Design and Implementation of a Passive Neutron Counter to Continuously Monitor Holdup in Glove Boxes

To improve the safeguards and nuclear material accountancy of holdup measurements, a continuous neutron monitor (CNM) was designed and implemented at Plutonium Conversion Development Facility (PCDF). The purpose of the CNM is to record the singles rate from holdup in glove boxes as a function of time to determine the long-term trend in the count rate. The holdup currently in the glove boxes at PCDF is from LWR recycled plutonium. In the near future, different Pu isotopic compositions from Fugen (MOX-B) spent fuel will be transferred to PCDF. The different Pu isotopics of MOX-B fuel will result in much higher neutron emission rates compared to that from LWR spent fuel. Thus, the CNM is needed to establish baseline measurements of the existing holdup as a function of time prior to the introduction of MOX-B fuel into the process equipment. These baseline measurements can then be compared to post- process measurements of holdup after the introduction of MOX-B fuel to determine the relative increase in holdup in the glove boxes. Monte Carlo N-Particle eXtended transport code (MCNPX) simulations were performed to optimize the detector design and location of the CNM such that the contribution to the singles rate from each glove box is uniform. In addition, the effect of LWR and MOX-B recycled Pu on the expected singles rate in the CNM was also assessed MCNPX simulations.