Introduction

1. WGRISK CSNI Activities in the level 2 PSA field : an international point of viewJeanne Marie LANORE*Emmanuel RAIMONDIRSN, BP 17 – 92265 Fontenay-aux-Roses* Ms Lanore is Chairman of the CSNI/WG-Risk

2. Introduction • The WGRISK is one of the Principal Working groups of the OECD/AEN/CSNI. • The main WGRISK mandate is to promote and help the use and development of PSA (Probabilistic Safety Assessment) in OECD member countries, by providing a forum of information exchange, and producing State-of-the Art Reports (SOAR), organizing Workshops, writing technical notes or Technical Opinion Papers (TOP).

3. Introduction • For a long time Level 2 PSA is an important field for WGRISK and several tasks were carried out on this topic. • 1997 : publication of a first SOAR • 2004 (Koln) : international workshop (level 2 PSA and Severe Accident Management) • 2005 (Aix en Provence) : international workshop (evaluation of uncertainties in relation to severe accident and level 2 PSA). • Presently an updating of the SOAR has been written, as well as a TOP. The TOP was published in April 2007 and the SOAR is envisaged before June 2007.

4. First State-of-the-art report (SOAR) on level 2 PSA (1997) • [NEA/CSNI/R(1997)11] • The SOAR in performing L2 PSA was based on existing studies concerning 19 PWR and BWR • Content • - Task description and working methodology • - Results and Insights from recent Level 2 PSA • - Key Severe Accident Issues • - Severe Accident Management • - Available methodology for qualitative Level 2 analysis • - Evaluation of Level 2 PSA models and quantification • - Integrated and PSA informed approach to decision making

5. First State-of-the-art report (SOAR) on level 2 PSA (1997) • [NEA/CSNI/R(1997)11] • Comments • The SOAR provides also a comparison of key features adopted in the US NRC NUREG-1150 • Although at the beginning of Risk-Informed approach for decision making, this first SOAR presents already several examples of safety improvement relying on Level 2 PSA results

6. International Workshop on Level 2 PSA and Severe Accident Management - 1/2 • This workshop has been organized in Köln (Germany) in March 2004, considering that since the 1997 SOAR : • A lot of work has been carried out (especially on severe accident codes) since 1997 • Many new Level 2 PSAs especially for VVER • Many applications (SAM…) • Several Level 2 PSAs for Low Power and Shutdown situations

7. International Workshop on Level 2 PSA and Severe Accident Management - 2/2 • Main finding • Overall methodology similar to NUREG 1150, but several important differences in the detailed application (supporting studies, level 1/level 2 interface…) • Importance of plant specific studies • Advanced methods under development • Many safety improvements linked to level 2 PSA results (VVER plants…) • Interest of updating the 1997 SOAR

8. International workshop on Evaluation of Uncertainties in Relation to Severe Accidents and Level 2 PSA (2/2) • Held in Aix-en-Provence (France) in November 2005. • Key points of the discussion : • The role of uncertainties analysis is very important in L2 PSA application, especially for : • PSA quality and credibility • Decision making (uncertainties that impact decision-making must be identified) for example on plant improvement definition • Definition of future R&D direction • Emergency planning definition • The source and nature of uncertainties have to be identified • Aleatory and Epistemic uncertainties are both important and a clear separation is difficult to make • Some new knowledge (experiments …) can reduce epistemic uncertainties • Aleatory uncertainties can only be reduced by modification of design (including procedures)

9. International workshop on Evaluation of Uncertainties in Relation to Severe Accidents and Level 2 PSA(2/2) • Severe accident codes need detailed analysis • Sensitivity studies, qualification limits identification… • Expert judgment is often needed and useful • But need to be structured and transparent • Future developments and recommendations • Level 2 PSA is a now common practice and uncertainty analysis is an integral part of the methodology • However there is no generally agreed and well established prescription for how it should be performed • An international effort towards harmonization is desirable (although it should incorporate some flexibility for including new or specific developments), including the development of guidance documents

10. New State-Of-The Art Report (SOAR)(2007) - 1/2 • The 1997 SOAR was updated to cover new Level 2 PSA results, methods and applications, especially following the two workshops of 2004 and 2005. • Some main new points are the following: • Results and insights are given from recent Level 2 PSAs, especially for VVER reactors (Paks, Dukovany,…) • Key severe accident issues: the report gives examples of the extensive research carried out worldwide for a better understanding of severe accident phenomena and improvement of knowledge for implementation of mitigation features for light water reactors. • Severe accident management: over the past 10 years many nuclear plants worldwide developed and implemented SAM (PWRs, BWRs, CANDU). Moreover the EU has undertaken studies on this topic ( SAMINE, SAMOS, OPTSAM).

11. New State-Of-The Art Report (SOAR)(2007) - 2/2 • Available methodology for qualitative Level 2 analysis: • Several developments were necessary to extend the level 2 PSA scope to low power and shutdown conditions • There are still differences in the modeling of accident progression: small Containment Event Tree (CET), large Accident Progression Tree (APET), Decomposition Event Trees. However the different approaches showed similar trends. • Differences were also noted in the level 2/level 3 interface • 5. Level 2 PSA models, uncertainties and quantification: • In the recent Level 2 PSAs different approaches are used for the treatment of uncertainties: Decomposition Event Trees or Phenomenological Fault Trees, Risk Oriented Accident Analysis Methodology (ROAAM), or a more extensive use of physical calculations (French IRSN approach). • 6. Integrated and PSA-informed approach to decision making • In the recent years the Level 2 PSA results were used to identify many plant improvements, in the framework of the wide use of risk-informed approach for decision making. • 7. Integrated severe accident analysis codes • The computer codes used for severe accident analysis have undergone several improvements, validation/benchmarking (MAAP, MELCOR, ASTEC, THALES-2)

12. 2007 - Technical Opinion Paper (TOP) • The technical Opinion Paper (TOP) issued in 2007 summarizes the present position of the WG-Risk for Level 2 PSA. • The main messages are the following: • Level 2 methodology may now be seen as mature. • Level 2 PSA is an essential part of safety analysis in a risk-informed framework. • There are still differences in the detailed approaches and standards and guidelines are being developed towards a more international harmonization. • The progress is largely due to the significant R&D activities and results. • In the future a wider range of applications can be expected with further improvement, especially with a better understanding and a reduction of uncertainties.