Incident Evaluation and Ranking of Operational Events · 2010. 9. 13. · • PSA computer code (e.g. RiskSpectrum, IRRAS, CAFTA, SPSA, etc.), allowing fast requantification. •

IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making

Incident Evaluation and Ranking of Incident Evaluation and Ranking of Operational EventsOperational Events

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LecturerLesson IV 3_II.13

LecturerLesson IV 3_II.13

IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 1

ContentsContents

1. Introduction.

2. International programs on probabilistic incident analysis.

3. NRC’s precursor analysis procedure.

4. CSN’s precursor analysis procedure.


– One of the first applications of PSA.

– Survey of possible consequences of an incident -- Other than those that occurred.

– Survey of alternate evolution of the incident.

– Most probable core damage paths.

– Complement of root cause analysis.

IntroductionIntroduction

Probabilistic Incident Analysis


– Obtain a quantitative measure of the risk associated with an event.

• Incident ranking

• Time evolution of the incidents

– Survey of other possible scenarios with severe risk

– Detection of plant vulnerabilities

– Obtain risk-based indicators

– Feedback to PSA models and tools


Objectives


– Computerised PSA model.

• PSA computer code (e.g. RiskSpectrum, IRRAS, CAFTA, SPSA, etc.), allowing fast requantification.

• PSA model: Reliability model of plant systems and initiating event evolutions.

– Incident screening process


Tools


– Systematic conduct of the analyses.

– Traceability of the analysis.

– Quality assurance.


Procedure

EdF 6 stages:1. Investigation of actual incident consequences.2. Investigation of potential incident analysis.3. Modelling within the PSA model.4. Quantification.5. Importance measures calculation.6. Interpretation of results.


US: NRC ASP program, assesses the importance of events since 1983; use of high level models with plant-specific data; NUREG/CR-4674.

Germany: GRS Two generic studies initially (1988); systematic analysis of incidents occurred since 1993 started in 1998.

France: IPSN/IRSN On-going program on the selection and investigation of incidents occurred since 1992; probabilistic quantification since 1996.

Belgium: AVN On-going program of probabilistic incident analysis since 1997.

Hungary: VEIKI Research institution performs analysis for the regulatory body since 1999.

International ProgramsInternational Programs

Regulatory Bodies


Czech Rep.: Rez Precursor analysis program starting in 1993.

Spain: CSN Activity since 1994; systematic analysis on behalf of the Incident Revision panel since 2000.

International ProgramsInternational Programs

Utilities

France: EdF Uses generic PSA models to classify incidents and identify weak points, since 1993.

Sweden: SwedPower Systematic activity for some Swedish plants (Vattenfall).

Hundary: PAKS NPP Probabilistic incident analysis within the Operating Experience group, since (1999).


– Started 1979 at Oak Ridge.

– First report published in 1982.

– Generic models, revised in 1988 and again in 1994.

– On going activity that assesses plants incidents.

– Trend analysis.

NRCNRC’’ss Precursor Analysis ProcedurePrecursor Analysis Procedure

Prior to 1999


– Standardised Plant Analysis Risk (SPAR) Models:

• Detailed models that have undergone a QA program in agreement with the utilities.

• Used for other risk-informed regulatory activities.

• 36 detailed models out of 72 already completed; 6 compared to plant-specific IPE results.

– Extensions:

• Shutdown and low power: 1 BWR - Grand Gulf - and 1 PWR - Surry.

• Level 2: about 10 models foreseen; scope to be defined.

• External events (seismic/fire) methodology to be defined.

NRCNRC’’ss PrecursosPrecursos Analysis ProcedureAnalysis Procedure

Since 1999


– Unexpected core damage initiators (LOOP, SGTR, and small-break LOCA).

– All events in which a reactor trip was demanded and a safety-related component failed.

– All support system failures, including failures in cooling water systems, instrument air, instrumentation and control, and electric power systems.

– Any event in which two or more failures occurred.

– Any event or operating condition that was not predicted or that proceeded differently from the plant design basis; and

– Any event that, based on the reviewers experience, could have resulted in or significantly affected a chain of events leading to potential severe core damage.


NRC Screening Criteria


– Incident selection by the Incident Revision Panel (IRP); Tools:

• Expert-based assessment of the safety importance of the incident.

• Preliminary quantification (importance measures).

• Computer help tool (under development).

– Information interchange required by the analysis may involve several individuals.

– Detailed analysis of the incident, including quantification of alternative scenarios.

– Discussion of the results within the IRP meeting:

• Classification as precursor incident (10-6).

• Classification as significant event (10-4).

• Rank within the indicator program.

NRCNRC’’ss PrecursosPrecursos Analysis ProcedureAnalysis ProcedureCSN Organisation


CSNCSN’’ss Precursor Analysis ProcedurePrecursor Analysis ProcedureIndicators

NRC Significant precursor events: >10-4.

IPSN/IRSN Requires separate and detailed assessment of precursor incidents with measure >10-4.

CSN Based on the number of incidents by probability interval.


– Further regulatory activities: in-depth assessment, specific inspections or requests to the utility.

– Support to the Operating Experience Group in:

• Evaluation of the Operating Experience.

• Trend Analysis, comparison among plants.

• Comparison with similar analyses done by the utilities.

– Training of plant personnel (examinations for operators’ licenses).

– Feedback to PSA - Living PSA.

– Complement other existing PSA applications in procedures/methodology/models/tools.

NRCNRC’’ss PrecursosPrecursos Analysis ProcedureAnalysis Procedure

Use of Precursor Analyses Within CSN


CSNCSN’’ss Precursor Analysis ProcedurePrecursor Analysis Procedure

Incident Classification

Initiators Transient that leads to a reactor trip.

Conditions Unavailability of systems or components of the plant that leads to a degradation of the plant safety for a given duration..

Potential Initiators Increase in the probability of occurrence of an initiating event due to operational reasons or equipment failure.

Complex incidents may involve more than one phase.



Failure Memory Approach

– Failures that actually occurred are modelled as logical failures (to apply Boolean logic rules).

– Partial failures increment the failure probability of the component.

– Initiating events that actually occurred are quantified separately.

– Potential initiating events increment the probability of occurrence.

– Success of component actuation and/or operator actions are modelled with their nominal probability.


CSNCSN’’ss Precursor Analysis ProcedurePrecursor Analysis ProcedureModification of the PSA Model: Recovery Actions

(NRC)A TRC-type correlation is used,

For an available time t, the probability of operator error is estimated as:



Risk Measures

Conditional core damage probability (Initiating events) Set initiating event probability to 1.

Instantaneous core damage frequency (Condition events) Set the conditions of the incident by setting the failed components to logical failure and/or modifying probability of others as appropriate.


– PSA model setup (base model quantification).

– Gather information about the PSA model.

– Calculation of the baseline core damage frequency:

• Used as a reference for condition events.

• Exclude test and maintenance unavailabilities.


CSN’s Precursor Analysis: PSA Model


1. Collect information on the incident.

2. Classify the incident.

3. Establish specific objectives for the analysis.


Task 1: Pre-analysis Activities

1. Review available documentation.

2. Identify incident phases.

3. Develop timeline diagram.

Task 2: Understanding the Event



Timeline diagram for the reactor trip due to the trip of 2 out of 3 RCP.


– Identify the applicable event trees.– Check that the models are adequate.– Identify the basic events in the PSA model that must be modified in

order to map the event:• Initiating events.• Components that failed (even partial failures).• Special events.• Common-cause basic events.• Test and maintenance activities.

– Make model modifications and identify the appropriate data settings.


Task 3: Modelling of the event


1. Preliminary quantification to assess the importance of recovery actions.

2. Modelling of recovery actions.

3. Request and use of additional detailed information.

4. Final quantification.


Task 4: Quantification


– Risk measure of the incident (Conditional core damage probability or instantaneous conditional core damage frequency).

– Cutset list: Identification of dominant sequences.

– Calculation of the importance measures to identify critical equipment.


Results:


– Sensitivity analysis for different status of the plant:• Variations of plant operational state.• Unavailable equipment.• Common cause failures.• Generally poor operator performance.• Operator and system failures.• Modelling of a similar event in a different location.• Modelling with a missed test.


Task 5: What if? analysis


1. Identify the dominant contributors to the risk from the event.

2. Investigate the sensitivity of the results obtained to reasonable (achievable) changes in the values of the data used..

3. Studying the effect of analysis uncertainties.


Task 6: Analysis and interpretation of results


– The estimated risk from the event:• The best-estimate conditional core damage probability.• Instantaneous core damage frequency (condition events).

– “What if” scenarios that were more risk significant.

– Conclusions from the task of Analysis and Interpretation of results.

– Feedback to the PSA model.


Task 7: Conclusions and reporting

Documents

Incident Evaluation and Ranking of Operational Events · 2010. 9. 13. · • PSA computer code (e.g. RiskSpectrum, IRRAS, CAFTA, SPSA, etc.), allowing fast requantification. •