NWAS. Developments for passive uranium assay system, 2

A uranium mass assay system, NWAS (Ningyo Waste Assay System), for 200-litter wastes drums applied by NDA method was developed and accumulated the data of the actual uranium bearing wastes drums. The system consists of the 16 pieces of Helium-3 proportional counters for neutron detection generated from ²³⁴U(α,n) reaction or ²³⁸U spontaneous fissions with polyethylene moderation and a Germanium solid state detector for gamma ray detection as to determine uranium enrichment. In previous report, some measurement experiences had been introduced briefly. After that satisfactory works had been continued and the uranium determination data of 850 drums had been accumulated approximately. On the other hand considerable problems on the system had been revealed technically or analytically through the measurements experiences, in specific terms, concerning various kinds of matrices, uranium chemical compositions and rage of uranium mass. Such experiences are to be described precisely, in addition newly gained knowledge will be marshaled. Furthermore the next improvement plans are now prepared that will be capable of active neutrons assay for uranium bearing wastes drums. The results of complications will lead us to the progressive next steps. (author)