Hideo Kuroi

Adjustment of Group Cross Sections by Means of Integral Data, (I):Theoretical Study

A theoretical treatment is developed for the adjustment of group cross sections making use of integral data such as critical mass, reaction rate ratio and sample worth ratio obtained from various fast critical experiments. The method of least squares is the usual practice in such treatment. In the present case, however, the total number of integral data available is usually smaller than that of the group cross sections to be adjusted. To overcome this difficulty, the observation equations for applying the least squares treatment are established by using both integral and differential data. In such treatment, the correlation between group cross sections can be easily taken into account. General formulas are presented on two kinds of such correlation, one based on nuclear theory and the other due to relative measurements of the cross sections. The χ2-test for the sum of squares of the residual is used as criterion to judge whether or not group cross sections carry systematic error. If systematic error exist...

A Proposed Concept for Actinide-Waste Transmutation

Underground nuclear waste disposal in stable geological formations requires predictions over periods of the order of 100,000 years; although these periods are not long by geological standards, it is difficult to get public acceptance for strategies based on such seemingly long-term forecasts. A concept for the transmutation of higher-actinide wastes from reactors is considered which might contribute to a significant reduction of the biological hazard of the wastes and thus help public acceptance of an expanding nuclear-power economy. Our present knowledge of the nuclear cross-sections of actinides is not sufficiently accurate for reliable forecasts of the behavior of the proposed Actinide-Burning Fast Reactor (ABFR) which will be characterized by a very hard neutron spectrum. Accordingly, a series of measurements is underway at the Japan Atomic Energy Research Institute to check seemingly favorable calculations on the possibility of operating an ABFR entirely on higher-actinide mixed-oxide fuel.

Effect of group collapsing in perturbation theory for sample worth analysis

The effect of group collapsing applied to the perturbation theory for sample worth analysis in fast reactor systems is theoretically and numerically examined assuming the validity of the thin sample approximation. As a result, the calculated worths of scattering predominant materials placed at the center of core are found to be strongly influenced by the group collapsing. The effect on the sample worth when the sample is placed in positions off the core center decreases with increasing distance from the center. It is noted that the reactivity perturbation due to inelastic scattering is also affected significantly by group collapsing especially near the core-blanket interface. Based on the above observations, it is concluded that the perturbation theory with about 70 energy groups appropriately arranged is necessary to reproduce the experimental values of Na, O and Fe sample reactivity worths with accuracy efficient for ordinary purposes.

DISTORTION OF NEUTRON COLLISION DENSITY DUE TO RESONANCE SCATTERING AND ABSORPTION

An analytical method for obtaining the collision density and the resonance integral in a homogeneous mixture of moderator and absorber is presented. The method incorporates ortho-normal expansion without any approximation such as narrow or wide resonance. A new ortho-normal set of functions of which the weight function is of shifted Breit-Wigner type is introduced for this purpose. Sample calculations are carried out for a mixture of 59Co and heavy water, which is a typical case far removed from the two approximations mentioned. Comparison of the results with numerically obtained exact solutions shows that the first four terms of the expanded series are sufficient to express the collision densities in this case.