PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

The relationship between risk analysis and event analysis – PSA based Event Analysis

P. De Gelder (AVN)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Content

• Background• General considerations• PSAEA Methodology• Some issues for further consideration• Conclusions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Background

• Importance of analysis of operational events• Originally “Deterministic” approach

– In-depth analysis, root causes, aggravating factors, measures to avoid reoccurrence

• Later also Probabilistic approach– E.g. well known ASP Programme of USNRC– Now commonly referred to as PSAEA = PSA based

Event Analysis

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Content

• Background• General considerations• PSAEA Methodology• Some issues for further consideration• Conclusions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA – General considerations – Types of events

• Types of event– Real initiating event– Potential initiating event– Condition event (unavailability of component or

system)

• Direct or transposed:– Direct event– Transposed event (occurred in NPP X; analysed for NPP

Y)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA – General considerations – Screening

• Screening policy depends on objectives of the PSAEA programme

• Screening can be based on:– Scope of the PSA– Number and nature of challenged barriers– Occurrence of common mode failures– Additional unavailability of safety equipment– Duration of the event– …

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Content

• Background• General considerations• PSAEA Methodology• Some issues for further consideration• Conclusions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Understanding the event (1/2)

• Clear understanding of:– Initial conditions of the reactor– Any demand for reactor trip– Any demands on (safety) systems and operator actions– …

• Timeline diagram, where needed– Several event phases– Event covering more than one plant operational state

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Understanding the event (2/2)

• Representation on component level:– Initial conditions– Components known to be failed, degraded or under

maintenance at initiation of the event– Components recovered later by the operator– Potential for common mode failures– …

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Modelling the event

• “Mapping of the event on the PSA model”:– Identifying the event trees to be used– No inappropriate simplifications in the model?– Basic events to be modified– Making model modifications (e.g. operator recovery

actions)– Modifications to data– …

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Quantification

• Typical steps:– Preliminary quantification– Investigation on more detailed modelling (recovery actions, …)– Final quantification

• “Failure memory approach”:– All failures modelled as such– Successes are “ignored” (nominal failure probability is applied)

• Typical severity measures:– Conditional core damage probability (CCDP)– Instantaneous core damage frequency (ICDF)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – “What if” analyses

• Optional task: Allows “sensitivity” analysis– Variations in plant operational state– Unavailable equipment– Common cause failures– Operator interventions– …

• Can yield additional insights for defining corrective actions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Analysis and interpretation of results

• Identification of dominant contributors

• Sensitivity of results (hypotheses, data, …)

• Uncertainties

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

PSAEA methodology – Conclusions and reporting

• Final report on the event:– Overview of the analysis performed– Modelling steps– Highlights of conclusions

• Risk estimates for the event• Identification of dominant contributors

– OEF: suggestion of safety improvements• Avoid reoccurrence • Reduce associated risk

• Within large PSAEA programme: statistical processing or trend watching

• Provides also feedback on PSA model

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Content

• Background• General considerations• PSAEA Methodology• Some issues for further consideration• Conclusions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Discussion – Input from OECD/NEA activities

• OECD/NEA activities:– WGOE– WGRisk– Joint WGOE/WGRisk workshop (Brussels; March 2001)

• Report NEA/CSNI/R(2003)11

• WGRisk report “Use and Development of PSA in NEA Member Countries”– PSAEA is now applied in many countries

• Report NEA/CSNI/R(2002)1

• CSNI Technical Opinion Paper No. 6: PSA based Event Analysis (2004)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Benefits of PSAEA

• Some quantitative measures of risk importance• Particularly suited for evaluating events with

multiple failures and/or unavailabilities• Insights in lines of defence left for that event• Helps to determine appropriate level of attention

to be devoted to that event• Helps to evaluate adequacy of potential

corrective measures• Can be used for trending analysis

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Development and harmonisation of methodology (1/2)

• Harmonisation on level of detail– Originally simplified models– Now almost exclusively full plant specific PSAs

• Some differences still exist, for instance:– Long lasting events

• impact over whole time span or over one year

– Events applicable to many plants• Impact to be multiplied by number of plants

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Development and harmonisation of methodology (2/2)

• Be careful about comparing “numbers” for different PSAEA programmes

• Also trend analysis not always straightforward:– Changes in screening criteria– Transition from simplified to more sophisticated

models– Periodic updates of PSA

• Potential issue for development:– Extension to the use of level 2 PSA

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Complementarity of approaches

• Complementarity of deterministic and probabilistic approaches– Detailed “deterministic” analysis needed as input for

PSAEA– Complementary focuses of both approaches– Not all events easy to quantify probabilistically (e.g.

organisational or safety culture issues, …)

• Under discussion: Event reporting to be risk-informed?– e.g. INES reporting

(although keeping in mind that INES is for communication purposes)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Input from international exchange of information

• Annual Technical Meeting of PSAEA– Hosted by AVN (this year: November 9 and 10)– Wide participation of PSAEA practioners– Reported towards WGOE and WGRisk through meeting

minutes

• Summary of insights:– See AVN presentation at PSAM 8 (New Orleans, May

2006): “Risk-based Precursor Analysis in the Nuclear Industry - Experiences on the National and the International Scene” (M. Hulsmans, AVN)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Input from international exchange of information - Achievements of precursor analysis

• Structured quantitative assessment of potential consequences of the event itself and other relevant scenario’s (what-if)

• More objective – possibly quite different – perception of risk significance

• Insights in structure of the risk– Dominant factors (POS/configuration/IE/equipment/operator

actions)– Useful for definition and evaluation of measures to prevent

reoccurrence / mitigate consequences

• Performance indicators, trending of risk• Feedback to PSA (e.g. model, IE frequency minutes)

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Content

• Background• General considerations• PSAEA Methodology• Some issues for further consideration• Conclusions

PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

Conclusions

• PSAEA now widely used for event analysis• Important to keep complementary

deterministic and probabilistic approaches• Possibility for future common work

between OECD/NEA WGOE and WGRisk