Space and energy effects in reactor fluctuation experiments

An analysis of the space and energy dependence of neutron fluctuations is presented, and applied to the results of reactor fluctuation experiments. Two experiments, measurements of Power Spectral Density and Variance to Mean, are singled out for special attention. These are of special interest, since previously published experimental results seem to be interpretable by space and energy-independent theories of neutron fluctuations.

Both infinite and finite-geometry reactor models are treated, and calculations are presented to illustrate the effects of detector size, shape and location, and delayed neutrons. In any event, agreement with actual experimental results is achieved only when the effect of fast diffusion is included through a two-energy group model. It is seen that when the migration length is sufficiently large by comparison with a typical reactor dimension, the results of the space-energy independent theory appear as a first approximation. Thus it is argued that experiments on larger reactors would yield results not in agreement with the space-energy independent theory.