Irradiated fuel assembly 235u and 239pu non-destructive control methods comparative analysis at high gamma background level
The potential small amounts (0,001 wt. %) of fissile materials determining methods at a high gamma background discussed and compared, the most appropriate for SFA nuclear material control was chosen. Passive neutron control possibility use for fissile materials Indirect detection (by 239Pu) with 242Cm, 244Cm pre-determination shown by numerical simulation. It is necessary to perform a number of conditions: 244Cm/242Cm/239Pu/240Pu ratio must be constant and known in advance; fissile materials should be evenly distributed within the container; background level should be significantly lower than its neutron radiation. Two detector types during passive method simulation setup were compared (3He-counters and 235U-based fission chambers). 3He-counters use prospects are shown. Neutron coincidence method with Am-Li and Pu-Be isotopic neutron sources does not allow to reliably determine the fissile materails content due to registered couples small amount. It was revealed that an active neutron monitoring method is optimal for the task. The calculated installation model shows more than 12 times signal excess on triple background error, indicating uniquely fissile materials (for 239Pu) minimum amount detection, wherein specimen vessel position does not affect on the recorded signal. Given the original structural material geometrical position heterogeneity in installation design amended as a pedestal moving bottom, allowing to set the specimen mass center in front of the 3He-counters and neutron source to increase signal registration efficiency, wherein it requires a preliminary gamma scanning height receptacle to determine structural materials spatial distribution. The direct detection, higher accuracy, less time detection and passive mode workability are proposed method advantages. After 239Pu determination evaluate the other isotopes content (Am, Cm, U, Np) is possible due to constant weight ratio 239Pu to detected actinide mass, depending on fuel burn-up degree and post-reactor excerpt. Thus, the proposed method allows to quickly find 239Pu, 242Cm, 244Cm content in SFA.
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