Abstract
The paper is devoted to non-destructive control of tiny amounts of fissile materials in large-sized containers filled with radioactive waste (RAW). The aim of this work is to model an active neutron interrogation facility for detection of fissile ma-terials inside NZK type containers with RAW and determine the minimal detectable mass of U-235 as a function of various param-eters: matrix type, nonuniformity of container filling, neutron gen-erator parameters (flux, pulse frequency, pulse duration), meas-urement time. As a result the dependence of minimal detectable mass on fissile materials location inside container is shown. Nonu-niformity of the thermal neutron flux inside a container is the main reason of the space-heterogeneity of minimal detectable mass in-side a large-sized container. Our experiments with tiny amounts of uranium-235 (<1 g) confirm the detection of fissile materials in NZK containers by using active neutron interrogation technique.
Highlights
One of the urgent challenges of today's nuclear power engineering is to detect and control fissile material (FM) in various radioactive waste (RAW)
The major part of RAW in Russia belong to low and very low level wastes (LLW) [1], which, as a rule, are packed into the NZK containers [2], the FM content there being estimated in the range ~ 10-2 to ~ 10 mg/kg of LLW [3]
It is clear that the existence of fission products, as well as the neutron emitting actinides in the certified RAW packages leads to increased the gamma and/or neutron background around the containers and the deterioration of FM control
Summary
One of the urgent challenges of today's nuclear power engineering is to detect and control fissile material (FM) in various radioactive waste (RAW). At the first stage of designing the required certifier, our theoretical calculations were made under the assumption that only uranium isotopes and no fission products and other actinides were present in the NZK The expediency of this assumption is supported by the fact that part of the nuclear fuel cycle facilities produce RAW containing uranium isotopes only. This work consists in modeling the active neutron control system for FM in the NZK containing LLW and determining the minimum detectable mass of FM depending on various factors (measurement duration, number of neutron detectors, type of matrix and uneven distribution of FM in the NZK), and in improving the proposed method. The simulation results allowed to determine the minimum detectable mass of FM as a function of its position in the NZK container as well as its dependence on the number of neutron counters used (Figures 3 and 4), provided that their backgrounds are the same. The wide range of the minimum detectable mass of uranium-235 (from 2 to 84 mg) shown in Figures 3 and 4 is due to the non-homogeneity of the thermal neutron field inside the container, which in practice can be partially neutralized by rotating the container during the measurement
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