CRITICALITY SAFETY OF ENRICHED UF6 CYLINDERS

The transport of enriched hexafluoride uranium (UF6) is carried out in large steel cylinders. In filled cylinders, a maximum of 0.5wt. % of impurities is assumed to be present in UF6. Usually, for nuclear criticality safety studies, these impurities are supposed to be HF and the cylinders content is then modeled by a UF6-0.1HF medium completely filling the cavity. The aim of this paper is to highlight parameters which have a significant and non-intuitive impact on reactivity and to present an evolution of the nuclear criticality safety assessment by taking into account uranium residues like UO2F2 in filled UF6 cylinders. The first part of this paper focuses on three main parameters (uranium mass, interstitial moderation between cylinders, cylinders steel wall thickness) which have an impact on reactivity. The calculations are performed for an infinite array of 30B cylinders (30 inches diameter) containing enriched UF6 up to 5wt. % of 235U. The main conclusions of this part are: the maximum of reactivity is reached when cylinders are not entirely filled with UF6, the interstitial moderation combined with a gap between cylinders lead to increase reactivity. Moreover, some air in-leakage in cylinders is possible and its moisture content results in the hydrolysis of UF6 and creation of non-volatile uranium residues, in particular UO2F2 complexes with H2O and HF. Because cylinders are not washed before each transport, the accumulation of such residues is possible in the cylinders. So, the second part of the study deals with the conservative composition of these residues in regard to reactivity and compares the reactivity of the packaging with and without their presence. According to scientific literature, the ratio H/U in UO2F2 complexes is variable. Furthermore, the mass of uranium residues depends on the amount of air moisture in-leakage in a cylinder which is difficult to quantify. The approach presented in this paper considers calculations with various spherical masses of UO2F2 per package with various H/U ratios surrounded by UF6-0.1HF. The results of these calculations show that the presence of uranium residues can increase considerably the reactivity of the packaging (depending on the H/U ratio in the complex residues).