Autonomous Mobile Directionally Sensitive Neutron Detectors for Managed Access Inspections

Managed access inspections are limited in time and inspector hours by the needs to avoid exposure of sensitive information, to minimize impacts on facility operations, and to conserve inspectorate resources. We are examining the possibility of using multiple autonomous, mobile, directionally sensitive neutron detectors (“inspector bots”) to support inspectors in detecting anomalous neutron sources. The two scenarios we are considering supporting are 1) Limited frequency unannounced access inspections at large gas-centrifuge enrichment plants under IAEA safeguards, and 2) Confirmation of the absence of undeclared treaty accountable items in sensitive military installations under future nuclear arms control agreements. Based on previous work of ours that demonstrated significant directional sensitivity of rows of moderated neutron counters, we have developed the concept for a one-foot diameter, two-foot high cylinder of polyethylene moderator, containing three one-inch diameter, 50 cm long, boron-coated-straw neutron counters located 120 degrees apart, half-way between the center-line and the surface of the cylinder. MCNP studies show that such a detector system provides high sensitivity and remarkable directionality: 0.37 cps per ng of bare Cf-252 at 2 m and 6:1 signal ratio between a counter directly facing the neutron source and each of the two counters at the back. This detector system will be mounted on a robotic transport mechanism to form an “inspector bot.” The bot could adjust its heading to maximize the signal in its most active counter, and to balance the signals in the two less active counters, providing a guidance system that can be used to facilitate smart mapping of neutron fields. A swarm of cooperative inspector bots could efficiently detect undeclared withdrawal stations in centrifuge halls, due to (alpha, n) reactions in enriched UF6, or undeclared warheads in storage facilities, due to spontaneous fission of Pu-240. We will report on progress towards the construction and calibration of a prototype bot.