The Effects of irradiation condition and microstructural change on lattice parameter, crystal lattice strain and crystallite size in high burnup UO pellet

高燃焼度UOペレットの格子定数,結晶格子歪及び結晶子径に及ぼす照射条件と微細組織変化の影響

天谷 政樹  ; 中村 仁一 ; 更田 豊志

Amaya, Masaki; Nakamura, Jinichi; Fuketa, Toyoshi

ノルウェーのハルデン重水炉で照射された3種類の燃料棒からペレット試料(平均燃焼度:37-62GWd/t)を採取し、その微細組織変化を光学顕微鏡観察、SEM/EPMA及び微小X線回折により調べた。測定された格子定数は以前に報告された値より小さい傾向を示し、格子定数が照射試験中の温度条件や照射中に試料内部に生じた微細組織変化の影響を受けたと考えられる。均一歪及び不均一歪の燃焼度依存性を考慮すると、次のことが示唆される。均一歪の原因となる格子間原子が、照射欠陥の回復過程の一つとしてまず転位を形成し始め、転位密度が増加する。燃焼度が増加するにつれて結晶子内部の転位蓄積量が飽和し、結晶子内部の照射欠陥の回復過程として転位の移動が支配的になる。

Pellet samples (average burnup: 37-62 GWd/t) were prepared from three kinds of fuel rods which were irradiated in the Halden Heavy Water Reactor in Norway, and microstructural changes in the pellet samples were investigated by means of optical microscopy, SEM/EPMA and micro-X-ray diffractometry. The measured lattice parameters tended to be smaller than the values reported previously, and it is likely that the measured lattice parameters were affected by the temperature conditions during the irradiation tests and the microstructural changes which occurred in the samples. Considering the burnup dependence of uniform and non-uniform strains, the following things are suggested: the interstitial atoms which cause uniform strain begin firstly to form dislocations as a recovery process of irradiation defect and the dislocation density increases. With increasing burnup, the accumulation of dislocations in the crystallite saturates and the migration of dislocations becomes dominant as a recovery process of irradiation defects in the crystallite.