228 Application of Probabilistic Methods for the Evaluation of Deterministic Deviations from Technical Specifications (PSAM-0277)

The German Atomic Energy Act stipulates damage precaution in line with the state of the art in science and technology for operating NPPs. Thus it could be expected, that existing nuclear power plants could not cope with updated Technical Specifications (tech specs) and regulations and would have deviations from the current state of the art concerning design and safety features. Nevertheless it is not necessarily the case that these plants are less safe and less available than newer ones. In this paper it is shown, how to assess deviations from the state of the art for an existing and operating NPP.

This assessment process is not part of the German supervising procedures and regulations. Some first steps to bring this process into legislation have been stopped due to political reasons. But there is an ongoing discussion in Germany how probabilistic methods could support deterministic ones in decision-making concerning necessity and time-frame of back fitting measures.

NPP Biblis Unit A, the first 4 loop 1300 MW PWR in Germany, has been selected to demonstrate this assessment process. The NPP was connected to the grid in 1974 and has produced more than 210 TWh of electricity. Biblis Unit A has most of the safety systems of modern plants. The incomplete physical separation of redundant trains and cross connections in cooling systems are plant specific. Due to back fitting of systems and accident management measures the CDF-result of the probabilistic safety analysis in 1990 was <3E-5 1/a. The deterministic part of the safety analysis revealed approx. 50 deviations from current regulations and tech specs. Most of them lead to back fittings. In 2000 a second safety analysis for Unit A was performed by RWE and reviewed by TÜV SÜD on behalf of the supervisory authority. The CDF-results were improved and well balanced but the number of deviations from tech specs, now based on the year 2000 had increased. This could lead to recommendations of the supervisory authority concerning additional back fitting measures.

To handle the deterministic results an integrated assessment using probabilistic methods has been performed. In this paper we will present examples for this assessment like

• back fitting of systems

• evaluation of operating experience

• evaluation of probabilistic relevance of “deterministic postulated” accidents

• comparison of old and new system engineering solutions

It is shown, that Unit A Biblis NPP reaches almost the same safety level as newer NPPs. The importance of the supervisory authority recommendations can be assessed based on PRA-results in terms of influence on CDF, necessity and ranking of modifications and optimisations.