LOVIISA NPP PRESSURIZED THERMAL SHOCK (PTS) THERMAL HYDRAULIC ANALYSES USING APROS SIMULATION CODE

摘要

Loviisa Nuclear Power Plant consists of two WER-440 type pressurized water reactor units and it is owned by a Finnish energy company Fortum Power and Heat Oy. Only some years after Loviisa Unit 1 start-up, during early 1980's, some reactor pressure vessel embrittlement by fast neutron irradiation was recognized. Embrittlement of the reactor pressure vessel materials increases the risk of a fast fracture under emergency core cooling conditions. This lead to an extensive study of pressurized thermal shock (PTS) scenarios, which included experimental research, large set of probabilistic and deterministic safety analyses, code development and plant modifications to both lower overcooling transient probability and to mitigate it's consequences. Operating licenses for Loviisa 1 and 2 Units reactor pressure vessels have now been applied by Fortum to cover operation up to the years 2027 and 2030, respectively. Operating licenses were granted as applied by the Finnish Radiation and Nuclear Safety Authority (STUK) according to application. However a constant know-how of PTS scenarios will be maintained. PTS-scenario related thermal hydraulic analyses are based on the original 80's decade analyses. The most important cases were recalculated using Apros (Advanced Process Simulator) because numerous plant modifications were changed the boundary conditions needed in the analyses. Probabilistic safety analyses included totally over 160 different PTS-sequences. The most limiting sequences were selected for deterministic analyses. PTS-analyses were performed in co-operation with Platom Oy and plant owner Fortum Power and Heat Oy. The main results of PTS-analyses are presented in the paper.