The second series of the EC funded project included five experiments for the investigation of the break location and CMT scaling (smaller CMT) on the PSIS behaviour. PACTEL operators made small changes to the CMT instrumentation to better detect CMT level and thermal stratification behaviour. The PSIS consisted of a Core Make-up Tank, which had connections to downcomer through an injection line and to one cold leg through a pressure balancing line. The break location and CMT scaling influences can be summarised as follows:

• Flow of cold water from downcomer to cold leg of the broken loop occurred in both hot and cold leg break experiments. The cold water did not, however, flow to the CMT through the PBL. So, no condensation problems occurred in the CMT due to flow of cold water to the tank.

• The saturated liquid layer in the CMT was very thin in the experiment where the break located in the cold leg close to downcomer. In the hot leg and cold leg inlet part break experiments, the saturated liquid layer was thick, due to flow of water to the tank during the injection phase. In all experiments, the thickness of thermally stratified region in the CMT increased during the experiments.

• The overall primary loop behaviour was similar in experiments with small and large CMT. As expected, the availability of larger amount of ECC water in the experiment with large CMT delayed the core heat-up.

The experiment series also investigated the CMT behaviour in a situation where the CMT is initially full of hot water. This may happen in the AP600 plant if the injection line check valve leaks. The programme also included an experiment without flow distributor (sparger) in the CMT. The results of these two experiments can be summarised as follows:

• Practically no recirculation flow occurred in the experiment where the CMT was initially full of hot water. The recirculation flow did not begun although there was a small initial density difference between the PSIS lines. This did not, however, cause any problems for the safety injection from the CMT.

• The fact that the CMT was full of cold water influenced the water distribution in the primary loop during the transient. This had affects on the water level formation in the vessel and the timing of the core heat-up.

• The experiment without sparger demonstrated the importance of the flow distributor on the CMT behaviour: the removal of the sparger led to rapid condensation in the CMT, which stopped safety injection from the tank. The PACTEL operators had to terminate the experiment due to condensation problems in the CMT.

• Condensation problems occurred in GDE-35 experiment when water flowing to the CMT broke saturated water layer in the CMT. This did not happen in the experiments with sparger, which distributed the incoming water horizontally into the tank.