Migration areas and effective delayed neutron fractions by critical experiments

Abstract

Migration areas and effective delayed neutron fractions are measured in critical assemblies of slightly enriched uranium rods in light water. The fuel consists of uranium rods, enriched to 1·3 per cent 235U, 4 ft long, 0·387 in diameter and clad in aluminium. Water to uranium volume ratios of 2 : 1, 3 : 1 and 4 : 1 are investigated. Migration areas are measured by observing the effect of fuel rod changes and boron poison in the water, separately and combined, on the excess reactivity. The analysis yields a value of M 2 which depends on the delayed neutron parameters, and one which is independent of them. Values of M 2 obtained by these methods are compared with those obtained earlier by sub-critical methods. A value of the effective delayed neutron fraction, γβ, in these assemblies is derived from the effect of uniform boron poison on the reactor periods. An average value of 00069 ±0·0001 is obtained for the three assemblies.