Safety Analysis for German Nuclear Power Plants

Transcription

1 Safety Analysis for German Nuclear Power Plants A Probabilistic View Martin Andernacht TÜV NORD GmbH, Hamburg Stockholm / September,

2 Structure of Content 1. The regulatory process in Germany 2. The role of the PSA in the regulatory process 3. The German Stress Test Safety Analysis by Reactor Safety Commission 4. Insights to the PSA from the Stress Test Stockholm / September, Andernacht Page: 2

3 Regulatory Process in Germany The regulatory process is based on a deterministic approach Probabilistic safety analyses are a supplement to the deterministic safety analyses Probabilistic safety analysis are used to verify the balance of the safety-related design and to identify potential weak points Decisions are made based on deterministic criteria. Stockholm / September, Andernacht Page: 3

4 Regulatory Process in Germany The main principle of the regulatory process is founded on the single failure concept. That concept requires that a redundant system has to work by assuming a random single failure in any part of the system, even if a redundant train of the system is out of order due to repair works. System train under repair Train 1 Train 2 Train 3 Train 4 System train with single random failure Stockholm / September, Andernacht Page: 4

5 Single Failure Concept Only a single failure in an event is considered. Common cause failures need not to be considered, if sufficient precautions are taken. These assumptions are based on probabilistic reflections, even if this is not mentioned in the regulations. System train under repair Train 1 Train 2 Train 3 Train 4 System train with single random failure Stockholm / September, Andernacht Page: 5

6 Probabilistic Safety Analysis (PSA) Probabilistic safety analyses are applied to assess the safety significance of substantial changes of the plant or its operation as well as major events. Probabilistic safety analyses are also applied to evaluate the influence of allowed outage times and regular in-service inspections. Stockholm / September, Andernacht Page: 6

7 Periodic Safety Review (PSR) In Germany, a periodic safety review (PSR) has to be performed every ten years by the utility. The PSA is a mandatory part of the PSR The German regulatory guide on PSA describes the fundamental requirements concerning the performance of PSA The technical details regarding the performance of PSA are set out in two technical documents (PSA Methods and PSA Data) that have been developed by a working group of PSA experts (FAK PSA) pictures from Bundesamt für Strahlenschutz (BfS) Stockholm / September, Andernacht Page: 7

8 Scope of PSA (according to regulatory guide) PSA Full Power Internal Hazards Internal Events External Hazards PSA Level 1 Core Damage Frequency PSA Level 2 Low Power Shutdown Internal Events PSA Level 1 Fuel Damage Frequency Source Term Risk Stockholm / September, Andernacht Page: 8

9 External Review of PSR and PSA The external review of the PSR in Germany usually lasts two years The main review tasks regarding the PSA are the check of the initiating event analysis corresponding to those initiating events used in the deterministic safety analysis and the events given in the regulations the accident sequence analysis against the plant design and the procedures written in the operating procedures (BHB) the success criteria against the license base or check new (best estimate) success criteria by recalculating them or checking their plausibility the system analysis against the system documentation of the plant pictures from VENE Stockholm / September, Andernacht Page: 9

10 External Review of PSR and PSA Of course the other parts of a PSA like probabilistic data, human reliability analysis, analysis of common cause failures and PSA quantification are included in the external review. But the most important task of PSA review is to check the significance of results. logo taken from the internet site of the organization pictures from VENE What insights can be gained from the PSA? This forms the basis for safety improvements of the plant often in discussion with a self-confident utility. Stockholm / September, Andernacht Page: 10

11 External Review of Probabilistic Safety Analysis (PSA) Since 25 years TÜV NORD has been the leading PSAreviewer in Germany with the NPP s: BWR Brunsbüttel BWR Krümmel PWR Stade PWR Unterweser PWR Grohnde PWR Brokdorf PWR Biblis pictures from Wikipedia Stockholm / September, Andernacht Page: 11

12 Lessons Learned from PSA in Germany Examples Additional Safety Systems Emergency safety valves in BWR (additionally to the safety and release valves ) Improving the systems which protect the PWR-reactor building (annulus) against flooding Optimizing of existing safety devices Change of the setpoints of pressurizer safety valves Optimizing of operating procedures, especially for the use during low power and shutdown states Optimizing in-service inspection intervals Introduction of accident management measures picture from VENE picture from Wikipedia Stockholm / September, Andernacht Page: 12

13 European Stress Test The scope and the main results of Probabilistic Safety Assessment should be provided. EU Stress test specifications: Format of the report, last sentence of the first part What is the meaning of the sentence? Is the PSA less than a supplement to the stress test? What are the experiences from the German stress test (Safety analysis by reactor safety commission)? What can PSA learn from the stress test? logos taken from the internet sites of the organizations Stockholm / September, Andernacht Page: 13

14 Insights to the PSA of the Fukushima Event The earthquake and the following tsunami was a massive external event (EVA) As we know today, these events did not have a low frequency The German reactor safety commission (RSK) performed a safety analysis by postulating enhanced external events without considering their probability The goal was to evaluate the robustness against cliff edge effects taking into account that the frequency for such events is prone to large uncertainties Approach taken by ENSREG is similar. Stockholm / September, Andernacht Page: 14

15 Insights to the PSA of the Fukushima Event Some PSA s show very low frequencies of the occurrence of several external events (EVA) Even if these frequencies of occurrence are set as core damage frequencies (CDF) the EVA-CDF are low compared with the CDF of internal events 1,E-5 1,E-6 1,E-7 1,E-8 1,E-6 1,E-6 8,E-7 6,E-7 4,E-7 1,E-9 2,E-7 1,E-10 internal events air crash explosion earthquake external flooding So the first view to these PSA s could show that external events are insignificant, but for cliff edge effects 0,E+0 internal events air crash explosion earthquake external flooding Stockholm / September, Andernacht Page: 15

16 Insights to the PSA of the Fukushima Event (Thesis) The core damage frequency can t be the only point to make a decision weather a scenario is insignificant or not. Additionally the consequences of a CDF have to be considered (it makes a difference if mitigation emergency measures are available or at least possible) To consider this the level 2 PSA is the right tool Only the level 2 PSA allows a real risk based and not a frequency based decision But it is not enough if the level 2 PSA is only focused on the large early release frequency (LERF) Stockholm / September, Andernacht Page: 16

17 Last But Not Least If you don t know much you have to suggest the worth This method led to several concepts (single failure concept, spatial separation) which ensure a high grade of robustness to the plants PSA can improve the knowledge of the plant which should be used for additional improvements of the plant itself pictures from Wikipedia Stockholm / September, Andernacht Page: 17

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