The first pressure peak occurred too early in the APROS calculation and one additional pressure peak was observed after the pressure had already started to drop in the experiment. The code simulated the general trend of the downcomer flow behaviour well until the PSIS stagnated. During the PSIS stagnation period, between 10000 and 12000 seconds, the downcomer flow decreased and stopped completely in the experiment. This did not happen in the APROS simulation. The CMT started to empty too early in the APROS calculations. This happened since too much water flowed into the pressuriser and the steam begun to flow to the cold legs too early. The recirculation flow through the PSIS decreased as the hot water filled the CMT in the APROS calculation, but the flow did not stop completely. The calculated recirculation flow at the end of the recirculation phase was about about 75% of the initial value in the APROS calculation.

The effects of numerical diffusion were clear in the APROS calculation of CMT water temperature (Fig. 3). The calculated temperature rise was smoother than in experiment. Too much water accumulated in the pressuriser in the all calculations during the primary flow stagnation periods, indicating problems in modelling pressuriser heat losses. At the end of the transient, too much water accumulated in the pressuriser in the APROS simulation. The accumulation of water in the pressuriser partly explains the fact that the PSIS flow stagnation did not occur in the APROS calculation. The core water level dropped too fast in the APROS simulation in the early phase of the transient and the codes did not predict the core water level lowering during the PSIS flow stagnation