An Evaluation of Cliff-Edge Effect Under SBO and Loss of EPS System Combined With Loss of UHS for the Aged CANDU-6 Reactor

Abstract

This paper deals with the thermal-hydraulic behaviors as affected by the degree of aging in the Primary Heat Transport System (PHTS) under Station Blackout (SBO) and loss of Emergency Power System (EPS) combined with loss of Ultimate Heat Sink (UHS). A cliff-edge effect occurs on a Nuclear Power Plant (NPP) under the severe accident condition. The aging in the PHTS that can affect the cliff-edge leads to lower heat transfer capacity from the primary side to its secondary side, and the overall PHTS behaviors are affected by the aged conditions. Assessments of the NPP under the severe accident condition have become essential to ensure the NPP safety after Fukushima accident. The sensitivity study on the postulated accident is conducted for the aged CANDU-6 reactor. This paper presents that the operator action time get faster to prevent proceeding to the severe accident as aging in the PHTS proceeds.