CAN WE CONSIDER THAT WATER IS COMPLETELY ABSENT FOR THE CRITICALITY ANALYSIS FOR PACKAGES EQUIPPED WITH MULTIPLE HIGH STANDARD WATER BARRIERS?

Applicants have to justify the criticality safety of packages loaded with fresh or spent fuel in the conditions of the 9 m drop test where there are high uncertainties about the integrity of fuel geometry. Due to the lack of knowledge about the behaviour of the fuel elements, the sub-criticality of the single package filled with water was hardly impossible to demonstrate. Therefore, French applicants considered special features in the package design that allow the exclusion of water penetration: in many cases (such as for commercial PWR UO2 fuel) large reactivity margins exist when the quantity of water is limited even if the integrity of fuel assemblies is lost after the tests. Until 2001, the transitional application of the 1985 edition of the IAEA transport regulations allowed the qualification of a unique watertight barrier in the package; but from then new designs had to integrate two watertight barriers that have to prevent water penetration even as a result of error. An interpretation of the first part of this requirement is presented in terms of mechanical resistance and water-tightness. Leakage tests performed after simulation of drop tests usually confirm that water penetration would not exceed a fraction of a litre. This quantity has to be added to residual water in the cavity after fuel loading. Then, for different types of commercial nuclear fuel, the increase of reactivity due to the presence of limited quantities of water is given. A complementary approach consists in analysing implications of operational errors during draining and drying the package cavities. Impact on reactivity of the presence of water in unlimited or limited quantities as a result of the chosen combinations of operational errors with normal and accident conditions of transport (in both cases of the isolated package and packages in arrays) is given. The implications on the special measures to be taken during package preparation before shipment are presented. These measures aim at guaranteeing full draining and drying of package cavity and compliance of each barrier with leakage test criteria with due account for human factors and materials redundancy.