Developing safety at Loviisa nuclear power station

The Loviisa nuclear power station (NPS) consists of two 445 MW PWR units. The main supplier was Atomenergoexport of the Soviet Union. The differences between the Finnish and Soviet safety philosophy resulted in a unique plant concept. During the 17 reactor-years of operation the high level of safety has not been affected by any incident. Process transients have led to a reactor scram only four times during the history of Loviisa NPS. The safety of Loviisa NPS has been further promoted by backfitting and improved operational practice. The Chernobyl accident led to no major immediate plant or procedural changes because of the different nature of Chernobyl and Loviisa nuclear power plants. However, the Chernobyl accident underlined the importance of some special areas and of severe accidents in general. Reactor pressure vessel rupture, a total loss of redundant safety features or core melt have not been included in the original design basis. However, severe accidents have been widely studied and mitigating measures have been provided. This development work was intensified since Chernobyl. Various containment pressure relief concepts are being investigated. In order to ensure decay heat removal, even after the total loss of the turbine hall, an extra independent emergency feedwater system with diesel generators of its own will be constructed. In order to improve the reliability of the back-up electricity supply a special transmission line from a nearby hydro power plant directly to the diesel-backed buses has been constructed. A probabilistic safety assessment study is being conducted. The integrity of the reactor pressure vessel, taking into account the embrittlement due to neutron radiation, rapid accidental cooling or pressurization has been the subject of extensive studies. It is expected that the lifetime of Loviisa NPS will be determined by the ageing of the reactor pressure vessels; until now no reason seems to prevent the operation of the plant up to the design age of 30 years. (author). 1 tab