First-principles calculation of mechanical properties of simulated debris ZrUO

模擬デブリZrUOの機械的特性の第一原理計算

板倉 充洋  ; 中村 博樹  ; 北垣 徹  ; 星野 貴紀 ; 町田 昌彦  

Itakura, Mitsuhiro; Nakamura, Hiroki; Kitagaki, Toru; Hoshino, Takanori; Machida, Masahiko

福島第一原子力発電所の炉内にある燃料デブリの機械的特性を明らかにするため、模擬デブリである二酸化ウラン・二酸化ジルコニウム溶融物についてその弾性定数と破壊靭性を第一原理計算で評価し、二酸化ジルコニウムの割合の影響を調べ、模擬デブリを用いた実験結果と比較を行った。その結果、実験で観測されている、二酸化ジルコニウム割合の増加に伴う急激な破壊靭性の増加は、二酸化ジルコニウムの複数の相の混在によるものと考えられるという結論を得た。

To elucidate the mechanical properties of fuel debris inside the Fukushima Daiichi Nuclear Power Plant, we use first-principles calculations to evaluate mechanical properties of cubic ZrUO, which is a main component of the fuel debris. We focus on the dependence of mechanical properties on the fraction x of zirconium, compare our results with recent experiment of simulated debris, in which dependences of elastic moduli and fracture toughness on the ZrO content showed deviation from a simple linear relation. We show that elastic moduli drop at around x=0.25 and increase again for larger values of x, as has been observed in experiments. The reason of the drop is a softening owing to disordered atomistic structures induced by the solute zirconium atoms. We also find that stress-strain curves for the x=0.125 case show marked hysteresis owing to the existence of many meta-stable states. We show that this hysteresis leads to slightly increased fracture toughness, but it is not enough to account for the significant increase of fracture toughness observed in experiments.