Oxide fuel fabrication technology development of the FaCT project, 4; Feasibility study of oxygen getter options for pellet type MOX fuel

FaCTプロジェクトにおけるMOX燃料技術開発,4; ペレット型MOX燃料用酸素ゲッターオプションの実現性検討

森平 正之 ; 水迫 文樹*; 坪井 靖*

Morihira, Masayuki; Mizusako, Fumiki*; Tsuboi, Yasushi*

被覆管内面腐食は高速炉MOX燃料の寿命制限因子の一つであり、燃料中の酸素ポテンシャルに依存する。この酸素ポテンシャルは照射中に生じる余剰酸素の蓄積により燃焼度とともに増加することから、高燃焼度化に向けて余剰酸素の吸収除去が必要になる。このため原子力機構では、従来、低O/M比ペレットの開発を進めてきたが、量産化の問題から代替案が必要となった。酸素ゲッター法は、燃料要素内に余剰酸素の吸収材としての金属片を装荷する方法であり、それによって1.98, 1.98といった通常のO/M比のペレットを利用することが可能になる。しかし、酸素ゲッター法をペレット型燃料に応用した例は限られていることから、本研究ではペレット型燃料用の酸素ゲッター概念の検討を行い、低スミア密度のペレットを燃料要素の上部軸方向ブランケット中に装荷する方式が最も推奨されることを示した。

Cladding inner corrosion is one of the life controlling factors of FBR MOX fuels and it depends on the oxygen potential in a fuel element. Oxygen potential increases with extension of burn-up due to the cumulated excess oxygen during fission. The oxygen getter method is idea way to locate metal fragments in a fuel element as an excess oxygen absorber. Since almost nothing has been reported concerning the application of an oxygen getter in pellet type fuels, conceptual development of the oxygen getter for pellet type MOX fuel and a feasibility study were done. For getter material, titanium was mainly evaluated in this study except for compatibility tests carried out for titanium and zirconium. Concerning the location of getter material in a fuel element, the pellet-cladding gap and axial blanket region are potential options to avoid melting of titanium or obtaining a eutectic solution with MOX fuel. At the same time, an adequate temperature for oxidation as well as compatibilities with cladding material and fuel must be realized. Three options were proposed for titanium and their potentials were evaluated from this viewpoint. As a result, locating the titanium pellets in the upper axial blanket region of the fuel element was identified as the most promising option and it could provide the required low smear density titanium pellet.