Validation of the THEMA corium spread modelling code on MST tests

Sammanfattning

Nuclear energy is an essential source of electricity, combining reliability with a reduction in greenhouse gas emissions. However, the serious accidents that have occurred in the past have highlighted the need for a better understanding of these phenomena in order to improve the safety of installations. The ROSAU project is part of this approach, studying the spread of corium under water using experimental tests (MST). At EDF’s request and in response to this need, LMAG is proposing, through the objective of this project degree, to qualify the THEMA modelling code on these tests and to improve our understanding of spreading phenomena. The MST tests involve prototypical corium masses of around, 300kg discharged by dam-breaking method or by a jet into a spreading chamber and whose final spreading, of the order of a metre, is reached in ~1s. The construction of the dataset for modelling the MST-3 test was based on an analysis of the test reports, a preliminary study of the MST-0 test (dry test) and a sensitivity study of the poorly known simulation parameters influencing the spread. THEMA predicts a final spreading length of 0.84m in 1.3s, i.e. a dynamic error of 100%, comparable to the MELTSPREAD’s results. The spreading dynamics are mainly determined by the initial phase of the injection, representing 30% of the total mass, underlining the importance of suitable boundary conditions for modelling a corium jet. The flow regime observed is turbulent in the flow front, suggesting the need for 2D modelling. However, the calculation presented a convergence problem and called into question the method used to take the upper crust into account. The sensitivity analysis identified the parameters with the greatest influence on corium spreading in the MST-3 tests. The injection temperature are the most important, but as it’s not well known due to the experimental conditions, that leds to an incorrect assessment of the corium solidification and its dispersion in the melt. In addition, the post-mortem analysis indicates significant abla- tion of the substrate by impact, having modified the composition of the corium via incorporation of concrete and degassing. Although THEMA incorporates a model of the swelling of the melt, adjustments are needed to better represent ablation and solidification, which are still poorly modelled at this stage.