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JRR-4反射体要素黒鉛反射材の照射成長に関する調査

Irradiation growth of graphite reflector installed in JRR-4

八木 理公; 堀口 洋徳; 横尾 健司; 大山 光樹; 楠 剛

Yagi, Masahiro; Horiguchi, Hironori; Yokoo, Kenji; Oyama, Koji; Kusunoki, Tsuyoshi

JRR-4において1体の反射体要素の溶接部に割れを確認した。調査の結果、反射体要素の割れの主たる要因は、要素内部の黒鉛反射材の膨張であり、膨張は高速中性子の低温照射によるものと考えられた。また、放射線透過試験の結果、割れを生じていない他の反射体要素についても黒鉛反射材が照射成長により膨張していることを確認した。JRR-4照射環境下における黒鉛IG-110の照射成長挙動を明確にするため、これまで使用してきた反射体要素を分解し、黒鉛反射材の外観観察及び寸法測定を行った。その結果、いずれの反射体要素においても黒鉛の寸法は高速中性子照射量の増大とともに大きくなることが確認できた。また、JRR-4環境下における照射成長係数(単位高速中性子照射量あたりの寸法変化率)は、高速中性子照射量2.5$$times$$10$$^{24}$$n/m$$^{2}$$以下において、最大7.13$$times$$10$$^{-25}$$%m$$^{2}$$/n,最低4.21$$times$$10$$^{-25}$$%m$$^{2}$$/n,平均5.71$$times$$10$$^{-25}$$%m$$^{2}$$/nであった。

A crack had been found on the weld of one reflector element in JRR-4. A survey revealed that the cause for the crack was the expansion of graphite reflector in the reflector element. It appeared that the expansion of graphite reflector was caused by fast neutron irradiation at low temperature. The survey confirmed radiographically that graphite reflectors in the other reflector elements without the crack expanded similarly by the irradiation growth. Irradiated graphite reflectors were carefully observed and were precisely measured the three dimensions after dismantling the irradiated reflector elements in order to understand quantitatively the irradiation growth behavior of IG-110 graphite under the JRR-4 operation condition. As the results, it was confirmed that growth of graphite reflectors increased with increasing of fast neutron fluence. The maximum irradiation growth per fast neutron fluence was 7.13$$times$$10$$^{-25}$$%m$$^{2}$$/n, the minimum was 4.21$$times$$10$$^{-25}$$%m$$^{2}$$/n, the average was 5.71$$times$$10$$^{-25}$$%m$$^{2}$$/n in the range of fast neutron fluence below 2.5$$times$$10$$^{24}$$n/m$$^{2}$$.

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