivities July 28, 2011 Office of Nuclear Regulatory Research

Overvie view o w of Sever ere Ac Accide dent nts Brian Sheron, Director Office of Nuclear Regulatory Research 2

Accident ident T Terminology inology • Design-basis accidents (DBAs) • Beyond-design-basis accidents • Core damage accidents • Severe accidents 3

Histor orical 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 4

Histor orical 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 5

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 onsite emergency ac pow er sources • Core injection and cooling capability become inadequate 6

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 7

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 8

SBO E Examp xample ( (con cont. t.) GE BWR RPV Pressure in a Long-Term SBO 1400 Operator manually opens 1 SRV SRV sticks open 1200 RPV Pressure 1000 Pressure [psia] 800 Large scale debris relocation into 600 lower head 400 Batteries exhaust - SRV recloses Lower head failure 200 0 0 2 4 6 8 10 12 14 16 18 20 22 24 time [hr]

TMI MI-2 Core ore Relocat ocation ion 10

Propo oposed d Level el 3 PRA RA Activi ivitie ies Dan Hudson, Project Manager Office of Nuclear Regulatory Research 11

Probabilis babilistic R ic Risk A Assessment ment PRA is a structured, analytical process that provides both qualitative insights and quantitative estimates of risk. Scenarios 1) What can go w rong? 2) How likely is it? Risk Triplet 3) What are the consequences? Likelihood Consequences 12

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 13

Overall V all Vision on 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 14

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 15

Pote otential 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 16

Pote otential 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 17

Primar mary O Options ons f 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 18

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 19

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 20

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 21

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 22

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 23

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 24

Opti ption 3: 3: Site Lev te Level el 3 3 PRA Scope 25

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 26

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. 27

Conclus Co usion on 28

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 29

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 30