Study on double heterogeneity effect of Pu-rich agglomerates in mixed oxide fuel using cross section homogenization method for particle-dispersed media

A cross section homogenization method for media containing randomly and uniformly dispersed particles, which was originally developed by Shmakov et al., has been applied to MOX fuels containing Pu-rich agglomerates. This method (Shmakov's method), which is incorporated into a continuous-energy Monte Carlo code MCNP, has been applied to lattice calculations of an infinite MOX fuel rod array. Shmakov's method can accurately reproduce the criticality calculation results for an explicit heterogeneous arrangement of Pu-rich agglomerates. A correction factor that Shmakov's method defines to obtain an effective microscopic cross section provides a proper quantitative indication of the double heterogeneity of MOX fuels containing Pu-rich agglomerates. The correction factors exhibit an obvious double heterogeneity effect of Pu-rich agglomerates dispersed in MOX fuel pellets. The effective microscopic cross sections of plutonium isotopes in MOX fuels containing Pu-rich agglomerates are significantly reduced due to the self-shielding effect as compared to the homogeneous MOX fuel model. However, the double heterogeneity effect of Pu-rich agglomerates on keff seems to be unexpectedly minor because the underestimate of the reaction rates in the resonance energy range is offset by the overestimate of the reaction rates in the thermal energy range.