Thermal Hydraulic Analysis of Pressurized Thermal Shock for Loviisa NPP Using Apros Simulation Code

Abstract

Loviisa Nuclear Power Plant consists of two VVER-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 led 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. Current operating licenses for Loviisa 1 and 2 Units reactor pressure vessels were applied by Fortum in 2012 to cover operation up to the years 2027 and 2030, respectively. The operating licenses were granted as applied by the Finnish Radiation and Nuclear Safety Authority (STUK) according to application. However a know-how of PTS scenarios shall be maintained. This becomes even more important in a light of any big power plant automation modernization projects targeting also safety systems. Currently Fortum is involved in an automation renewal project also known under its ELSA-project name. The project is scheduled to be ready in 2018. The project targets at replacing parts of old analogue automation with new digital systems and also extending their diversity by introducing new safety functions for emergency management scenarios. As a result, changes to some emergency operating procedures are inevitable. Due to the changes to emergency operating procedures PTS-scenarios for Loviisa NPP have been also updated. In 2016 Fortum launched PTS-project in connection with ELSA-project. Previous PTS-scenario related thermal hydraulic analyses were based on the original 80’s decade analyses, which were subsequently revised many times following other modernization projects that took place at Loviisa NPP. In this new round of PTS-analyses the most important cases were recalculated using Apros (Advanced Process Simulator). ELSA-project introduced changes are not only emergency procedures specific but also involve safety system related process and automation logic modifications. The changes affected therefore boundary conditions needed in the analyses. In total probabilistic safety analyses included 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.