AN EXPERIMENT TO MEASURE EFFECTIVE DELAYED NEUTRON FRACTIONS

>An experimental measurement of the effective delayed neutron fraction ( beta -bar) was made for a clean critical assembly by determining the asymptotic period associated with introduction of a known amount of reactivity. The "known amount" of reactivity was obtained by replacing, uniformly throughout the reactor, a small quantity of U/sup 235/ with an alloy of B/sup 10/ and Hf designed to match the absorption properties of U/sup 235/. The replacement was thus equivalent to a uniform reduction in nu , the number of neutrons emitted per fission from the fuel. Such a reduction introduces a reactivity change equal exactly to delta nu / nu /sub 0/. Two analyses of the experiment were made using different high energy cross sections in conjunction with four group, two dimensional diffusion theory. The measured value of beta lay between the results of these computations, the error spread (an average rms error of plus or minus 5.2%) being too great to permit any conclusion regarding the significance of the comparison. (auth