Modelling of fuel-coolant interaction in stratified configurationKokalj, Janez (Avtor) Leskovar, Matjaž (Mentor) Uršič, Mitja (Komentor) nuclear safetysevere accidentfuel-coolant interactionvapour explosionmelt-coolant premixed layermelt-coolant stratified geometryDuring a hypothetical severe accident in a light water nuclear power plant, the molten reactor core may come in contact with the coolant water. One of the consequences can be a vapour explosion. Typically in nuclear safety, the vapour explosions are mostly analysed in the melt jet-coolant pool geometry. In the stratified melt-coolant configuration, which was less analysed, a layer of melt lies below a layer of coolant. The stratified melt-coolant configuration was believed to be incapable of producing a strong energetic interaction between the melt and coolant.. This belief was based on the conclusions from the past theoretical and some experimental research, where the lack of the interfacial instabilities and consequently the lack of an explosive premixture was determined. In the recent fuel-coolant interaction experiments performed in the stratified configuration at the SES and PULiMS facilities (KTH, Sweden), a premixed layer formation of ejected melt drops in water was clearly visible and was followed by strong spontaneous vapour explosions. The purpose of our research was to improve the knowledge, understanding and modelling of the fuel-coolant interaction phenomena in the stratified configuration. Firstly, the phenomenon of the vapour explosion is presented. An overview of the experimental work is given. The SES and PULiMS experiments (KTH, Sweden) and other experiments of vapour explosion and premixed layer formation in stratified configuration as well as some other relevant experiments are presented. Existing modelling approaches for mixing in case of the vapour explosions in the stratified configuration are reviewed. Possible mechanisms for the premixed layer formation are discussed and assessed according to their relevance to the reviewed experiments. Based on the visual observations and some available mechanisms from the literature, models for the premixed layer formation are presented. Due to the uncertainties and the lack of detailed information, two different approaches are followed in the modelling. Both approaches are based on the bubble collapse mechanism, where coolant micro-jet is formed. The first modelling approach describes the melt drops ejection due to the coolant micro-jet penetration in the melt and its vaporization. The second modelling approach describes the melt drops ejection due to the pressure perturbation, when the coolant micro-jet hits the melt surface. Both models and experimental observations are discussed and joined into the third model, which describes the considered phenomena the best. The developed model is implemented into the MC3D code (IRSN, France) and validated against the experimental results. To enable the implementation of the proposed model into the Eulerian fuel-coolant interaction codes, a description of melt drops with a two-melt-drop-group approach is introduced. The behaviour of the implemented model is verified with numerical and physical tests. The implementation of the model into the fuel-coolant interaction code enables the simulation of the observed fuel-coolant interaction in the stratified configuration. The presented analyses demonstrate the model capability to describe the premixed layer formation in a qualitative agreement with the available experimental data. The indirect comparison of the simulated premixed layer with the SES S1 and PULiMS E6 experiments via the strength of vapour explosion shows underestimation in simulations’ explosion strength. This underestimation is further discussed and other possible contributions, e.g. contribution from the melt jet break-up and mixing during the explosion are highlighted.20212021-12-24 08:15:02Doktorsko delo/naloga134126sl