Critical separation between subcritical clusters of 4.29 wt % 235U enriched UO2 rods in water with fixed neutron poisons. Technical topical report

Measurement data are presented for a series of criticality experiments on systems simulating fuel element shipping containers and fuel storage basins. The conditions investigated involve clusters of 4.29 wt% 235U enriched UO2 fuel rods immersed in water. The number of rods required for criticality in a square lattice spacing (25.4 mm) near optimum neutron moderation and the critical separation between three subcritical clusters aligned in a row was determined. The effect on this critical separation between clusters was determined for the following neutron absorbers: 304L Steel with 0, 1.1, and 1.6 wt% boron; boral; copper with 0 and 1 wt% cadmium; cadmium; aluminum; and Zircaloy-4. Fuel rods are 12.649 mm in diameter, clad in 0.660 mm thick, 12.827 mm ID aluminum tubing and varied in length from 914 mm to 927 mm. The neutron absorbers are in the form of plates and are positioned between the fuel clusters such that no line of sight existed between the clusters. Measurements were made with each absorber plate located approximately midway between the fuel clusters and with the plates near the respective cell boundary of the center fuel cluster