Measurements of multiple control rods effect in graphite moderated reactors

Measurements on the effects of multiple control rods arranged in ring geometry were undertaken with application of the pulsed neutron technique in SHE, a heavily reflected graphite-moderated 20% enriched uranium critical assembly. The aggregate number of experimental cases exceeded 100, which systematically covered the four fundamental factors to determine the control rod effects: 1. The atomic ratio of 235U to C in the core region (C/235U = 2,226 and 5,378)2. The neutron absorbing substances in the control rods and their densities (B4C powder and Cd plate. Density of B4C: 0.1 and 1.0 g/cm3)3. Geometrical factors of the control rods (Cylindrical shape, 20 or 44 mm in diameter)4. Number of control rods (One to six control rods inserted in circular geometry) The experimental results are compared with calculations based essentially on a system of simple two group Nordheim-Scalettar method used as tool to solve the three-dimensional diffusion equation. Agreement within about 10% was obtained between measurement and calculation utilizing extrapolation distances for the fast neutron group, determined semi-experimentally.