Effect of radial zoning of 241Am content on homogenization of denatured Pu with broad range of neutron energy based on U irradiation test in the experimental fast reactor Joyo

Abstract

The design consideration of depleted U and Am (DU-Am) oxide fuel pin was performed for Pu denaturing in the framework of the protected plutonium production based on the irradiation analyses of the DU samples irradiated in the environment of broad range of neutron energy in the experimental fast reactor Joyo. From the results of irradiation analyses of DU, it was confirmed that there is a strong dependence of transmutation behavior of DU on the ratio of neutron flux in the resonance region to the total neutron flux, namely resonance neutron ratio, even in a fast reactor. Also, it was confirmed that there is a strong effect of sample material form and shape on generated Pu nuclide inventory especially near the reflector area (>20% resonance neutron ratio), because of the intensive self-shielding of 238U, though less is expected for 241Am. Sensitivity study of hypothetical DU-Am oxide fuel pellet irradiation on neutron energy and burn-up was performed to evaluate significant gradient of radial 238Pu isotopic composition profile (e.g., from 12% to 18% distribution in 3% Am doping, in 30% resonance neutron ratio and in the neutron fluence of 4.0×1022n/cm2 inside a large pellet with softened neutron spectrum), and vulnerability of the fuel pellet surface in terms of Pu denaturing was revealed. Design consideration of radial zoning of 241Am content was introduced to flatten the radial distribution of isotopic composition of Pu. The results of radial zoning of 241Am (4% and 3% Am in the outer and inner zone of DU-Am oxide fuel pellet) in hypothetical irradiation neutronics analysis showed the radial profile of produced Pu is over 15at.% of 238Pu isotopic composition in any zone and meets the criterion of Kimura et al. based on decay heat of Pu to impede utilization to fission explosive devices.