On-line determination of the prompt fraction of in-core neutron detectors in CANDU reactors

Paper in proceeding, 2004

This paper describes a new method for determining the prompt fraction of in-core neutron detectors in CANDU reactors. The method is based on noise analysis, and thus does not require any perturbation of the reactor operation. This method is therefore very well suited to on-line monitoring, and could detect early detector degradation as the detectors age. The prompt fraction estimation of the in-core neutron detectors is of prime importance in CANDU reactors, since these detectors are used by the reactor shutdown systems, and should consequently respond very quickly to any flux change. This new method is based on the fluctuations of the light water levels in the water compartments at a frequency of roughly 0.25 Hz. These fluctuations are due to the control cycle of the regulating system of the reactor, and are equivalent to a spatially-distributed noise source of variable strength. The induced neutron noise can be monitored by the in-core neutron detectors, but only the prompt component of these detectors is able to follow these fluctuations at 0.25 Hz. Comparing the measured detector signals to the neutron noise estimated by core calculations allow determining the prompt fraction of the detectors.