A moisture probe using neutron moderation for PuO2 canister inspection

We have been developing a sensor to quickly and non-destructively measure the moisture content of canisters containing PuO₂. These canisters will be retained in long-term storage facilities and, for stability reasons, must have moisture contents <0.5 wt%. The technique relies on the moderation of neutrons emitted by a radioisotopic source (Cf-252) to determine the hydrogen content of the material. Modeling was used to help optimize the sensor design and to predict the sensitivity to moisture content. A benchmodel was first constructed and tested at SAIC with simulated PuO₂ using Fe ₃O₄ and B₄C; moisture was simulated with nylon rods. The benchmodel was also tested at Los Alamos National Laboratory (LANL) using actual PuO₂-filled canisters modified to accept polyethylene rods to simulate moisture. The results indicated that the detection limit (one statistical standard deviation) for a 10 minute inspection was less than 1.2 g of water, or 0.04 wt%, in 3 kg of PuO₂. A prototype system was recently constructed for testing a variety of PuO₂ samples with various material matrices. Here we present the calculations, sensor designs, tests, test results, and discuss future plans