Proposal of Direct Alpha Estimation Technique by using Ring Ratio in the Continuous Neutron Monitor (CNM)

摘要

To improve the safeguards and material control and accountancy (MC&A) of holdup measurements, a continuous neutron monitor (CNM) that monitors the singles rate from holdup in glove boxes continuously was designed and implemented at Plutonium Conversion Development Facility (PCDF). Using totals neutron counting, it is impossible to get Pu mass directly from the singles count rate without predetermination of the counting efficiency, alpha value and sample multiplication (M). If these unknown factors can be measured directly by the CNM, it would be very useful to improving the accuracy of MC&A measurements at PCDF. In case of the holdup deposits, M can be defined as 1 due to the thin distribution of plutonium inside glove box, or M can also be easily determined by Monte Carlo N-Particle transport (MCNP) code simulations. Thus, the determination of the alpha value by direct measurement of the holdup is the main focus of this research. To address this subject, we have developed a new approach called Direct Alpha Estimation (DAE) technique. DAE is based on the efficiency curve of the neutron detector as a function of the neutron energy difference caused by (α,n) reactions in the measured holdup material. To establish the relationship as an experimental trial, the ring ratio (inner ring / outer ring) of the Fast Carton Assay System (FCAS) which has dual ring 3He tubes was applied instead of the current CNM. We took 22 samples directly from the holdup deposits in the glove boxes, and then measured them with the Plutonium Scrap Multiplicity Counter (PSMC) to obtain a more accurate alpha value. The alpha values ranged from 0.87 to 63.4. As a result, a good correlation between the ring ratio and the alpha value was obtained. Thus, since the ring ratio can estimate alpha value easily if M is fixed, we would propose that this technique with dual ringed CNM be extended to not only Pu mass determination, but also to monitor the impurity contents in holdup. Specifically, this would include the low-Z elements related to the (α,n) and moisture content, as well as Pu isotopic composition change (~(241)Am buildup).