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 （ 235 U） 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 235 U 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.

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