Impact of transmutation issues on interpretation of data obtained from fast reactor irradiation experiments

The subject of fission–fusion correlation is usually cast in terms of reactor-to-reactor differences, but recently the fusion community has become aware of the impact of differences within a given surrogate facility, especially in constant time experiments when different dose levels are attained in different positions of one reactor. For some materials, it is not safe to assume that in-reactor spectral variations are small and of no consequence. This point is illustrated using calculations for fusion-relevant materials that were irradiated in the Fast Flux Test Facility–Materials Open Test Assembly (FFTF–MOTA) over a wide range of in-core and out-of-core positions spanning more than two orders of magnitude in dpa rate. It is shown that although both the neutron spectrum and flux changes, the spectral effectiveness factor, dpa/1022 n/cm2 (E>0.1 MeV), remains remarkably constant over this range. The transmutation rate per dpa varies strongly with reactor position, however.