Considerations to Improve Decay Heat Removal by Natural Circulation Under Accident Scenarios for Gentilly 2 NGS

Abstract

A piping integrity study has been carried out to assess the integrity of the Primary Heat Transport System (PHTS) piping which could be subject to vibrations that may be caused by PHTS pumps operating under two-phase conditions after a postulated loss of coolant accident (LOCA). The study has shown that under accident scenarios, the PHTS pump trip is not required in the short term to preserve the Gentilly-2 PHTS piping integrity. Nevertheless, Hydro-Quebec has decided to proceed and study the implementation of an automatic PHTS pump trip system.In the event of a loss of forced circulation of the coolant in the primary heat transport system, natural circulation of the coolant may be relied upon to transport the decay heat from the horizontal core to the boilers located above the core.With the implementation of the Gentilly-2 automatic PHT pump trip system,(reference 1), several additional accident scenarios will be added to the existing list of accident scenarios where the decay heat removal is relied upon through the effectiveness of thermosyphoning. In this paper, several considerations to improve the decay heat removal by natural circulation under accident conditions will be presented. The recommended and preferred modifications are discussed in details.The modification of the loop isolation logic, which would enable the feed system to continue providing flow to both loops, the installation of two check valves between the feed pumps and the loops and the elimination of the thirty seconds delay on the steam generators crash cooldown are shown to provide a major improvement in the thermosyphoning capabilities of the intact loop.