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

二酸化珪素媒質中のアクチニド臨界濃度の計算

奥野 浩  ; 佐藤 庄平; 川崎 弘光*

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

U, Pu, Am, Cm及びCfを含む26アクチニドの金属-SiO及び金属-HO混合物の臨界濃度を計算した。ここで、臨界濃度は、無限中性子増倍率kが1になる濃度と定義される。計算は、モンテカルロ中性子輸送計算コードMCNP5と核データライブラリJENDL3.3の組合せで行われた。計算結果は、調べたすべての核分裂性核種において、金属-SiOの臨界アクチニド濃度が金属-HOの臨界アクチニド濃度の約1/5であることを示した。対応する臨界濃度の半分を未臨界アクチニド限度と想定し、この濃度での金属-SiO及び金属-HOのkが検討したすべてのアクチニドで0.8未満であることを見いだした。部分和の法則を6核分裂性核種のアクチニド濃度と未臨界アクチニド濃度の比に対して適用し、高レベル放射性廃棄物の報告された例について未臨界性を確認した。臨界アクチニド濃度の計算結果に対する異なる核データの影響が大きいことをCm, Cm及びCfに対して見いだした。

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.