PRA INSIGHTS RELATING TO THE LOSS OF ELECTRICAL SOURCES A WGRISK SURVEY

Transcription

1 PRA INSIGHTS RELATING TO THE LOSS OF ELECTRICAL SOURCES A WGRISK SURVEY Jeanne-Marie Lanore, François Corenwinder, Gabriel Georgescu, IRSN, France Attila Bareith, NUBIKI, Hungary Per Hellström, SSM, Sweden Kevin Coyne, USNRC, USA Milan Patrik, UJV REZ, Czech Republic Marina Röwekamp, GRS, Germany

2 Contents Introduction Steps of the study PSA information Safety insights and improvements PSA challenges and unresolved issues Concluding remarks 2

3 Introduction (1) The loss of electrical power sources is generally recognized as an important contributor to the risk related to nuclear power plants (NPPs) In particular, the importance of external hazards leading to a loss of electrical power sources has been further underscored by the Fukushima Dai-ichi reactor accidents The OECD Nuclear Energy Agency (NEA) Working Group on Risk Assessment (WGRISK) indicated that a review of current Probabilistic Safety Assessment (PSA) studies would be a useful method in order to identify safety insights associated with losses of electrical power sources 3

4 Introduction (2) Two types of risk and safety insights were sought: Insights for plant safety related to results and applications of risk calculations: This includes insights related to the overall risk of losses of power sources, potential safety weaknesses, and safety benefits brought by modifications already implemented or planned (including possible post-fukushima modifications) Insights into PSA methodology: This includes insights related to the identification of good practices, potential methodology gaps, and differences in the methodologies used or developed by member countries 4

5 Steps of the Study Preparation of a survey questionnaire that was distributed to the entire WGRISK members in December 2013 Questionnaire answers were received from 19 countries (with answers often including the results from several PSAs) In total, the questionnaire responses covered 38 PSA studies A writing group analyzed the answers and developed a first draft of the report After a task group meeting (March 2015) and a complementary questionnaire, the final report was reviewed and endorsed by the WGRISK members The final report was subsequently approved for publication by NEA CSNI (Committee on the Safety of Nuclear Installations) in June 2016

6 PSA Information (1) Initiating events (IE) In most cases, various categories of power supply faults were analyzed, but with differences in the categories definitions The duration of a LOOP was taken into consideration in most analyses External hazards as LOOP initiators were considered in most of the analyses The frequency of total or partial LOOP typically falls into the range of 10-3 /reactor year to 10-1 /reactor year in terms of order of magnitude (Fig. 1), although a direct comparison is not meaningful due the differences in plant design, grid layout/connections and in the definition of power failure events (including failure duration).

7 LOOP Frequency Representation 7

8 PSA Information (2) Results The initiating events considered are site specific and grouped differently However a global LOOP frequency ranges from 10-1 /reactor year to 10-3 /reactor year for all the responding countries The conditional core damage frequencies (CDF) resulting from LOOP events, when provided in the survey, have a wide variability (10-4 /reactor year 10-6 /reactor year), with no particular tendency related to the design The relative contribution of LOOP to CDF has also a large variability (Fig. 2)

9 LOOP Contribution to CDF vs. Total CDF

10 PSA Information (3) Importance measures Batteries have a relatively low Fussel-Vesely importance (FV) due to their high reliability, but a high risk achievement worth (RAW) due to their high functional importance The diesel generators have various importance, but CCFs of diesel generators have a high importance in all the studies Decay heat removal systems in case of LOOP have generally various FV importance but a relatively high RAW due to low functional redundancy.

11 Safety Insights and Improvements The task provides a large number of insights on safety improvements related to the use of PSA: Improvement of external sources Addition and/or improvement of diesel generators, of back-up equipment (fixed or mobile) Interconnections between units and/or systems Improvement of batteries Protection of primary pumps seals

12 PSA Challenges and Unresolved Issues LOOP frequency data analysis related to CCF Timing of accident sequence development in LOOP event trees Determination of the proper sequence durations for LOOP IE modelling (24 hrs, 72 hrs, etc.) Determination of the specific external grid recovery times and probabilities Credibility of reactor circulating pump (RCP) seal integrity Interaction between units in a multi-unit site following LOOP Lack of data to quantify CCFs of busbars

13 General Insights (1) This survey shows that (more generic) plant response insights are more interesting than initiating event (various LOOP categories, more site specific) PSA insights are more interesting for levels 3 and 4 of defense-in-depth (DiD, see IAEA definition) than for DiD levels 1 and 2 Challenges related to the plant response to LOOP (i.e. plant recovery from LOOP or from station blackout following LOOP) can be key contributors in PSA, so particular attention needs to be paid to them

14 General Insights (2) The following recommendations are proposed based on the survey: Need for appropriate data collection including LOOP frequences, recoveries, CCFs Justification of safe end state for sequences and corresponding appropriate mission times Proper considerations to timing of an accident development including important time breakpoints (loss of RCP seal integrity, battery depletion, etc.) Modeling of multi-unit interactions: impact of common initiating events (including hazards), support or adverse impact of other units Importance of repair and back-up equipment (fixed or mobile)

15 Concluding Remarks The insights of this survey can be used within several NEA activities (by CNRA or CSNI) as well as in other international or national activities relating to PSA or more generally to safety of NPPS The WGRISK will use these insights in its program of work: The study of multi-unit interactions is already in progress with the Task Status of Site Level PSA (including Multi-unit PSA) Developments The challenges regarding HRA in case of external hazards are addressed by the Task HRA in External Events PSA Survey of Methods and Practices Recent PSA and safety improvements (e.g., post-fukushima actions) will be reviewed in the Task Use and Development of Probabilistic Safety Assessment in Member and Non-member Countries WGRISK reports are publically available, see OECD/NEA website 15

16 Thank you for your attention! 16