Tadahiro Ohnishi

Reactivity Worth of Fuel Elements: Measurement and Perturbation Analysis

Reactivity worths of fuel elements were measured in the Ozenji Critical Facility (OCF) and analyzed with three group perturbation method. The result shows that the worth of one single fuel pin can be well predicted by calculation over a very wide range of the core spectrum, namely from a lattice of 2.5% enrichment and 0.43 volume ratio to that of 1.5% enrichment and 3.5 volume ratio. The analysis indicates the importance of thermal neutron flux peaking remaining after the removal of a fuel pin. Only by incorporating this effect can the reactivity worth of a fuel pin be correctly evaluated. In the present study, the neutron spectrum in the water hole where the peaking occurred was assumed to be the same as in the reflector. The reflector spectrum seems to provide better agreement with experiment than the core spectrum. Validity of the analysis was extended to a bundle of sixteen fuel pins by measuring the reactivity worths of bundles of fuel pins as well as the thermal neutron flux distributions. One dimen...

Physics Studies on BWR Void Simulated Cores, (I)

In an attempt to obtain more information concerning steam void effects on the physical behavior of BWR cores, voids ranging from zero to 24% volume fractions were statically simulated by inserting polyethylene tubes between fuel rods of simulated BWR core, and measurements were made of local flux distribution, neutron spectrum, reactivity, reactivity coefficient of void and so forth. The results were compared with calculations. Improved methods of calculation were developed, which assured better agreement with measured results. While this paper only deals with local distributions of the thermal neutron flux and spectrum within the assembly cells, it has been shown that spectrum variation near water gaps could be well described by an overlapping model which provided an improvement on the analysis. An additional improvement was obtained by the incorporation of an approximate transport correction.