Application of risk-informed approach to the in-service inspection programme of Unit 1 of Loviisa NPP

The pre-service and in-service inspections (ISI) of the primary circuit components and piping of Loviisa NPP (VVER440) have been carried out since the start-up of the plant following the ISI-programme which is based on the requirements presented in the ASME Code Section XI. The latest 10 years' ISI programme was completed in 2007 and the preparation of the new ISI programme for following years was started in 2005. The basis for this new ISI programme is the new YVL-guideline issued in 2003 by the Finnish Radiation and Nuclear Safety Authority STUK. The guideline requires that utilities shall utilize risk-informed methodology when planning new ISI-programmes for safety class 1, 2, 3 and 4 as well as for non nuclear safety class classified piping systems. The approach applied in drafting the ISI-programme was in principle the same as in ASME XI Supplement R, Method B but the approach was applied for the whole plant. The consequence assessment was based on the re-sults of existing extensive probabilistic safety assessment (PSA) analysis according to the values for Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP). The failure assessment was performed on a qualitative basis by a group of Fortum's experts familiar with Loviisa NPP associated materials integrity and in-service activities. The selection of elements for inspection was based on the risk categorization and identified degradation mechanisms of the piping section in concern. In addition plant specific failure types and experience with their locations were taken into account. The operational history of the plant since 1977 and the experience gained through in-service inspections and maintenance work during the years passed gave valuable additional information for the selection of items to be inspected. The new approach has brought several new features compared to the existing ISI programme. New systems, system portions and small diameter instrumentation piping of the primary systems were included in the programme.