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Zircaloy-4の高温酸化挙動に及ぼす固体ホウ酸の影響

Effect of solid boric acid on the high temperature oxidation behavior of Zircaloy-4

小宮山 大輔; 天谷 政樹

Komiyama, Daisuke; Amaya, Masaki

PWRの冷却材喪失事故(LOCA)において、流路の閉塞等により燃料棒の冷却が十分に行われない場合、燃料被覆管表面に冷却材中のホウ酸が析出する可能性が考えられる。通常運転温度域では、実機での実績からホウ酸水はZircaloy-4の酸化挙動に影響を及ぼさないと考えられるが、LOCAを想定した高温域におけるホウ酸とZircaloy-4との反応に係る知見は十分に得られていない。本研究では、固体ホウ酸を載せたZircaloy-4の板材を900$$^{circ}$$Cまでの温度及び複数の雰囲気で酸化させることにより、固体ホウ酸の高温時挙動、ホウ酸とZircaloy-4との反応の有無、及びホウ酸がZircaloy-4の酸化挙動に及ぼす影響を調べた。実験結果から、高温酸化雰囲気においてZircaloy-4表面に固体ホウ酸の脱水により生成する無水ホウ酸が存在すると、この無水ホウ酸がZircaloy-4と雰囲気との接触を断つことでZircaloy-4の酸化を抑制することが示唆された。また、酸化膜付きZircaloy-4の表面に固体ホウ酸が付着し高温まで加熱された場合は、形成している酸化膜の空隙に無水ホウ酸が浸透することでその後の酸化を抑制することがうかがえた。

The boric acid in the coolant may precipitate on the fuel cladding surface in the case that the blockage of coolant flow path etc. occurs and/or the cooling of fuel becomes insufficient during a LOCA (Loss-Of-Coolant Accident) in PWRs. While there is much knowledge about the compatibility between Zircaloy-4 and boric acid under normal operation conditions, such knowledge under high temperature condition, e.g. LOCA, has not been sufficient. In this study, isothermal heating tests were carried out by using Zircaloy-4 plates with solid boric acid in various atmospheres at up to 900$$^{circ}$$C; and the stability of boric acid, the reaction between Zircaloy-4 and boric acid and the effect of solid boric acid on the high temperature oxidation behavior of Zircaloy-4 were investigated. From the results obtained, it was suggested that, if boric acid anhydride remained on the surface of Zircaloy-4, the boric acid anhydride prevented the surface of Zircaloy-4 from contacting oxidizing atmosphere and mitigated the high-temperature oxidation of Zircaloy-4. In the case that solid boric acid adhered to the surface of pre-oxidized Zircaloy-4 and they have been heated up to high temperature, it was indicated that the solid boric acid penetrated into the gaps in the pre-oxide layer and the penetrated solid boric acid mitigated the oxidation of Zircaloy-4 thereafter.

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