Monitoring and Detection of Plutonium Movement in Storage Vaults

The neutron monitoring experiments conducted at ZPPR successfully demonstrated the feasibility of using 3He proportional counters to measure the neutron field in a plutonium storage vault. During the experiments, count rate stability at the ±0.04 to ±0.05% level was maintained for extended periods of time. A removal sensitivity of about 1 kg of plutonium from a nominal vault inventory of 2500 kg was achieved. This sensitivity applies to removal from any storage cell in the vault and is based on 4000-s counting periods. Statistically, for this situation, we had predicted a false-alarm rate with a 1-kg trip level to result in one false alarm per 2 years of continuous monitoring. The 8-month vault monitoring experience at ZPPR indicates that, with only modest hardware improvements, this level can be achieved.