Non-Destructive Assay of Spent Nuclear Fuel Using Gamma-Ray Mirrors
as a Narrow Band Pass Filter

As a material of concern for proliferation, we are interested in measuring the Pu content of spent fuel by comparing the ratio of fluorescence K-shell emissions from U and Pu. With emission stimulated by higher energy background, these lines have the advantage that they remain visible above the copious background. However, the low-intensity Pu K1 line (103.7 keV) sits where the background is highest, creating detector deadtime and resolution issues while requiring very high spectral resolution to separate it from the adjacent 105.3-keV line from 155Eu. Unfortunately, around 100 keV, only tight collimation can be used to control the direct flux reaching the detector, so the low intensity of the Pu line means long integration times are needed. We are exploring high-efficiency, gamma-ray mirrors as defined band-pass filters to deflect the K-shell spectral region (94 to 120 keV) onto a point-contact, high-purity germanium detector shielded from the fuel’s direct flux, completely removing rate issues. Such mirrors are manufactured by depositing hundreds of alternating layers of SiC and WC onto flat surfaces. By varying the layer spacing throughout the depth of the coating, reflectivity can be tuned to cover an angle/energy range suitable to the measurement. Several mirrors have been fabricated with over 400 aperiodic bi-layer pairs providing a ~0.20 reflection with a flattop response over a bandpass of 94 to 110 keV. The mirrors were recently tested at the ORNL Irradiated Fuel Examination Laboratory, providing almost an order of magnitude flux increase over earlier results obtained with a mirror made of a periodic multilayer, with a single bi-layer thickness. A two-mirror stacked system yielded statistically significant results for the Pu content from several ~ 7-cm-long locations on LWR spent fuel pins with different burn-up values. This paper presents the overall system and the results of those measurements.