Calculation of critical concentrations of actinides in an infinite medium of silicon dioxide

Okuno, Hiroshi  ; Sato, Shohei; Kawasaki, Hiromitsu*

The critical concentrations of metal-SiO and -HO mixtures were calculated for 26 actinides including U, Pu, Am, Cm and Cf, where the critical concentration was defined as the concentration that the infinite neutron multiplication factor, k being calculated to be 1.0. The calculations were performed using the Monte Carlo neutron transport calculation code MCNP5 combined with the evaluated nuclear data library JENDL3.3. The results showed that the critical actinide concentration of metal-SiO was ca. 1/5 of that of metal-HO for all the fissile nuclides investigated. The k's of the metal-SiO and metal-HO at a half of the respective critical actinide concentration, which concentration was assumed as the subcritical actinide concentration limit, were found to be less than 0.8 for all the actinides considered. Applying a sum-of-fractions rule with respect to the ratios of actinide concentration to the subcritical actinide concentration limit for six fissile nuclides, subcriticality of high-level radioactive wastes was confirmed for a reported sample. The effect of different nuclear data libraries on the results of critical actinide concentrations was found large for Cm, Cm and Cf.