The study of pore and defect structures of prestressed nuclear graphite by the radiotracer method using krypton-85

Radiotracer method by use of 85Kr was applied to non-stressed and prestressed nuclear graphites for the purpose of examining its effectiveness for detecting any microstructural difference among the graphites used in the study or any changes in pore and defect structures caused by prestressing. Materials used were three kinds of nuclear graphites, grades SM1-24, 7477PT and H327. After impregnation of specimens with 85Kr at 3.4 x 102 kPa, the counting rates for β- and γ-rays which resulted from the residual 85Kr atoms were measured as a function of exposure time in air for the specimens prestressed to 0, 0.2 or 0.9 σm, where σm is the mean compressive strength. Three kinds of graphites behaved in different ways: In the case of SM1-24 graphite, the initial counting rates for β- and γ-rays increased with increasing prestress level, whereas the apparent half-life, which was defined as the time necessary for the counting rate to be a half of the initial rate, decreased as the prestress level increased. For 7477PT graphite, prestressing to 0.2 σm made the initial rate smaller than that for the non-stressed. In the case of H327 graphite no strong influence of prestress was observed. From these results, it was concluded that the changes in pore and defect structures caused by prestress can be detected by the technique employed here at least for SM1-24 graphite. (author)