The PACTEL experiment GDE-11 was calculated with APROS 2.11 codes. This experiment contained a rapid condensation and long lasting mixing period in the CMT. A steep thermally stratified layer was already formed before the break opening. All simulations used previously created and tested models for PACTEL, which were modified according to the needs of the PSIS. The results of the calculations have been presented in references [6, 12, 13, 14, 15, 16].

The APROS calculation used 5-equation model. The drift flux model was used to describe the velocity difference between phases. The CMT was modelled with five equal sized nodes. Sensitivity analyses with different number of nodes was not performed. With five nodes, it was not possible to model exactly the thermal stratification in the CMT. In the base case calculation, the top node liquid temperature was initiated to 140°C, which corresponds to the average temperature of the upper part of the CMT.

• CMT level did not drain until the top node liquid temperature had reached saturation. Draining stagnated to the level of the node boundary and continued after the liquid in the node next below was in saturation. During the stagnation the injection flow decreased dramatically and started to oscillate.

• This stepwise draining mode was observed also with the initial temperature of 50°C in the CMT top node.

• The CMT behaviour dominated the primary system behaviour.