EXPERIMENTS AND ANALYSES ON SODIUM VOID REACTIVITY WORTH IN MOCK-UP CORES OF METALLIC FUELED AND MOX FUELED FAST-REACTORS AT FCA

Sodium void reactivity worth in two metallic fueled fast reactors and a MOX fueled one was measured at the Fast Critical Assembly (FCA). Usi ng the measured reactivity worth, bias factors to be applied to the ca lculation were determined for the non-leakage term and the leakage ter m separately by a least squares method. Based on these bias factors, t he calculation accuracy for each term was assessed. For the non-leakag e term, JENDL-3.2 showed good agreement with the experiments. No diffe rence in the calculation accuracy was observed between the metallic fu el and the MOX fuel, though their neutron spectra were largely differe nt. When the enriched uranium was contained in the void region togethe r with plutonium, however, JENDL-3.2 overestimated the non-leakage ter m by about 6 %. The effect of the plutonium isotope composition was al so examined. For the leakage term, it was found that the accuracy of t he current calculation depended upon the void fraction in the cell. Th e directional diffusion coefficient based on the Benoist's theory was validated by transposing the plate direction. Moreover, the design con cept of LMFBR with the sodium plenum, which aims at the reduction of t he sodium void reactivity, was also examined.