Measurement of the U-235 Content of Concreted Waste Drums

摘要

A challenging assay situation recently arose, whereby the fissile (i.e. total plutonium plus U-235) content of a population of 164 historical waste drums containing concrete needed to be measured, to comply with nuclear safety limits for transport to, and interim storage within, an Engineered Drum Store.BIL Solutions Ltd has developed a new methodology for measurement of the U-235 content of these "concrete" drums, because the approach normally used by the in-situ Drum Monitors was found to be overly pessimistic. Initial investigations indicated significant quantities of uranium were present in these drums (but negligible plutonium), mixed with Np-237 and / or Ra-226. These initial measurements also indicated that the uranium was likely to be depleted in enrichment. The U-235 content was therefore determined by measuring the U-238 mass via the passive coincident neutron emission, and combining this with the U-235 and U-238 isotopic abundances, obtained by analysis of a gamma spectrum.Of the uranium isotopic analysis codes available, the FRAM (Fixed energy, Response function Analysis with Multiple efficiencies) software was selected as being most suitable for this application. A wide gamma-ray energy range is used (i.e. 120 keV to 1200 keV) which was considered more likely to yield results when there is significant attenuation. The software is also user configurable, enabling interferences from the other radionuclides present (i.e. Np-237 and Ra-226) to be accounted for. A series of test measurements were performed with well-characterised uranium sources attenuated by concrete shielding, to gain confidence in the performance of FRAM under such conditions. These test measurements indicated that FRAM was able to correctly determine the enrichment of heavily shielded uranium.The new U-235 measurement methodology was then applied to the population of concrete drums; successfully yielding U-235 results despite the dense waste matrix and significant interference from Np-237 and / or Ra-226. The U-235 mass results obtained were significantly more accurate than those that would have been obtained had the traditional approach (i.e. transmission corrected 186keV count rate) been used, facilitating safe transport and storage of the drums.