On performance experience and measurements with Ningyo Waste Assay System (NWAS). 3

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 U-234(α,n) reaction or U-238 spontaneous fissions with polyethylene moderation and a Germanium solid state detector (Ge-SSD) for gamma ray detection as to determine uranium enrichment. In previous report, some measurement experiences had been introduced briefly. After that the measurements campaigns against the actual wastes drums stored in URCP had been carried out successfully, the uranium determination data of 850 drums had been accumulated approximately. Those characteristics were rich in variety including various kinds of matrices, uranium chemical compositions and range of uranium mass and so on. These works have contributed the decrease of the MUF in URCP, for which was the first purpose of introduction of NWAS. On the other hand several considerable problems on the system or methodology had been revealed technically or analytically through the measurements experiences. Such experiences are to be described precisely, in addition newly gained knowledge will be marshaled. Furthermore as the next improvement plans, the active neutrons assay for uranium bearing wastes drums are now progressing. The results of complications will lead us to the progressive next steps. (author)