Year
2009
Abstract
Monitoring the amount of HEU in large (metric tons) storage arrays with passive time coincidence distribution measurements is complicated by a variety of effects. These include (1) time correlated background from HEU outside the particular localized section of the storage array being monitored, (2) cosmic radiation induced fission of nearby array materials, and (3) how large a portion of the array can be monitored with a set of detectors at a particular location in the array. Such a large storage array is typically rows of birdcages two wide and three high (metal structures that maintain safe nuclear criticality spacing between stored HEU castings) separated by 2 ft wide aisles between rows. This evaluation investigates how large an array of birdcages can be monitored by two 4 × 4 arrays of proton recoil scintillators. Monte Carlo simulations provided the time distribution of coincidences, multiplets (number of times n detection events occur in a time interval), and Feynman variance as a function of the length of the array. These calculations were performed for hypothetical arrays of 18 kg HEU standard Y-12 storage castings spaced 20 in. apart. These types of arrays are used because the spacing between HEU castings are loosely coupled in that the castings at the ends of the arrays only interact with adjacent castings; this makes monitoring of the entire array difficult with detectors at a particular location in the array.