Instrumental methods for neutron spectroscopy in the MIT blanket test facility

Abstract

U.S. Atomic Energy Commission contract

The energy spectrum of the neutron flux in a realistic mockup of the blanket region of a large liquid-metal-cooled fast breeder reactor was measured using three different spectrometers: He-3 and Li-6 semiconductor detectors and a Proton-Recoil proportional counter. The He-3 detector was operated in the sum and difference modes, and the Li-6 detector in the sum, difference and triton modes. The experimental data was unfolded using direct, integral and derivative techniques. Methods were developed or perfected to enable use of the He-3 detector over the neutron energy range from 10 keV to 1.3 MeV and the Li-6 detector from 10 keV to 3.1 MeV; the Proton-Recoil detector was operated in the region from 2 keV to 1.5 MeV. In general, good agreement was found between the experimental measurements for all detector types, modes of operation and methods of unfolding, except for the low-energy He-3 data. The present experimental results and previously reported data obtained using a method based on gamma line broadening are in relatively good agreement in the high energy region above 0.8 MeV. The measured neutron spectrum is also similar in shape to neutron spectra measured at ANL in critical assembly mockups of large LMFBR cores, but systematically softer, as expected.. However, there is a large discrepancy in the energy region from 10 keV to 50 keV between the present results and either spectra unfolded from foil data or those numerically calculated using the 1-D ANISN code in the S8 option with 26 energy groups.