Modelling of the behaviour of a UF6 package in a fire - Dibona code - description and validation

The International Atomic Energy Agency (IAEA) issued in 1996 a revision of the regulation requiring that UP6 packages must withstand a specific fire test. The purpose of this study was to set up a two-dimensional numerical model, in order to obtain the pressure and temperature results necessary to assess the required performances of the package. The scenario and the phenomenology were introduced in the model from the analysis and the interpretation of the TENERIFE tests. The model, which uses the finite element computer code ANSYS, takes into account most of the physical phenomena which take place in this kind of problem: radiation through an absorbing gas, thermal contact resistance, pool boiling, convection, thermal stratification, effect of bubbles agitation in the liquid, solid core sinking, UF6 melting, UF6 condensation, UF6 sublimation, UF6 evaporation, and pressurisation. The experimental results of three of the TENERIFE tests were successfully used in defining the numerical model, which was then used to predict the behaviour of a 48Y container engulfed in the IAEA fire test. The time when the critical pressure is reached within the package, was calculated to be twenty five minutes, i.e. before the end of the regulatory test. (author)