Fission gas release in BWR fuel with a burnup of 56 GWd/t during simulated reactivity initiated accident (RIA) condition

燃焼度56GWd/tのBWR燃料における反応度事故条件下での核分裂生成ガス放出

天谷 政樹  ; 杉山 智之 ; 永瀬 文久 ; 更田 豊志

Amaya, Masaki; Sugiyama, Tomoyuki; Nagase, Fumihisa; Fuketa, Toyoshi

商用炉で照射されたBWR燃料(燃焼度56GWd/t)から調製した試験燃料棒に対し、反応度事故条件を模擬したパルス照射実験を行った。実験後の燃料棒パンクチャ試験及び燃料ペレットのEPMA分析結果に基づき、パルス照射実験中のFPガス放出挙動を調べた。パルス照射後のペレット局所Xe濃度は、ベース照射後のペレットのそれに比べ相対半径で00.8の範囲で低下した。この局所Xe濃度の低下は11%のFPガス放出率に相当し、パルス照射実験後のパンクチャ試験結果と同等であった。燃料ペレットの組織変化と結晶粒界でのFPガス保持量を考慮すると、パルス照射実験中のFPガス放出は燃料ペレットの周辺部よりも中間部で生じた粒界分離の影響を受けたものと考えられる。

Pulse irradiation simulating reactivity initiated accident (RIA) condition was conducted for the test rod prepared from the BWR fuel rod with a burnup of 56 GWd/t irradiated in a commercial reactor, and fission gas release during the pulse irradiation was investigated based on the result of rod-puncture test and electron-probe-microanalysis of fuel pellet. The local xenon concentration of pulse-irradiated pellet decreased compared with that of base-irradiated pellet in the relative radius of 0 to approximately 0.8. The decrease corresponds to a fractional fission gas release of approximately 11%, and this value was comparable with the rod-puncture test result. Considering the microstructural change in fuel pellet and the amount of retained gas in grain boundary, it is likely that the fission gas release during pulse irradiation was affected by the grain boundary separation which occurred in the mid-radius rather than the peripheral region of fuel pellet during pulse irradiation.