Qualitative and Quantitative Criteria for RIDM Stanislav Husťák Nuclear Research Institute Řež plc, Czech Republic Reliability and Risk Department INFRA Risk informed decision making on nuclear power plant safety Technical Assistance Information Exchange Instrument & State Nuclear Regulatory Committee of Ukraine 27 – 28 January 2011, Kyiv, Ukraine

2 Presentation Outline  Types of decisions and associated criteria  Overview of risk measures and criteria  Issues related to comparison with probabilistic criteria  Examples of PSA applications and related criteria  Examples of probabilistic criteria with discussion  Conclusions

3 Types of Decisions and Associated Criteria  From the point of RIDM subject  Acceptability of permanent changes –design or operational changes/modifications –the current and proposed states are compared  compliance with safety goals  criteria for risk increase  criteria to limit activities –for the proposed state –evident risk decrease is generally acceptable  Acceptability of operation –including temporary changes  criteria to limit activities/temporary risk increase  Acceptability of design or procedures –compliance with safety goals –criteria for risk/safety significance

4 Support for Decisions  Applicable for all types of decisions  Risk-informed categorization –absolute or relative risk/safety significance  basis for acceptability  selection for a change  Risk optimization –comparison of alternatives or strategies  to select better one  to find the minimum of risk –no numerical probabilistic criteria needed –well balanced PSA/reliability model is necessary  the final result should not be determined by different levels of conservatism for the compared alternatives

5 PSA Top Events  PSA undesired events –basis for risk measures used for criteria  Level 1 PSA –reactor core damage  core damage frequency (CDF) –fuel damage (in selected or all NPP locations)  fuel damage frequency (FDF)  to address impact on spent fuel pool cooling or other facilities  Level 2 PSA –unacceptable release  large early release frequency (LERF)  large release frequency (LRF), etc. –definitions may vary significantly  Level 3 PSA –individual or group risk  not used in PSA applications at this time

6 Types of Criteria  Probabilistic Safety Criteria (PSC) –instantaneous (point-in-time) vs. average (long-term) –frequencies vs. probabilities –absolute criteria  fixed numeric value  a percentage of the safety goal, etc. –relative criteria (fractions)  a percentage of the current values of risk measures –however, such criteria should not penalize plant for enhancing safety  importance measures  Deterministic criteria –design and operation rules (legislation)  single failure criterion, safety margin, etc. –number of available redundancies  especially for the cases not covered with Technical Specifications (TS) –number of available levels for defense-in-depth –level of diversity, etc.

7 General Types of PSC  Safety limits –safety goals  yearly CDF, LERF, etc.  instantaneous CDF, LERF, etc. –system reliability criteria  e.g. number of failures per demand –containment failure criteria –etc.  Differential risk measures –incremental of risk metrics –average or instantaneous (conditional) risk increase  ΔCDF, ΔLERF, etc. –incremental of conditional probability  ΔCDP (ICCDP), ΔLERF (ICLERP), etc.

8 General Types of PSC (cont.)  Importance measures –used for relative ranking according to risk & safety significance –commonly used importance measures are:  Fussell-Vesely importance (FV) –safety significance  Risk Achievement Worth (RAW) –risk significance –sometimes called Risk Increase Factor (RIF) –not defined for component failures resulting in initiating events  Risk Reduction Worth (RRW) –sometimes called Risk Decrease Factor (RDF)  Conditional probabilities –used e.g. for ranking based on the severity of failure consequence  CCDP, CLERP, etc. –preferred for initiating events

9 Some Issues Related to PSC  Applicability of adopted criteria –consistency with criteria in national regulation –consistency with plant safety goals (targets, objectives)  e.g. to reflect INSAG-12 quantitative safety targets  Comparison with PSC –scope of the PSA model –scope of the calculation –data (parameter) uncertainty  mean values are usually compared –model (epistemic) uncertainty  sensitivity analyses  expert judgment of possible impact necessary –the technique for comparison  calculation algorithm  considered effects

10 Main Principles of US NRC RG  An example of methodology for RIDM  The use of PSA for RIDM on changes to the licensing basis –licensee can use risk-informed approach for justification of licensee -initiated change  especially for justification of relaxation –five key principles 1.The proposed change meets the current regulations unless it is explicitly related to a requested exemption or rule change. 2.The proposed change is consistent with the defense-in-depth philosophy. 3.The proposed change maintains sufficient safety margins. 4.When proposed changes result in an increase in core damage frequency or risk, the increases should be small and consistent with the intent of the Commission's Safety Goal Policy Statement. 5.The impact of the proposed change should be monitored using performance measurement strategies.

11 RG Probabilistic Criteria  Impact of change on CDF

12 RG Probabilistic Criteria (cont.)  Impact of change on LERF

13 PSA Evaluation to Compare with RG Criteria  Scope of PSA model –full scope PSA required –justification of missing scope  should not affect results  bounding assessment –full scope not needed to compare with the current risk level  if risk increase is very small (Region III) and  there is not an indication of exceeding CDF 10-4/y (LERF 10-5/y) significantly –comparison with LERF criteria during shutdown operation  alternatively, criteria for LERF (10 times lower) may be applied for CDF –it is equivalent to Level 2 PSA bounding assessment with LERF = CDF  Uncertainty of PSA model –data (parameter) uncertainty  mean values to be compared –model (epistemic) uncertainty  should be addressed

14 Evaluation of Technical Specifications  Example of typical PSA application for RIDM –Allowed Outage Time (AOT) extension  limits on incremental of conditional probability (single AOT risk) –usually applied on ICCDP (ΔCDP), ICLERP (ΔLERP) –limits applied on CCDP would penalize AOT with low conditional CDF in comparison with the high one in case when ICCDP is the same  risk impact on the average risk level –ΔCDF, ΔLERF for average (yearly) risk is usually limited –includes frequency of Limiting Conditions for Operation (LCO) entry –Surveillance Test Interval (STI) extension  risk impact on the average risk level –one-time AOT extension (TS exemption)  limits on incremental of conditional probability –may be relaxed due to expected rare occurrence of exemption –AOT adequacy  limits on incremental of conditional probability (single AOT risk) –other changes  risk impact on the average risk level

15 Risk Measures Associated with AOT

16 Risk Measures Associated with AOT Extension

17 Main Principles of US NRC RG  An example of methodology for TS evaluation  Permanent licensee-initiated TS changes –licensee can use risk-informed approach for justification of TS relaxation  AOT or STI extension  Three-tiered approach 1. insights from PSA 2. avoidance of risk-significant plant configurations 3. risk-informed configuration management  risk monitoring  Acceptance criteria –ICCDP < 5 x and ICLERP < 5 x for AOT extension  ICCDP < 1 x and ICLERP < 1 x suggested in a new revision of RG (DS 1227) –consistency with the criterion acceptable for Maintenance Rule –the criteria for allowed average risk increase are taken from RG 1.174

18 Configuration Management  Another example of typical PSA application for RIDM  Risk monitoring of instantaneous risk level –Operational Safety Criteria (OSC)  limits on instantaneous risk level  exceeding instantaneous CDF /y or LERF /y is commonly considered unacceptable  colored schemes and criteria to distinguish low/moderate/high/ unacceptable levels of instantaneous risk are usually used  Criteria for incremental of conditional probability –per configuration  ICCDP < and ICLERP < from NUMARC are applicable for Maintenance Rule  not clear for overlapping maintenances –per period (week, month, outage)

19 Risk-informed Categorization  Equipment categorization based on relative significance –uses PSA importance measures –PSA applications  Graded QA - classification of Structures, Systems and Components (SSC)  some In-service Inspection (RI-ISI) methods –consequence categorization for passive pressure boundary components  In-service Testing (RI-IST)  Reliability Centered Maintenance (RCM) –common criteria for high safety significant (HSS) component:  FV > 0,005 (RRW > 1,005) or RAW > 2, see IAEA-TECDOC-1200 –NEI applies for CCF relaxed RAW by factor of 10  Equipment or event categorization based on absolute values –severity of failure consequence –PSA applications  event analysis  some RI-ISI methods –consequence categorization for passive pressure boundary components

20 Conclusions  Presentation gives quick overview of risk measures and criteria associated with RIDM –more numeric criteria with discussion can be found e.g. in:  IAEA-TECDOC-1200  NKS reports (Nordic Nuclear Safety Research)  NEA/CSNI/R(2004)20 - state of the art of risk monitors –reported OSC from many OECD countries  NEA/CSNI/R(2009)16 - reported PSC from many OECD countries  RIBA project of European Commission (overview of RI-ISI approaches) –analysis of the criteria applicability needed prior their adopting  Probabilistic criteria should not be the exact threshold values –the state-of-knowledge (epistemic) uncertainties in PSA models should be taken into an account when calculation results are compared with numerical values  The final decision should consider deterministic point of view as well

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