U²³⁸ Epithermal Capture in PWR-1 Blanket Clusters

A measurement of the epithermal radiative capture in U²³⁸ has been carried out in a natural UO₂-fueled blanket cluster of the nuclear mock-up of PWR Core 1. Analysis indicates that a substantial increase (∼20%) in epithermal captures in a natural uranium metal plate fuel cluster should occur in the fuel elements adjacent to a wide intercluster water channel. The experiment shows that the captures in a cylindrical UO₂ fuel element at the edge of the bundle is only 7% greater than in a neighboring fuel element. However, the radial distribution of captures in the first fuel rod shows that the captures near the wide intercluster water channel are 65% greater than at an equivalent position on the side of the rod away from the water channel. Calculations of the relative epithermal U²³⁸ captures in the cluster have shown that diffusion theory predicts the spatial dependence of the captures in the interior of the cluster but fails near the edge of the bundle. Monte Carlo analysis confirms the observed increase in the captures in a fuel rod at the edge of the bundle, although the precision of the analysis does not make a quantitative comparison feasible.