Abstract
The critical equation in the 3-group diffusion approximation was improved by taking into account all the epithermal reactions. For the effective cross section, Westcott's formulation is followed, wherein the s-factor was modified so as to give the epithermal effect accurately. In connection with evaluating the modified s-factor, simple formulas for the effective resonance integrals of U²³⁵ and U²³⁸ were derived for a very dilute homogeneous system and for a natural uranium rod. The present method was applied to the criticality calculation for JRR-3 (Japan Research Reactor No. 3), a natural uranium-heavy water moderated heterogeneous reactor, which went critical on Sept. 13, 1962. The prediction as to the criticality was verified fairly well. (auth)
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