A PWR boron dilution benchmark

The presentation describes the outline, assumptions and preliminary results of a boron dilution benchmark for pressurized water reactors. In this benchmark the neutron kinetic code DYN3D in combination with three different thermal hydraulic models has been used. Besides the own thermal hydraulic model of DYN3D, models based on the ATHLET and the RELAP5 code were created. The calculations were carried out for a generic PWR core for the subcritical hot zero power state at beginning of an equilibrium cycle. All control rods except the most effective one are inserted into the core. The deboration scenario is based on the start-up of the first main coolant pump with a slug of deborated coolant in the loop. Time dependent core inlet distribution of the boron concentration used as boundary condition for the calculations is based on corresponding experiments at the coolant mixing test facility ROCOM.
Stationary core calculations for the initial state show an excellent agreement between the three calculations. The influence of the different thermal hydraulic models reveals after the return to power. The deboration front causes in all three cases a super-prompt overcriticality jump. The time and the height of the first power peak depend on the ability of the codes to suppress the numerical diffusion of the deboration front. DYN3D and DYN3D/RELAP5 shows with about 30000 MW a considerably lower value than the 53000 MW of DYN3D/ATHLET. This difference has also an influence on the maximum cladding temperature. DYN3D/ATHLET shows with nearly 300 °C the highest value, what is still far from safety relevant conditions.