Effects of Water Pool Subcooling on the Debris Bed Spreading by Coolant Flow
2011 (English)In: ICAPP2011: Proceedings of the 2011 international congress on advances in nuclear power plants, American Nuclear Society, 2011Conference paper (Refereed)
Deep water pool in the reactor pit is considered an effective means of arrest and long term cooling thecorium released from the reactor pressure vessel in the case of a hypothetical severe accident with lightwater reactors. Corium melt interaction with water is expected to result in the material fragmentation andformation of a porous debris bed on the containment basemat. From the safety point of view, it isessential to quantify the conditions for debris bed coolability without corium remelting due to the internalheat release. Previous research results have clearly demonstrated that, given the total mass of the material,the conditions for the occurrence of local dryout are contingent on the particle diameters, porosity, as wellas on the debris bed shape. In particular, a tall heap-shaped debris bed would be more prone to dryoutthan that uniformly spread over the pool basemat.For the BWR designs adopted in Sweden and Finland, one of the severe accident scenarios to beconsidered is the gradual melt release, for which the molten corium is released over a rather long time(hours) rather than as a concentrated jet. This case was studied recently by using the dedicated computercode DECOSIM (DEbris COolability SIMulator) developed at KTH. The code implements the multifluidmodel for liquid-vapor flows in the heat-releasing debris bed and in the volume of pool, where effects ofturbulence are taken into account by the k-epsilon turbulence model. The model used in DECOSIMdescribes the following phenomena: i) filtration of water and vapor in the porous debris bed with heatrelease; ii) turbulent natural convection flows in the water pool; iii) sedimentation of melt particles andtheir interaction with circulatory flow in the pool due to drag and turbulent dispersion; iv) fallout ofparticles, their packing and growth of debris bed. To address the multiscale nature of the problem, acomputationally efficient “gap-tooth” algorithm was developed to speed up considerably the simulationsof long transients typical of gradual melt release mode.Previous DECOSIM simulations of debris bed growth from falling melt particles were performedassuming saturated conditions in the pool at the system pressure. It was shown that the large scale naturalcirculation flows developing in the pool due to vapor production in the porous debris bed, affectsignificantly the debris bed shape because they capture the particles and cause their spreading over thebase-mat of the containment, making the debris bed more flattened and increasing its potentialcoolability. However, an important question remained on how these results would be affected if the poolwater is subcooled. In this case, condensation of vapor in the pool reduces the mixture buoyancy, whichcan decrease the effectiveness of particle spreading.In this work, numerical simulations by DECOSIM code are presented, in which the effects of poolsubcooling on the formation of debris bed are studied. It is shown that, in the subcooled case, thereduction of void due to vapor condensation is offset by the localized heating of water above the debrisbed. The density non-uniformities in the water pool caused by water heating are shown to be sufficient forthe development of large scale natural circulation flows, strong enough for the particle spreading to beefficient. The effect of water subcooling on the ultimate shape of debris bed is demonstrated bycomparing the results obtained for high and low subcooling. This work is performed within the MSWI project supported by APRI/ENSI/NKS
Place, publisher, year, edition, pages
American Nuclear Society, 2011.
IdentifiersURN: urn:nbn:se:kth:diva-53593OAI: oai:DiVA.org:kth-53593DiVA: diva2:470389
2011 International congress on advances in nuclear power plants - ICAPP 2011, Nice, France, 2-5 May, 2011
QC 201201092011-12-282011-12-282012-01-09Bibliographically approved