Temperature coefficients are calculated and compared with measured values for the three loading patterns in a light-water-moderated core of the Kyoto University Critical Assembly (KUCA). The H/235U ratio varies from 159.2 to 315.6 for these loadings. The methods employed are (a) one-dimensional criticality calculation, (b) two-dimensional criticality calculation, and (c) onedimensional perturbation calculation.
Group constants are obtained for five temperatures after convergence is reached in the iterative process of criticality calculations combined with energy spectrum evaluations. The calculation based on these group constants successfully reproduced the decrease of the coefficients with increasing H/235U ratio. lt also reproduced the slope of the curve in the temperature-coefficient vs. temperature plot, although discrepancies remained in the magnitude of the quantity. The perturbation calculation revealed that the core region makes a negative contribution, in contrast to the positive contribution of the reflector. The main contribution to the former comes from the decrease of thermal fissions, while the latter from the decrease of thermal and fast absorptions.
The mentioned agreement between the tendencies of the theoretical and the measured values is attributed to the consistent determination of the reflector saving δ, and of the fast-group inverse relaxation length κ of the reflector in the process of fast-group constant production.
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