Optimization of Reactor Shutdown Margin Measurements in High Gamma Fluxes

Since the quality of subcriticality measurements using neutron noise analysis in power reactors is degraded by high gamma-ray intensities produced by residual fission products, a study was made to derive an equation that can be used to predict this degrading effect quantitatively. For simplicity, this study is based on one-detector measurements but is equally appropriate to two-detector cross-correlation measurements. Equations derived to give insight into the behavior of ionization chambers operating in high gamma environments have led to the experimental development of a relatively low-cost, optimized chamber.