Improvement of Detection Limit in 14 MeV Neutron Direct Interrogation Method by Decreasing Background

Abstract For the improvement of the detection limit in the 14MeV neutron direct interrogation method, the simulation calculation using the continuous-energy Monte Carlo transportation code (MVP) was carried out. From the results, it was clarified that the background neutrons appearing in the nuclear-fission neutron counting area can be greatly decreased by adopting a stainless steel SUS-304 (an iron-based alloy) reflector in place of a graphite moderator in the conventional detection system. The background level of this modified detection system becomes 1/10,000 of that with the conventional system and method. Namely, the background counting of 5,000 in the graphite detection system by the conventional method can be decreased to 0.5 by adopting the stainless steel SUS-304 detection system using the direct interrogation method for a measurement time of 10 min. As for the detection limit, it can be improved to 1/1000 of that using the conventional detection system and method. The detection limit for the cementation waste that is contaminated with natural uranium is 0.000122 Bq/g using the stainless steel detection system. For the cellulose waste, it is 0.000451 Bq/g, and for the metallic waste, 0.000702 Bq/g. These measurements could be performed in 10 min. KEYWORDS: nondestructive measurementTRU radioactive wasteuranium wastecemented wastefissionable materialactive neutron assaydirect neutron interrogationhigh detection sensitivitydetection limit14 MeV neutronsposition sensitivity difference