Core Exit temperature response during an SBLOCA event in the ASCO NPP

Abstract

Given the difficulties in placing measurements in the core region, core exit temperature (CET) measurements are used as a criterion for the initiation of safety operational procedures during accidental conditions in pressurized water reactors (PWR). However, the CET response has some limitation in detecting inadequate core cooling simply because the measurement is not taken in the position where the cladding exposure occurs. The Group of Thermal Hydraulic Studies of the Technical University of Catalonia has conducted analytical studies to assess the performance of RELAP5 and the nodalization approaches for CET predictions through post-test analyses of the OECD/NEA ROSA-2 Test 3 experiment. This experiment was conducted at the Large Scale Test Facility (LSTF), a facility operated by the Japanese Atomic Energy Agency (JAEA), and represented an SBLOCA in the hot leg. The studies carried out have led to deriving an updated nodalization approach for the core region and upper plenum. The knowledge acquired with post-test analyses has been transferred to a full plant model of the Ascó nuclear power plant (NPP) through Kv scaling calculations. The scalability between the LSTF and the Ascó NPP has been analyzed. The necessary changes in the nodalization to correctly reproduce the CET response, as indicated by the post-test calculations, have been added to the Ascó NPP model. The final step of the work presented here was to adapt the boundary conditions to a more realistic situation taking place in the Ascó NPP. CET and PCT readings were seen to present large differences similarly as it occurred in the ROSA-2 Test 3. When the CET reached the safety criteria, the PCT measured was in the range of [777, 906] K depending on which CET measurement was considered as a reference. Due to the high temperatures at the time the set point is triggered, the effectiveness of the accident management actions are at stake and therefore future studies should be focused on the analysis of the evolution of the scenario after the CET signal is reached and the assessment of the CET set-point value.