INFLUENCE ON CRITICALITY SAFETY OF THE PENETRATION OF HYDROCARBONS INTO PACKAGES CONTAINING FISSILE MATERIALS

According to the IAEA transport regulation, the nuclear criticality safety demonstration for a package containing fissile materials must take into account water presence in it. But some other materials as hydrocarbons, which could be better neutron moderators than water, could ingress and mix with the fissile materials, in the case of a road accident involving, for example, a portable tank container. The aim of this paper is then to show influence on the reactivity of this kind of scenario. The present work is focused on a study for IF type packages which, in general, contain non irradiated UO2 fuel in physical form of powder or sintered pellets conditioned in fuel assembly and, therefore, do not need water or containment barriers of high standard level. First of all, to cover the existing different types of hydrocarbons, a bounding composition is determined based on literature gathered information. This composition is defined by the following chemical formula CH2.1 and by a density equal to 0.86 g/cm3 . Criticality calculations show low impact of the slight presence of impurities (oxygen and sulphur) in hydrocarbons. Comparisons between critical masses and volumes are performed for UO2 powder (for several 235U enrichment) and between the keff of an isolated UO2 assembly (for several 235U enrichment too) considering water, CH2 and hydrocarbon as moderators. Results show that, without being as efficient as CH2, the considered hydrocarbon is a better moderator than water. Finally, the same work is made for packages for UO2 powder or assemblies. Results show that, for one case, an isolated package can be critical in case of penetration of hydrocarbons. In other cases, the penetration of hydrocarbons can only lead to critical conditions for an array of packages, but not for an isolated package. This study demonstrates the interest in considering penetration of hydrocarbons into packages containing fissile materials for criticality safety assessment, which potentially might be reflected in the IAEA transport regulation.