BEHAVIOUR OF A PACKAGE FOR TRANSPORT OF SPENT FUEL ASSEMBLIES EXPOSED TO BEYOND REGULATION FIRES

IRSN is performing a study relative to the thermal behaviour of a new TN International package design for transport of spent fuel assemblies called TN®112. The aim of this study is to evaluate the behaviour of the package submitted to fires, with durations and temperatures different from those required in the IAEA regulation TS-R-1 (respectively 30 minutes and 800 Celsius degrees). Its main objective is to provide quantitative data available for safety assessment in emergency situations involving fires. Moreover it can also be used for a cross comparison with the analysis of the thermal behaviour of the package during the IAEA regulatory fire test presented by the applicant in the package design safety report. This study is based on numerical calculations performed with the code THERMX-PROTEE. The three-dimensional model used represents a quarter of the upper half of the package, where is located the closure system. The thermal behaviour of the resin located in the plug, the trunnions and between the inner and outer shells was modelled considering endothermic reactions of vaporization. During the heating phase of the fire test, the water vapour produced in heated elements is transferred and condensed in the nearby colder elements; the associated thermal transfers can increase very fast the temperature of the latters. The part of the vapour which cannot be condensed when most of the nearby elements reach a temperature above 100 Celsius degrees is evacuated through the holes that are distributed throughout the external envelope of the packaging and closed by fusible plugs. A specific calculation module has been developed to take into account the corresponding energy transfers. This module was qualified by comparison with the results of experimental fire tests. The calculations performed in the framework of this study cover temperatures of fire between 400 and 1000 Celsius degrees. One of the results of those calculations is the time necessary to reach the maximum allowable temperature of the elastomer gaskets.