Numerical study of the thermal behaviour of two types of packages exposed to long duration fires

The thermal behaviour of two types of package exposed to long duration fires is studied. The TNTM 12/2A and TNTM 28VT packages, respectively used for spent fuel and vitrified waste transports, are modelled. Threedimensional meshes are used. Attention was paid to the model of the thermal protective resin of the packages because of its complex thermal behaviour. During heating several endothermic reactions occur: water vapour is produced and a part of it diffuses though the resin and condensates on the cold parts of resin, increasing the global heat transfer within the material. The other part of the water vapour exits the package by fusible holes. The thermal characteristics of these reactions have been established thanks to specific tests performed in a laboratory. A model taking into account all these phenomena was developed and integrated to the global thermal model of the packages in order to simulate the thermal behaviour of the packages exposed to long duration fires. Four fire temperatures were considered and, for each of them, the maximum fire duration that packages can withstand without activity release was calculated. The results show safety margins regarding the IAEA regulatory thermal test (800°C-30 min). The use of the complex model of resin led to calculate safety margins about 40% greater than those calculated with a model of resin taking only conduction into account. The results were used to prepare a guideline for safety assessment in emergency situations involving fire. This emergency tool provides safety limits for containment according to fire duration, fire temperature, package heat power and ambient temperature.