Detecting nuclear materials smuggling: using radiography to improve container inspection policies

This paper proposes a layered container inspection system for detecting illicit nuclear materials using radiography information. We argue that the current inspection system, relying heavily on the Automated Targeting System (ATS) and passive radiation detectors, is inherently incapable of reliably detecting shielded radioactive materials, especially highly enriched uranium (HEU). This motivates the development of a new inspection system, which is designed to address a fundamental flaw of the ATS-based system, allowing for improved defense against sophisticated adversaries. In the proposed inspection system, all cargo containers go through x-ray imaging equipment first. From the x-ray image, a hardness measure of the container is computed. This hardness measure characterizes how likely it is that shielded HEU, if it does exist in the container, will not be detected in a subsequent passive detection step. Depending on the value of the hardness, the lower-hardness containers are sent to passive detection and the high-hardness containers are sent directly to active detection. This paper explores the trade-off between the detection probability of the new inspection system and the expected sojourn time a container spends in the system. The solution details and decision-making tools for using such a system are provided. Comparisons are made between the proposed system and the current ATS-based nuclear inspection