LTO Modifications to Reduce the PTS Issue -- Results of TH Calculations in the Context of the APAL European Project

Abstract

Numerous long-term operation (LTO) projects have been initiated across the European Union (EU) in the recent years to support extension of the lifetime and increase the safety level of the existing fleet of nuclear power plants (NPPs). A phenomenon of pressurized thermal shock (PTS) has been identified as one of the most critical and limiting safety issues to be considered when preparing for LTO. In a project Advanced PTS Analysis for LTO (APAL) funded by the Euroatom research and training programme the impact of identified LTO improvements relevant to PTS in a pressurized water reactor (PWR) are being investigated with the help of advanced PTS assessment methods. Thermal-hydraulic (TH) system codes RELAP5, TRACE, and ATHLET as well as mixing codes KWU-MIX, GRS-MIX and the ECC-MIX component are used to simulate an accident scenario initiated by a 50 cm2 break in the hot leg (HL) with coincident loss of offsite power (LOOP). The analyzed NPP used in the study is based on a 1300 MW four-loop PWR of German design. The TH simulation results indicate that in this kind of small-break loss-of-coolant accident (SBLOCA) sequence the heating of water in high-pressure safety injection (HPSI) tanks and the reduction of HPSI flow by the operator will bring the largest benefits from PTS point of view. The results obtained in the TH analysis will be used in deterministic and probabilistic fracture-mechanics benchmarks and in the final stage of the APAL project the effect of the selected LTO improvements and human actions on the safety margin will be quantified.