Multidimensional fluid mixing capabilities of ATHLET 3.1A during an overcooling transient in a generic PWR KONVOI

In the presented work, a simulation of a 10% main steam line break (MSLB) in steam generator (SG) 1 in a generic German PWR KONVOI model is carried out and investigated by means of the system code ATHLET 3.1A. The accident analysis is focused first, on a thermal-hydraulic transient characterization, in order to subsequently study the multidimensional fluid mixing in the reactor pressure vessel (RPV), and further verification against suitable experimental data. With this aim, in the ATHLET simulation, the nominal plant operational parameters of the generic KONVOI reactor are transposed with the boundary conditions from the test PKL G3.1. The obtained results show an increase in the heat removal through the U-tubes of SG 1during boil-off, giving rise to an asymmetric overcooling in the reactor coolant system. At the arrival of the overcooled water to the RPV, the cold water stream mixes with the ambient coolant in the downcomer and eventually spreads across the whole region. In the core region, the overcooled water propagates from the periphery towards the core centre. The obtained behavior is in good agreement with the experimental results from the ROCOM and PKL test facilities.