Analysis of Thermalhydraulics characteristics on constant and variable models for pressure tube radial expansion by using the ASSERT code in a CANDU6 channel

Abstract

Because of extreme conditions such as high temperature, high pressure and irradiation, gradual radial expansion of pressure tube in the CANadian Deuterium and Uranium 6 (CANDU6) reactors occurs in the radial direction. Due to the decrease in the thermal margin of the channel, the Trip Set Point (TSP) for reactor trip should be decreased as well, to ensure a sufficient magnitude of safety margin for the reactor. Then operating power should be reduced as time goes by. Making precise thermal margin predictions is therefore a crucial activity. The ASSERT code has sub-channel level resolution and 2-dimensional analysis is possible. For these reasons, a lot of activities are performed using the ASSERT code. In this study, the ASSERT code calculation results from different pressure tube radial expansion models were compared with each other. With this comparison, it was examined how details in the model affect the critical results, such as Critical Channel Power (CCP) and T/H characteristics. One model used a constant radial expansion quantity along the axial flow direction, and the other using varying radial expansion quantity along the axial flow direction. The expansion area or volume was conserved so that consistency was maintained in this study.