The Performance of Self-interrogation Neutron Resonance Densitometry in Measuring Spent Fuel

The Self-Interrogation Neutron Resonance Densitometry Technique(SINRD) technique is one of the fourteen nondestructiveassay (NDA) techniques investigated under the Next GenerationSafeguards Initiative (NGSI) effort. SINRD shows promising capabilityin determining the 239Pu and 235U content in spent nuclearfuel. SINRD is a relatively low-cost and light-weight instrument,and it is easy to implement in the field. SINRD makes use of thepassive neutron source existing in a spent fuel assembly, and ituses ratios between the count rates collected in fission chambersthat are covered with different absorbing materials. These ratioswere correlated to certain attributes of the spent fuel assemblysuch as the total mass of 239Pu. Using count rate ratios instead ofabsolute count rates makes SINRD less vulnerable to systematicuncertainties. Building upon the previous research performed byLafleur et al., this work focuses on the underlying physics of theSINRD technique: understanding the neutron energy spectrumat various locations inside and at the edge of the fuel assembly;understanding which isotopes have major impacts on the SINRDsignal, by weighting the resonance absorption caused by each ofa few isotopes deemed important in certain energy windows (dictatedby the absorbing material covering the fission chambers);and understanding the spatial dependence of the count rate oneach row of the fuel rods. The results of these studies show thatwater gap of 0.5 cm or more smears out the structure of the spectrumin the energy ranges important to SINRD; 239Pu is the mostimportant nuclide that affects the count rate in the “Gd-Cd”window, and 235U has a significant impact as well, especially inthe low burnup case; only the first three rows next to the detectorhas a significant impact on the signal. In short, this work providesinsights into the factors that affect the performance of SINRDmost and it will help to improve the hardware design and thealgorithm used to interpret the signal for the SINRD technique.