The development of fast neutron assay of low radioactivity samples collected from Dounreay decommissioning site is described, the development aims to identify a plutonium detection limit with neutrons. To inform the neutron measurements, a greater analysis of the chosen samples is presented.
Analyses used were fast neutron counting, gamma spectrometry and accelerator mass spectrometry. Analysis showed that fast neutron detection can distinguish samples at a radioactivity of approximately 1 Bq/g of 240Pu origin from samples at background levels of natural uranium radioactivity.
Further, AMS and gamma spectrometry has showed that the Dounreay samples are consistent with the global fallout averages for the main plutonium isotopes 239Pu, 240Pu, 241Pu, and 242Pu. The 240Pu/239Pu ratio measured in this research is lower than average however for most Dounreay site samples, leading to a conclusion that this 240Pu/239Pu ratio is symptomatic of the Dounreay site hosting breeder reactors, which could boost the levels of 239Pu in comparison to 240Pu.