Heterogeneous effects of sodium and $sup 238$U and the ratios of some cross- sections in BFS-22

The role and magnitude of the effects of the heterogeneous arrangement of materials in the central reactor zone is important in reactor planning. In various zones of a critical assembly BFS-22 the distribution of the U-238 (n, gamma) and Na-23 (n, gamma) reactions was investigated in samples of natural uranium and metallic sodium. The increase in gamma rays from Np-237 and Na-24 was followed. The following reaction cross-section ratios were determined: Na-23 (cap)/U-235(fis), U-238(cap)/U-235(fis), and U-238(fis)/U-235(fis). The Na-23 (cap)/U-235(fis) ratio was calculated to be 0.77 x 10/sup -3/, but found to be 0.580 x 10/sup -3/. The U-238(cap)/U-235(fis) was calculated as 0.133, and found to be 0.128 experimentally. The U-238(fis)/ U-235(fis) ratio was calculated to be 0.029, and found to be 0.029 experimentally. The results indicate that in some cases it is necessary to take into account the effects of resonance blocking with a heterogeneous arrangement of materials in the central reactor zone. (SJR)