Boron dilution transient simulation analyses in a PWR with neutronics/thermal-hydraulics coupled codes in the NURISP project

The detailed 3D calculation of a boron slug transient with neutronics/thermal hydraulics coupled systems has been a highly demanding and difficult exercise because of the trouble coupling with boron transport models. Within subproject 3 of the FP7 European Project NURISP, two neutron kinetics codes, COBAYA3 and DYN3D, coupled with the thermal hydraulics code FLICA4 in the NURESIM platform were employed to simulate boron dilution transients.
Three transients were defined in the project, involving increasing volumes of diluted water entering the core inlet, to test the adequacy of the coupling between the codes. This paper contains the results obtained with COBAYA3/FLICA4 and DYN3D/FLICA4 couplings for the PWR boron dilution benchmark defined. Additionally, results from the coupled codes DYN3D/FLOCAL are applied for further verification in some cases.
A thorough sensitivity study of the solutions accuracy to the time step and the axial mesh size was performed, and optimal options applicable to all three boron slugs were obtained. The results verify the applicability of the implemented couplings to this type of problems accurately, where peak powers reached can be very high during short periods after which the reactor stabilizes at a few per cent of the nominal power.