Coupled Neutronic-Thermalhydraulic Transient Behaviour of a Pressure Tube Type Supercritical Water-cooled Reactor

Abstract

The Generation IV International Forum has established several goals for the next generation of nuclear energy systems, which are to be substantial improvements over contemporary designs. In Canada Generation IV research efforts have focused on developing the Pressure Tube type SuperCritical Water-cooled Reactor (PT-SCWR), an evolution of CANada Deuterium Uranium (CANDU) technology. An integral part of the PT-SCWR is the High Efficiency Re-entrant Channel (HERC), wherein coolant first travels downward through a centre flow tube and then upward around the fuel. The large density variation of supercritical fluids, combined with the negative Coolant Void Reactivity (CVR), make the concept similar to a Boiling Water Reactor (BWR). The objective of this study was thus to apply the state-of-the-art in BWR analysis to the PT-SCWR. Models were created using the DRAGON (neutron transport), DONJON (neutron diffusion/spatial kinetics), and CATHENA (channel thermalhydraulics) computer codes. A procedure for DONJON-CATHENA coupling was developed to enable simulation of coupled transients. The specifications of the HERC necessitated multiple coolant reactivity feedbacks be included in the model, in turn requiring extensions to the DONJON source code. The model created for this work is thus among the first to incorporate multiple coolant feedbacks in core-level coupled spatial kinetics and thermalhydraulics transient analysis, and is uniquely capable of simulating such transients in the PT-SCWR. This work found that while the total CVR was negative as required, the reactivity effect of coolant void solely around the fuel was positive. As a consequence additional heat delivered from fuel to coolant, which decreases the coolant density, has a positive reactivity effect making BWR-like coupled instabilities impossible. On the other hand, in some postulated transients, such as Loss-OfCoolant Accidents (LOCAs) or Loss-Of-Flow Accidents (LOFAs), this positive reactivity results in temporary power excursions. A fast-acting shutdown system is potentially necessary to limit damage to the fuel in such transients.