FABRICATION OF A SOURCE TO EVALUATE SNM PORTALS

Spectroscopy portals that detect special nuclear material (SNM) are an important part of the US effort to interrogate and interdict material entering the country as well as material leaving secure facilities. These portals use a variety of methods to detect SNM. One of the methods is gamma ray spectroscopy. This paper describes the fabrication and testing of an SNM portal monitor that simulates the gamma ray signature of a sphere of 2.5 kg of highly enriched uranium (HEU), using a much smaller amount of HEU. Using only a Category IV quantity of HEU (with Attractiveness level C), this spherically symmetric portal monitor produces a gamma ray spectrum that matches the main responses of 2.5 kg of HEU to better than 90% and is spatially invariant to less than 10%. Additionally, the object conforms to sealed source requirements. Because of the relatively small amount of enriched uranium in the object and the fact that it is a sealed source, manufacture, transportation, storage, and use requirements are greatly reduced. The economy of use of HEU is achieved by alloying the enriched HEU with aluminum. Radiography and chemical analysis confirmed the uniformity and overall composition of the casting. Although modeling of the gamma ray spectrum of the aluminum-uranium alloy predicts a 186 keV gamma ray response that is indistinguishable from 2.5 kg of HEU, the predicted Compton continuum deviates from the desired response. In order to better match this response, it is necessary to add depleted uranium. The depleted uranium is fixed at the center of the sphere using mating aluminum spacers. In order to mitigate the potential for rupture of the device during thermal testing (described below), the aluminum spacer components contain features to provide free volume to accommodate volume expansion of the aluminum. The final component used for this device is a shell of titanium. This titanium cladding serves to contain the radioactive uranium components, and satisfies the sealed source requirements. In addition to the HEU source described above, two additional objects were fabricated using an aluminum-depleted uranium alloy. Besides serving as metallurgical development objects, they were also used for sealed source testing. These tests involved thermal, puncture, and impact tests, followed by a leak test to evaluate the integrity of the objects. The objects met these requirements.