A Graphite Moderated Critical Assembly--CA-4

An essentially bare graphite moderated critical assembly having dimensions of 51.0"x51.0"x44.11" and a critical mass of 52.48 kg of U235 was constructed at the DRNL Critical Experiments facility. Using a value for the buckling, B2, of 0.0018628 cm-2 and an extrapolation distance of 2 cm, a multigroup calculation for the assembly gave an effective multiplication of 0.9912. The self-shielding factor due to lumping of the fuel in 0.01” discs was measured and found to be 0.94 Ł .04 compared to a calculated value 0.95. Values of 0.15 ev for the mean fission energy and 27.66% for the fraction of fissions in the thermal group were calculated. Core removal type control rods were calibrated by the "rod-drop" method and by the observation of stable reactor periods. The form of the control rod calibrations curves indicate a contribution due to neutron streaming in the void formed by withdrawing the rod as well as the expected cosine squared distribution. A comparison using a flat strip of cadmium as a poison rod, which left essentially no void when removed. Showed good agreement with a cosine squared sensitivity curve. The loss in reactivity as a function of the separation of sections of the assembly showed a loss of multiplication of 0.00635 due to a gap 0.3" wide, in fair agreement with a recent calculation, but approximately double the value calculated by an earlier method. Bare indium and cadmium covered indium foils were exposed in various parts of the assembly to observe the flux distribution both macroscopically and microscopically. Power distributions were observed by means of aluminum catcher foils in contact with the uranium. An experiment showed the catcher foil measurements to be insensitive to the neutron spectrum. Comparison between fission rates of bare, cadmium covered, and cadmium-indium covered fuel discs gave values of the ratio.