TEPSS related PANDA tests (ESBWR)

A number of test series to investigate passive safety systems for the next generation of Light Water Reactors have been performed in the PANDA facility at the Paul Scherrer Institute (PSI). The large scale thermalhydraulic test facility allows to investigate Passive Containment Cooling Systems (PCCS) and the long-term containment behaviour after a Loss of Coolant Accident (LOCA). After successful completion of the ALPHA phase-I test series, where the PCCS performance of the Simplified Boiling Water Reactor (SBWR) was examined, phase-II was initiated in 1996 with new projects, all with international participation (EC Fourth Framework Programme on Nuclear Fission Safety). One of these projects is entitled 'Technology Enhancement for Passive Safety Systems' (TEPSS). TEPSS is focused on the European Simplified Boiling Water Reactor (ESBWR). Several new containment features and PCCS long-term response under different LOCA scenarios were investigated in PANDA. The PCCS start-up was demonstrated under challenging conditions. The effect of nitrogen hidden somewhere in the drywell and released later in the transient was simulated by injecting air for a certain period into the drywell. The effect of light gases on the PCCS performance was investigated by helium injection to the drywell. Finally, the influence of low PCC pool levels on PCCS and containment performance was examined. The main findings were that the PCCS works as intended and shows generally a favourable and robust long-term post LOCA behaviour. The system starts working even under extreme conditions and trapped air released from the drywell later in the transient does only temporarily reduce the PCCS performance. The new PANDA test series provided an extensive data base which will contribute to further improve containment design of passive plants and allow for system code assessment in a wide parameter range. (author)