Relativistic density-functional study of solid solubitilty of tansiton metal/-uranium alloys; The Roll of d-d orbital interactions

相対論密度汎関数法による遷移金属/-ウラン合金の固溶性の研究; d-d相互作用の役割

栗原 正義*; 尾上 順*; 平田 勝 ; 鈴木 知史 

Kurihara, Masayoshi*; Onoe, Jun*; Hirata, Masaru; Suzuki, Chikashi

相ウランにおける遷移金属(TM)の固溶挙動を、相対論DV-X分子軌道法を用いて検討した。固溶性の評価には、遷移金属のd軌道のエネルギー(Md)、及び、遷移金属とウランの原子軌道間の軌道結合次数(OOP)を用いた。この分子軌道法による計算結果から、U 6dとTM dとの相互作用が、-U/TM合金の最大固溶度(MSS)について、重要な役割を果たしていることがわかった。また、MSSの値は、d-d軌道相互作用による安定化エネルギーにより説明され、MdとOOPはこの安定化エネルギーに影響していることがわかった。

The alloying behavior of transition metals (TMs) in solid -phase uranium (-U), which is expected to be used as fuel for next-generation nuclear reactors, is investigated using the discrete-variational Dirac-Fock-Slater molecular orbital method. Using a model cluster, U/TM, as the minimum unit of -U/TM alloys, we evaluate the d-orbital energy of the TM (Md), the bond order between the TM and U atoms, and the orbital overlap population (OOP) between the atomic orbitals of the TM and U atoms. We subsequently examine the effect of these quantities on the maximum solid solubility (MSS) of the -U/TM alloys. Md and the OOP (U 6d-TM d) exhibit good correlations with the MSS for -U/TM alloys, so that the interaction between the U-6d and TM-d atomic orbitals is found to play a key role in determining the MSS of the -U/TM alloys. The magnitude of the MSS can be explained in terms of the stabilization energy caused by U 6d-TM d orbital interactions for -U/TM alloys, which is affected by the Md and the OOP. The exponential dependence of the MSS on Md and the OOP is explained in terms of the equilibrium constant obtained using a substitution cluster model for -U/TM alloys. We also map the MSS of -U/TM alloys using the Md and the OOP as the alloying parameters. These results will assist the quantum design of nuclear fuel materials for other alloy systems besides the present alloy system.