CONSEQUENCES OF THE USE OF THE AVERAGE PU-CONTENT FOR CRITICALITY EVALUATION OF PWR MOX-FUEL TRANSPORT AND STORAGE PACKAGES

Currently, criticality studies of Pressurized Water Reactor (PWR) Mixed OXide (MOX) (uranium and plutonium) fuel assemblies in transport and storage packages are based on conservative hypothesis by assuming that all rods have the same plutonium content corresponding to the maximum value. In that way, the real heterogeneous mapping of the assembly is masked and covered by a homogeneous assembly, with a plutonium content at the maximum value. As this calculation hypothesis is extremely conservative, Transnucléaire and Cogema have developed a new calculation method based on the use of the average Pu-content value in the criticality studies. The use of the average Pu-content, instead of the real Pu-content profiles, provides higher or equivalent reactivity values that make it globally conservative. This method can be applied for all MOX PWR complete or incomplete fuel assemblies, which Pu-content in mass does not exceed 15 %; it provides advantages which are discussed in the paper.