Gamma-Sensitive Self-Powered Detectors and Their Use for in-Core Flux-Mapping

In-core gamma sensitive self-powered detectors have desirable characteristics such as instantaneous response and low burnup, but they do not directly measure the thermal neutron flux. Their usefulness thus depends on how well the in-core gamma flux distribution follows that of the thermal neutton flux, or more precisely how the distribution in the detector current follows that of the thermal neutron flux. To determine the behavior of these detectors we have computed the response of Pt and Pt-clad Inconel self-powered detectors to gammas from .1 to 8 MeV including the effect of recoil electrons produced in the surrounding material. Using these results and gamma spectra obtained from reactor design calculations the distributions over a PWR core of the current from each detector was determined and compared with that of the thermal neutron flux. The comparison showed that the distributions from either detector and the thermal neutron flux remained consistent within about 5% over most of the core but deviations of 10-15% occurred near the radial boundary of the core and 20-30% occurred at the very top and bottom. Thus a single calibration constant can be used to relate detector current to neutron flux for all the self-powered detectors in the core except those located very near the core boundaries.