Abstract
The absolute fission rates of 235U, 237Np, 238U and 232Th were measured in four types of spherical blanket assemblies containing lithium and/or natural uranium and/or graphite. The results of measurement are compared with those of one-dimensional transport calculations employing 100-group neutron cross-sections obtained from the ENDF/B-IV data file. It is shown that the ratios between calculated and experimental values of 232Th, 238U and 237Np fission rates decrease with distance from the assembly center, where D-T neutrons are generated. An overestimation of about 50% is observed in the calculated 235U fission rate for the graphite reflector region. One of the main sources of the disagreement proves to lie in the inability of the codes adopted for generating the multi-group cross-section to take account of the angular distributions of the secondary neutrons emanating from nonelastic reactions. The results of the analysis indicate that the method of calculation currently employed in fusion reactor neutronics overestimates the reflection of neutrons and underestimates the penetration of fast neutrons when a graphite reflector is used.
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