A theoretical model of boiling water reactor neutron noise

The calculated results of the neutron noise model show good agreement with experimental data when the noise sources driving boiling are assumed to be axially uncorrelated and the resultant void fluctuations propagate through perturbing the attenuation characteristics of the core. The results are, in general, not dependent on the neutron energy group structure.

The model results are compared with previous theoretical studies. It is demonstrated through a one-dimensional radial neutronic model of the core that the local component of BWR noise results from a shielding effect (attenuation noise) due to steam bubbles passing between the fuel and the neutron detector, while global noise is due to the reactivity (fission chain) effect associated with boiling and spatially correlated driving noise sources.

A one-dimensional axial neutron transport model with radially homogenized cross sections is shown to overestimate the detector response to perturbations propagating through the fission chain. The resulting increase in the detector spatial field of view when applied to the void fluctuation model produces oscillations in the phase angle versus frequency between axially separated neutron detectors. When perturbations are propagated primarily through attenuation effects in a one-dimensional model, the phase angle of axially separated detectors is linear with frequency displaying the characteristic lag time of steam bubble transport.