FAST ENRICHMENT SCREENING FOR ‘GO/NO GO’ CLASSIFICATION OF LARGE CONTAINERS

Methods for rapid non-destructive uranium enrichment classification of large containers are of importance to waste consignors, safeguards and counter-terrorism agencies. There is often a need to quickly categorize and segregate suspect containers as ‘depleted’ or ‘enriched’ on a ‘Go/No Go’ basis. Recent improvements in gamma spectroscopy technologies have provided the capability to perform rapid field analysis using portable and hand-held devices such as battery-operated medium and high resolution detectors (including lanthanum halide and high purity germanium). Furthermore a new generation of portal monitors are currently under development with advanced spectroscopic capabilities. Instruments and technologies that were previously the domain of complex lab systems are now widely available as touch-screen “off-the-shelf” units. Despite such advances, the task of enrichment analysis remains a complex exercise. This is particularly so when surveying large items such as historic waste drums and crates containing debris of unknown density and composition contaminated with uranium. The challenge is equally applicable to safeguards inspectors evaluating large items and for interdiction of illicit special nuclear materials in mass transport e.g. shipping containers at ports and borders. The variable shielding, container size, lack of matrix knowledge, wall thickness and self-shielding compound this problem. Performing an accurate assessment within the short count time window demanded of the field operative, leads to the need for a reliable method that can adapt to such conditions and is robust to a wide dynamic range of counting statistics. Several methods are evaluated with reference to the performance metrics defined in applicable standards (such as ANSI N42.43-2006). The primary issue is to minimize the bias that can result from attenuation effects, particularly as the gamma emissions from U235 are low energy and therefore highly susceptible to absorption in large containers with metal scrap. Use of other radiometric signatures such as bremsstrahlung radiation and neutron emissions are considered in addition to photopeak ratio analysis. Benchmark comparisons are performed against well-established enrichment analysis methods such as PC-FRAM, MGAU, and the Enrichment Meter. The suitability/limitations of the more traditional methods is discussed.