Investigations of passive safety systems in LWRs : melt coolability with bottom coolant injection, light gas effects on a passive containment cooling system

Abstract: The key objectives in introducing passive features in thesafety systems for Light Water Reactor (LWR) are to increasetheir reliability, to simplify the design of the plants and toimprove performance and economics. Component and integralsystem tests in scaled facilities are required for optimizingthe design and for certification of these safety systems.Experimental programs are also needed for a betterunderstanding of all significant physical phenomena on thebasis of which the passive safety systems function.In thefirst partof the thesis the results of an investigationon melt coolability with bottom coolant injection arepresented. Ex-vessel coolability of a molten pool and/or debrisis, perhaps the most vexing remaining issue, needingresolution, in the beyond the design-base safety assessment forthe current and the future light water reactor plants. Latephase of the progression of a severe accident is associatedwith corium-melt discharge from the Reactor Pressure Vessel(RPV) and its relocation on the concrete basemat in the form ofa debris bed consisting of liquid/particulate corium. The coredebris generates decay heat and attacks the concrete basematand the containment structures and continues to do so, untilthe coolability of debris bed is achieved. Failure to assurethe coolability of the debris bed is tantamount to the failureto assure the termination of the severe accident. A researchprogram named DECOBI was developed at the Nuclear Power Safetydivision of the Royal Institute of Technology to study thecoolability of melt pools with bottom coolant injection whichoccurs passively through nozzles embeddedin concrete whichopen after ablation of a predetermined depth of concretebasemat. The research objective was to explore the mechanism ofhigh heat transfer and in particular the large scale porosityformation. In the DECOBI program first a series of experimentswere performed at low temperature, by using transparent pooland coolant that allows the visualization of the flow patternduring the pool-coolant interaction. The parameters governingthe early stages of the coolant phase change and dispersion inthe pool are identified through these visual experiments.Experiments were performed also with metallic (Pb) andbinary oxide simulants (CaO-B2O3, MnO-TiO2, CaO-WO3), such that a substantial range of parametersinfluencing coolant-melt interaction and the subsequent debrisstructure was investigated. The molten metal (Pb) has highconductivity and low viscosity, which results in a high heattransfer rate and mixing of pool and coolant. The coolingprocess occurs rapidly and the porosity, after solidificationhas a fine and quite uniform structure. On the contrary theoxide mixture CaO-B2O3has high temperature, low conductivity, highviscosity and glassy material structure, which resulted inlower heat transfer rate and more difficult pool-coolantmixing. In the coolant-contacted regions, directly above thenozzle locations, an interconnected branched channel typeporosity was obtained. These regions had high porosity and theyquenched due to the continuous passage of coolant, even longafter solidification. The other regions did not developporosity because the crust formed outside the quenched regions,prevented the direct contact of coolant with these regions.These regions, however, could conduct heat to the adjacent coldregions and cooled eventually. The experiments performed usingtheoxide mixtures CaO-WO3and MnO-TiO2(which have lower viscosity compared to that ofCaO-B2O3and a ceramic material structure) showed betterpool-coolant mixing and rapid quenching of the melt. Theporosity obtained was uniformly distributed throughout themelt. DECOBI program, thus, reveals that, through this schemeof coolant injection the debris arriving in the containment asa result of vessel failure can be quenched in a relativelyshort time. Analysis performed, based on the insights gainedfrom the experiments, estimates the amount of porosity that canbe obtained with a single nozzle.Thesecond partof the thesis is focused on the issue of thelight gas effects on the efficiency of a Passive ContainmentCooling System (PCCS) for a Boiling Water Reactor. The presenceof hydrogen affects the efficiency of PCCS, sincenon-condensable gases reduce the steam condensation rate andconsequently the efficiency of the condensers. The gasdistribution not only influences the PCC performance but alsoaffects the containment pressure build-up. The data and someanalysis of a series of four system transient tests performedin the PANDA facility, show that retention of non-condensablegas in the DW could mitigate the system pressure. The PCC(Passive Cooling Condenser) operation modes are qualitativelydiscussed and the PCCS performance is analyzed for one test byquantifying the major heat sinks (PCC pools, wetwell pool, RPVwater, structural materials, etc) and comparing them during thetest period with the decay heat (heat source) generated in theRPV. The heat transfer coefficients in one PCC have beendetermined for one test at selected times before, during andafter helium injection. In this way, the effect of light gashas been quantified in term of variation of heat transferrate.Keywords:DECOBI, nuclear severe accident, PANDA,PCCS.