Monitoring the flow of fissile liquids

Monitoring the flow of fissile liquids between tanks or facilities can be accomplished by inducing fission in the fissile material with a modulated neutron source and detecting delayed gamma rays downstream. The modulation of the neutron source produces wares of activation whose delay can be detected in gamma sensitive detectors downstream to obtain the flow velocity. The detector count rate can be related to the fissile mass in the liquid. This system can be attached to the outside of pipes and utilizes a polyethylene moderator, a neutron absorbing material that can be moved in and out between the pipe and the surrounding moderator, and a detector of delayed gamma rays downstream. This system is similar in principle to systems that have been used to monitor the flow of UF ₆ gas in the blend-down of HEU to LEU in Russia. The high density of the fissile material in the liquid (grams to 100's of grams per liter) of solution compared to the very low density of UF ₆ gas results in an implementation of this method that uses much smaller sources and much less moderator since the fissile liquids usually contain moderator. This paper describes the Monte Carlo neutron and gamma transport calculation used to design the system, the system design, and expected performance for some typical liquids and flows. This type of system has applications for nuclear safeguards. It also has application for criticality safety since flow into non-geometrically safe tanks can and has resulted in criticality accidents. Although these simulations were for uranyl nitrate, the methodology could be applied to Pu solutions with slightly longer measuring times (or larger source sizes).