Heterogeneity Study for an Intermediate-Energy Core Programme

An intermediate energy spectrum core programme has been proposed and will be carried out very soon in the JAERI fast critical facility, especially for obtaining information on the group constants in the resonance-energy region. Various analytical results, obtained for the programme by use of the ABN cross-section set, are discussed. The intermediate-energy core will be composed of 20%-enriched uranium metal plates and a suitable amount of hydrocarbon compound or beryllium. To obtain information on the cross-sections from the so called integral experiments, it is necessary to carry out the experiments in the varied series of an appropriate system. One such series can be achieved by changing the moderator atomic density NBe/N235 in the range 0 to 100. It is shown that valuable information on the cross-sections in the resonance-energy region may be obtained from various experimental results, including those on the criticality, the heterogeneity effect on the criticality induced from the configuration of fuel plates, and the Doppler effect, because the contribution from the resonance-energy region to the experimental results is large. The heterogeneity effect on the criticality in the intermediate-energy core is shown to be interesting and different from that in the fast reactor system. This is caused by the difference in the fission-capture ratios of the heterogeneous and homogenous system due to the different self-shielding factors of fission and capture cross-sections. Doppler measurement by heating a sample is discussed with emphasis on the heterogeneity effect of the sample. Highly sensitive Doppler reactivity change of the sample and considerable amount of the moderator used in the system are useful for measuring the heterogeneity effect of the sample. The importance of the contribution of gross neutron spectrum variation to the Doppler effect of the system are also discussed in a sample calculation. (author)