Assessment and Optimization of the Canadian SCWR reactivity control systems through reactor physics and thermal-hydraulics coupling

Abstract

Canadian Nuclear Laboratories conceptualized a new reactor design to fulfill the goals set forth by the Generation IV International Forum. Unlike the traditional CANDU™, the Canadian Supercritical Water-cooled Reactor (SCWR) stands vertically and uses the batch fueling technique. Despite the CANDU-like assembly design, the traditional code system is not adequate to model this concept. Moreover, the current design lacks reactivity control mechanisms to remove the initial excess reactivity and manage the core power distribution. Fuel-integrated burnable absorber and cruciform control rods were designed for the Canadian SCWR as part of this work. A new genetic algorithm technique has been applied to optimize the control rod pattern by trying to minimize the maximum cladding surface temperature (MCST) and the maximum fuel centerline temperature (MFCLT) through the use of unique surface responses. The results of the study showed that the MCST and MFCLT did not meet the safety margins during normal operation. Therefore, the concept was enhanced by optimizing the fuel enrichment and burnable absorber concentration along the fuel assembly, shortening the cycle length, reducing the reactivity worth of the individual control rod, and finally lowering the core thermal power. The control rod pattern was re-evaluated and considerably improved the results. PARCS and RELAP5/SCDAPSIM/MOD4 were coupled through a series of external scripts to simulate 3D neutron kinetics/thermal-hydraulics control rod drop accidents. Every assembly was individually modeled in the coupling to accurately represent the evolution of these localized transients. Fuel melting occurred when the reactor SCRAM was not available. On the contrary, the safety margins were largely met when the reactor tripped on the overpower signal. However, this detection method demonstrated its limitation when one power pulse did not reach the SCRAM signal and led to fuel melting. A local neutron overpower signal could be implemented to ensure the reactor SCRAM in every control rod drop accident.