Void Reactivity Decomposition Analysis for LWR Fuel Assembly having MOX and FCM rods

The TRU nuclides are recycled through the MOX fuel rods while the TRUs produced from FCM ones are not recycled due to their deep burning. From view point of neutronics, one of the most important issues is the positive void reactivity which can occur in a PWR fuel assembly containing TRU nuclides. The purpose of this work is to give the physical understanding of the void reactivity in the specialized PWR fuel assemblies having MOX and FCM fuel rods with the reactivity decomposition method. The void reactivity was decomposed with normalization to total integral flux into nuclide-wise contributions for each reaction type. From the decomposition analysis, it was shown that 1) the decomposition method gives the almost exact reactivity within about 3.6 pcm displacement bound compared to the direct reactivity change calculated with nominal and voided states, 2) the positive contribution to the void reactivity comes from the reactions in capture and fission (in particular, capture contribution is much larger than the fission one) for high nominal voiding levels, and 3) the plutonium isotopes having even mass number, Np-237, Am-241, and Am-243 give much positive contributions while the negative contributions come from the fissile nuclides due to the reaction in the fission production under voiding.