PTS Re-Evaluation Project for Czech NPPS

Reactor pressure vessel (RPV) is a key component of nuclear power plant (NPP) that can limit the NPP lifetime. Radiation embrittlement due to neutron fluence is the most significant aging mechanism for RPV. For the RPV lifetime assessment, material degradation limit (i.e. limits that are allowed to be approached by material degradation, without endangering the RPV integrity) has to be established. This limit is established on the basis of pressurised thermal shock (PTS) analyses. PTS is an event in NPP that is characterized by rapid cooldown in the primary coolant system with (usually) high primary pressure. Two NPPs are operated in the Czech Republic by the ČEZ company, namely Dukovany NPP (4 units of WWER 440/213 type) and Temelín NPP (2 units of WWER 1000/320 type). Within preparation of both these NPPs for long term operation, PTS re-evaluation project was started. The original assessments of PTS were performed for NPP Dukovany during 1996 - 2004, for NPP Temelín during 2001 – 2004. It is obvious that from that time some changes in the NPP equipment, procedures etc. occurred and also the applied methodology for PTS assessment has been improved. Moreover, significant improvement of software tools and hardware capabilities proceeded, which enabled improvement of applied models. This was one of reasons for starting the PTS re-evaluation project. The project is performed during 2016 - 2020. PTS analyses are a multidisciplinary task. They consist of a series of thermal hydraulic and structural analyses. The chain of PTS assessments starts in the selection of scenarios to be assessed. The selection of scenarios is based on expert judgement with support from probabilistic analysis. Groups of scenarios of Loss-of-Coolant Accidents (LOCA), secondary leaks (MSLB), primary-to-secondary leaks (PRISE) and others (namely pressurizer safety valve opening and reclosure) are assessed. Within the PTS re-evaluation project, the main focus is given to the worst scenarios assessed in past, but from all above mentioned groups some representative scenarios are selected and re-evaluated.