On use of multi-chambered fission detectors for in-core, neutron spectroscopy - 7569

With their small size, it may be possible to use several, co-located sub-miniature or micro-pocket fission detectors to measure a neutron spectrum at a point in a reactor core. In previous studies, it was proposed to deploy two-chamber devices loaded with U and Th, respectively, in order to distinguish between thermal and fast fluxes. However, the threshold energy for Th fission is approximately 1 MeV, too large to provide sufficient detail in the epithermal region. Here, alternative reactants, in greater number, were explored to understand how the resulting signals from multiple chambers could be used to resolve the neutron spectrum. In particular, it was hypothesized that a sufficient number of chambers with unique reactants can act as a real-time, foil-activation experiment. An unfolding scheme based on maximizing (Shannon) entropy was used to produce a flux spectrum from detector signals that requires no a priori information. To test the method, integral, detector responses were generated for single-isotope detectors of Th-232, U (233, 234, 235, and 238), Np-237, and Pu (238, 239, 240, 241, and 242) using a simplified, pressurized-water reactor spectrum and flux-weighted, microscopic, fission cross sections, in the 69-group format. An unfolded spectrum was found from subsets of these responses that had a maximum entropy while reproducing the responses considered and summing to one (that is, they were normalized). Three, specific cases were considered: (1) use of only U-235, U-238, and Th-232, (2) use of U-235, U-238, Th-232, Np-237, and Pu-238, and (3) use of all nuclides listed previously. The three cases led to average, relative, group-wise errors of 201%, 162%, and 63%, respectively. Although inclusion of Pu-238 substantially reduced major errors between energies of 0.78 and 2.1 eV, residual errors in this energy range and at lower energies suggests greater information in the thermal regime is needed. (authors)