Development of Non-destructive Assay System using Fast Neutron Direct Interrogation Method for Actual Uranium Waste Drums

Abstract

 A non-destructive assay system using the fast neutron direct interrogation method has been designed and developed to be put into practical use for the determination of the uranium (²³⁵U) mass contained in actual uranium-contaminated waste drums. The method is capable of measuring the fissile mass in a drum by counting the number of fission neutrons resulting from nuclear fission reactions between the fissile materials contained in a drum and thermal neutrons generated by 14 MeV fast neutrons irradiated from outside the drum. A performance test employing simulated metal waste drums demonstrated that a natural uranium mass of as low as approximately 10 g could be detected within an error of ±20% regardless of the distribution of uranium samples in the drum, and the total number of fission neutrons was proportional to the ²³⁵U mass. A demonstration test employing actual waste drums could determine the uranium mass by using a newly developed correction method for deriving the fissile mass in a drum. It has been proved by the experimental validation tests that the assay system equipped with the correction method is very useful for the accountancy of waste drums.