Experiences with applications of PSA results for optimization/revision of technical specifications for operation of nuclear power plants

Over the past few years, NPCIL has performed comprehensive Level-1 Probabilistic Safety Assessment (PSA) for a 220 MWe Pressurised Heavy Water Reactor (PHWR) at Kakrapar Atomic Power Station (KAPS) and for the first 540 MWe PHWR at Tarapur Atomic Power Project (TAPP- 3 and 4). The major objective of these PSAs was to present an integrated picture of the safety of the plant to identify and understand key plant vulnerabilities. As a result of the availability of these PSAs, there is a desire to use them to operate the plants in the most efficient manner practicable. In recent years, the operation of Indian Nuclear Power Plants has been characterized by improved availability/capacity factors and reduced forced outages. Frequency of planned outages is also being reduced. In order to achieve this, the PSAs are now being used as an engineering tool for optimization of Technical Specifications with regard to Allowed Outage Time (AOT) and Surveillance Test Intervals (STIs). USNRC Regulatory Guides have published Probabilistic Safety Criteria (PSC) which can be used as guidelines for optimization of technical specification, taking into consideration Instantaneous (conditional) Risk, Cumulative Risk over the AOT period, and Average Risk over a long period. This paper discusses the approaches followed for such optimization. Case studies are also presented. (author)