Boron dilution analyses at reactor shutdown conditions using the coupled code DYN3D/ATHLET

A parameter study of a hypothetical boron dilution event during the work of the RHRS was performed. Based on a validated coolant mixing model for the lower plenum of the RPV, realistic boron concentration curves at the inlet into each fuel assembly were obtained and used for a number of stationary and transient core calculations. For the generic core loading pattern used in the calculations, in the stationary calcu-lations recriticality is reached for slug volumes of about 8 m3 if the realistic core inlet boron concentration distribution is used. Transient core calculations from 8 to the maximum initial slug volume of 15 m3 showed, that the maximum calculated fuel and cladding temperature values are below critical values. Although in all calculations local coolant boiling occurs over several tenths of seconds, the integrity of the fuel is always ensured according to the calculation results.