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(Pu,U)O$$_{2}$$の酸素ポテンシャルI UO$$_{2+X}$$ 及び(Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2-X}$$の測定と格子欠陥理論によるモデル化

Oxygen potential of (Pu,U)O$$_{2}$$ I -Oxygen potential measurement of UO$$_{2+X}$$ and (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2-X}$$

加藤 正人   ; 田村 哲也*; 遠藤 秀男

Kato, Masato; Tamura, Tetsuya*; Endo, Hideo

UO$$_{2}$$及びMOXの酸素/金属比(O/M比)は、照射挙動に影響を与えるため、重要な燃料仕様の一つである。酸化物燃料の酸素ポテンシャル測定は、照射挙動や燃料製造条件を最適化する目的で、様々な手法により行われている。本報告では、示差熱-熱重量計(TG-DTA)を用いて酸素ポテンシャル測定を確立するとともに、UO$$_{rm 2+X}$$及び(Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{rm 2-X}$$の酸素ポテンシャル測定を行ったので報告する。試験はAr/H$$_{2}$$/H$$_{2}$$O系のガスによる気相平衡法により実施し、800$$^{circ}$$C, 1000$$^{circ}$$C, 1200$$^{circ}$$Cにおいて定比組成近傍のデータ測定を行った。O/M は、混合ガスの割合をステップ上に変化させ、重量変化からO/M変化量を求めた。酸素分圧は、装置の前後で安定化ジルコニアの酸素センサーにより測定し、測定温度での酸素分圧を評価した。各温度における測定結果は、文献値とよく一致した。測定結果より$$Delta$$Go$$_{2}$$, $$Delta$$Ho$$_{2}$$, $$Delta$$So$$_{2}$$を評価した。また、測定結果と文献値から格子欠陥モデルを適用して酸素分圧とO/Mの関係を整理した。UO$$_{rm 2+X}$$, (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{2pm x}$$及びPuO$$_{rm 2-X}$$について、Xを温度と酸素分圧で表す式を作成し、測定データと計算結果がよく一致することを確認した。

The ratio of oxygen to metal is one of important fuel specifications on UO$$_{2}$$ and MOX fuels, because it effect on irradiation behavior. Oxygen potential of oxide fuels have been measured by various methods on the purpose of optimization of irradiation behavior and fabrication condition. In this repot the oxygen potential of UO$$_{rm 2+x}$$ and (Pu$$_{0.3}$$U$$_{0.7}$$)O$$_{rm 2-x}$$ was measured by thermal gravimetry and differential thermal analysis (TG-DTA). Measurements of oxygen potential were carried out at 800$$^{circ}$$C, 1000$$^{circ}$$C and 1200$$^{circ}$$C in Ar/H$$_{2}$$/H$$_{2}$$O mixture gas flow. Ratio of O/M was obtained from the weight change of the sample according to the partial oxygen pressure that was controlled by H$$_{2}$$/H$$_{2}$$O ratio in atmosphere. The partial oxygen pressure in atmosphere was measured by stabilized zyrconia oxygen sensor. The experimental results agree approximately to the other works. Thermodynamic data, $$Delta$$Go$$_{2}$$, $$Delta$$Ho$$_{2}$$, $$delta$$So$$_{2}$$, were evaluatedfrom the experimental data. The oxygen potential of UO$$_{rm 2+x}$$, (Pu,U)O$$_{rm  2pm x}$$ and PuO$$_{rm 2-x}$$ was modeled by lattice defect theory using the data of the literature and this work. The resulting equation well reproduce the large amount of oxygen potential-temperature-composition data for the Pu-U oxide system.

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