Effect of the composition of radiation on the radiation damage to graphite

A statistical analysis is performed of the results on the determination of the critical neutron fluence in MR, SM-2, and BOR-60 with different irradia tion temperature. It is shown that the critical neutron fluence depends nor only on the irradiation temperature but also, and to an even greater exten t, on the radiation composition factor (ratio of the neutron and gamma-ray- flux densities). Thus the critical neutron fluence for irradiation at 600 d egrees C in MR (radiation composition factor; 0.13) is 17.10(21) cm(-2) and in SM-2 (radiation composition factor 0.1) 11.10(21) cm(-2) at the same te mperature. When the same graphite is irradiated in the region of the outer corner of a working block of RBMK, where the radiation composition factor i s 0.55, it is expected that the critical neutron fluence will be 31.7.10(21 ) cm(-2). In summary, taking account of the effect of gamma-radiation introduces subs tantial corrections: the experimental results obtained in research reactors are found to be at least a factor of 2 too low. This gives hope of substan tiating the substantial increase in the service life of the RBMK graphite m asonry.