報告書番号

T03069

タイトル（和文）

核分裂環境下での蛍石型セラミックスの照射挙動解明 -- タンデム加速器を用いた模擬照射試験 --

タイトル（英文）

Radiation effects in fluorite structure ceramics CeO2 under fission energy ion irradiations as simulations of fission field in nuclear fuels

概要 （図表や脚注は「報告書全文」に掲載しております）

軽水炉燃料の高燃焼時に発生する燃料細粒化現象の形成機構を解明するために、UO2模擬物質かつイナートマトリックスの候補材であるCeO2結晶にタンデム加速器を用いた高エネルギーイオン照射試験を行った｡本報告では特に高エネルギー核分裂片による電子励起の効果についての知見を得た｡(1)Xe, I, Zrイオン照射試験から核分裂生成物で生じる電子励起の影響範囲は約5～7 nmφと推測でき、またイオントラック径の2乗と電子的阻止能が比例する傾向を示した。(2)照射温度300℃はイオントラック形状に大きな影響を及ぼさなかった｡(3)薄膜試料ではイオントラック重畳により結晶粒の微粒化または非晶質化が進行する事が示された｡(4)バルク試料では高照射量でも結晶構造の乱れは観察されず、バルク効果と電子励起の効果により照射欠陥の回復が発生した可能性が示された｡(5)高照射量までXe照射したバルク試料表面で低燃焼時の軽水炉燃料に類似した凹凸状の粒界表面変化が観察された｡

概要 （英文）

High burnup extension of LWR fuel is progressing to reduce the amount of total process flow in the nuclear fuel cycle and eventually to reduce the fuel cycle costs. In the periphery region of high burnup fuel pellets in LWRs, a crystallographic re-structuring is commonly observed, as named "rim structure", which is characterized by the existence of highly dense small sub-grains whose size is approximately 200 nm, and the accumulation of small pores with average size around 1 μm. This structure shall be formed by the accumulation and mutual interactions of radiation damages, fission products (FPs) and electronic excitations deposited partially by nuclear fissions. In order to separate each processes and understand the mutual interactions among them, 70 ‐ 210 MeV FP ions (Xe, I, Zr) irradiation examination on CeO2, as a simulation fluorite ceramics of UO2, have been done at JAERI-Tandem facility.This paper shows the effects of electronic excitation on microstructural evolution in CeO2, as follows; 1) High energy Xe, I, Zr ion irradiations clarify that the effective area of electronic excitation by high energy fission products might be around 5-7 nmf, and the square of ion track diameter tend to be proportional to the electronic stopping power (Se), 2) There are few effects on the inner-microstructure of ion tracks under the irradiation temperature 300 degree C, 3) In the microstructural evolution in wedge-shaped TEM specimen under high energy ions irradiation, the elliptical deformation of diffraction spots and faint "halo-pattern" become clear as the fluence becomes higher, > 1 x 1012 ions/cm2. These results indicate that the inner structure of ion tracks under room temperature shall be amorphous or grain-subdivided. 4) By the cross-sectional observation of bulk specimens under high fluence ion irradiation, some recovery effects of radiation damage by "bulk-effect" and "electronic excitation" have been indicated. 5) Under the irradiation with 210 MeV Xe ions whose fluence is 1 x 1015 ions/cm2, some typical surface transients that the grain surfaces become crenellated are observed, and the similar transients have been shown in the low burnup UO2 fuel.

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