Coupled Neutronics - Thermohydraulics Simulations of Fast Transients in CANDU

Abstract

The neutron kinetics code CERBERUS and the thermohydraulics code FIREBIRD-III MOD1-77 have been integrated into a computer-program package capable of performing accurate coupled simulations of fast power transients. The package executes on a desk-top computer (APOLLO) for convenience and economy. The integrated package consists of several interlinked programs, including the lattice code POWDERPUFS-V, the reactor-modelling code MATMAP, the neutron 3-d kinetics code CERBERUS, the thermohydraulics code FIREBIRD-III MOD1-77, and the detector- and electronics- modelling code POINTSIM. The program package was used to study hypothetical large-loss-of-coolant accidents in an equilibrium CANDU 6 core. The transients were assumed to be terminated by shutdown-system-1 action. In these simulations, an unprecedented level of detail was built into the thermohydraulics model. The pass downstream of the break (in which the coolant density changes are rapid) was modelled by 5 representative channel groups, to take into account zones of different power and of different channel elevation. The other three passes were modelled by one representative channel group each. Different coolant-density and fuel-temperature transients applied in different parts of the core. Results show that even under very conservative assumptions at all stages of the calculations, the power-pulse energy deposition in the fuel easily meets safety-related criteria.