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LWR MOX fuel irradiation tests; HBWR irradiation with the instrument rig, IFA-514/565

軽水炉用MOX燃料照射試験; IFA-514/565計装リグを用いたHBWR照射試験

小澤 隆之

Ozawa, Takayuki

IFA-514照射試験は、軽水炉用MOX燃料の照射特性を把握することを目的として、ノルウェー国ハルデン炉(HBWR)で実施した。燃料要素の仕様はBWR 8$$times$$8燃料に準じ、PuO$$_{2}$$富化度を5.8wt.%としたMOX燃料を用いた。本照射試験では、ペレットの形状(中実/中空)や表面粗さ(研磨/未研磨)をパラメータとした6本の燃料要素が集合体平均燃焼度約45GWd/tまで照射され、その間の炉内挙動データ(被覆管伸び,燃料スタック伸び,燃料中心温度,燃料要素内圧)を取得するとともに、燃料要素に顕著な腐食,変形等は認められなかった。IFA-565照射試験は、IFA-514照射試験で用いた6本の燃料要素のうち3本を集合体平均燃焼度約56GWd/tまで継続照射したものであり、燃料要素に顕著な腐食,変形等は認められなかった。一連の照射後試験結果から、FPガス放出挙動はBWR UO$$_{2}$$燃料及びATR MOX燃料のそれと類似しており、FPガス放出挙動に差は認められなかった。また、中空ペレットのFPガス放出率(約13%)は中実ペレット(約16%)より低かった。さらに、金相試験結果及び燃料スタック伸び量から、いずれの燃料要素でも顕著なPCMIは生じておらず、ペレット形状(中実/中空)がPCMI挙動に及ぼす影響に明確な差は認められなかったが、中空ペレットの方が中実ペレットに比べて被覆管外径変化率はやや小さく、外径変化の抑制効果があるものと考えられる。

IFA-514 irradiation test was performed in Halden Reactor (HBWR) in Norway to study the irradiation performance of LWR MOX fuels. The fuel specifications for this irradiation test were decided in accordance with those of BWR 8 $$times$$ 8 fuels, and plutonium content of MOX fuels was 5.8 wt.%. Six MOX fuel rods, of which parameters were pellet geometry (solid or annular) and surface roughness (grinded or as-sintered), were irradiated to the assembly average burn-up of 45 GWd/t, and the instrument data during irradiation, i.e,. cladding elongation, fuel stack elongation, fuel center temperature, and rod inner pressure, were obtained, and subsequent post-irradiation examinations indicated no remarkable corrosion and deformation on the irradiated fuel rods. The irradiation for three of six fuel rods irradiated in IFA-514 irradiation tests were continued to the assembly average burn-up of 56 GWd/t in IFA-565 irradiation tests, and the results of this irradiation test inditated no remarkable corrosion and deformation on the irradiated fuel rods. The FP gas release behavior of LWR MOX fuels was similar to that of BWR UO$$_{2}$$ and ATR MOX fuels, and no difference was confirmed in the FP gas release behavior. Also FP gas release rate of annular pellets (13 %) was lower than that of solid ones (16 %), and no difference in the effect of pellet geometry, i.e. solid and annular, on PCMI behavior was confirmed since no evidence of remarkable PCMI taking place was observed in any fuel rod from the instrument data of fuel stack elongation and the results of ceramography, but the pellet geometry is expected to have an effect of the cladding diameter change reduction since the cladding diameter change of annular fuel rods was less than that of solid ones.

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