Modelling and simulant experiments of severe accident phenomena (MOSES): Experiments on dryout heatflux in volumetrically heated granular particle bed with the STYX facility

Core debris coolability in the containment is studied to verify the severe accident management strategy adapted in Olkiluoto BWRs. The molten core material discharged from the failed reactor pressure vessel falls into a several meters deep water pool in the Olkiluoto containment. It is most likely, that the melt will fragment on its way down in the subcooled water pool and form a self-heating particle debris bed on the pedestal floor. The coolability of this particle bed is the key question to be answered. The problem of particle bed coolability has been approached first by defining the representative particle size distribution for Olkiluoto plant [1]. The mass-averaged particle size of this distribution was determined to be 3.46 mm with the particle size ranging from 0.25 mm to 11 mm. The particles formed in melt-coolant interaction tests were generally non-spherical. Second, a literature review on existing debris bed coolability experiments was carried out [3]. An extensive dryout heat flux database exists for beds of single-sized, spherical particles. Some data is available for homogeneously mixed or stratified beds with particles having a narrow size distribution from POMECO tests [4], and for homogeneous and stratified beds with wider particle size range from DCC tests [5],[6]. However, the particle shape may have been more uniform in the POMECO tests (natural sand) and DCC tests (UO2 particles) than resulted e.g. FARO experiments and MACE tests with formation of particulate. The available calculational models were sensitive to the particle size and bed porosity, yielding results from easily coolable to non-coolable situation with a relatively small parameter range. Most of the measured data and all the calculational models suggested that a bed with average particle size of 3.5 mm would have a dryout heat flux close to 1 MW/m2 and thus would be easily coolable. For investigating of the effects of Olkiluoto specific particle bed characteristics on dryout heat flux an own simulant material test facility, STYX-1 [2] was constructed at VTT as a combined effort between VTT Industrial Systems and VTT Processes. The particles in STYX bed follow the representative size distribution constructed for Olkiluoto case and the shape of particles is irregular. The bed depth is determined to be the same as expected in Olkiluoto plant if the whole corium inventory would spread uniformly on pedestal floor. VTT Industrial Systems designed and built a test rig capable of heating a pressurised test bed immersed in water to a heat flux up to 1 MW/m2 and run the tests. The resulting power and temperature distribution data is utilised by VTT Processes to model and assess the coolability concepts related to severe accident management in the Olkiluoto BWR.