Bounding plutonium vectors for pwr-MOX burnup credit applications
Congress title :ICNC 2007
Congress location :Saint-Petersbourg
Congress date :28/05/2007
It is easy to determine a conservative composition of the fresh fuel for PWR-UOX Burnup Credit applications to perform criticality studies: for a given burnup, the maximal initial uranium enrichment leads to the more reactive spent fuel for all applications. For PWR-MOX fuels, if an isotopic vector of the plutonium is conservative for a set of fresh fuel, it won't be automatically conservative after irradiation. Therefore, it seems necessary to determine, for a given ratio of Pu/(U+Pu), a bounding plutonium vector for the fresh fuel that gives, after irradiation, the most reactive fuel, whatever the irradiation history can be. But, how can we determine the bounding plutonium vector for an application taking Burnup Credit into account? The first step of this study was to identify if the definition of such a bounding isotopic vector was achievable. And then, a method leading to a bounding isotopic vector was set out. The calculations were performed with the French DARWIN 2.0 package based on APOLLO 2 and PEPIN2 depletion codes and with the criticality calculation scheme APOLLO 2 MORET4 from the French CRISTAL V.1 package. Various initial plutonium isotopic vectors were studied. Firstly, the plutonium was supposed to be only composed of 239Pu and 240Pu. Then, calculations with plutonium initially composed of 239Pu, 240Pu and 241Pu were performed. These calculations showed that: -the configuration leading to the maximal neutron multiplication is always obtained with a minimal amount of 240Pu; -the determination of bounding initial isotopic vectors of the plutonium mainly depends on the spent fuel cooling time, mainly due to the beta- decay of the 241Pu (which half-life is 14.4 years) in 241Am, a neutronic absorber for thermal spectrum. This paper will present the first approach set to determine the bounding plutonium vector for fresh fuel, depending on the cooling time of the fuel of interest after irradiation. Indeed, studies pointed out that, for long cooling times, the bounding initial plutonium vector would minimize the amount of 241Pu, and for short cooling times, the bounding vector would depend on both the burnup value and on the weight ratio Put/(Ut+Put). The conservatism of this hypothesis has been verified for various usual MOX fuels. A gain estimation of MOX Burnup Credit was also achieved with comparisons between the keff values obtained with a fresh PWR-MOX fuel for various usual plutonium isotopic vectors and with a spent PWR-MOX fuel (containing actinides and fission products) with the bounding plutonium isotopic vector set up in the study. We can notice that, even if the plutonium isotopic vector is chosen in order to be conservative for the fuel composition after irradiation, an overall important reduction in reactivity associated with the irradiation of the fuel remains.