Multi-layer neutron analysis of a filled dry storage cask

Monte Carlo simulations of dry storage casks tend to be resource-intensive due to the complex nature of the fuel inside and the large amounts of shielding surrounding the fuel. For monitoring applications of these casks, detection efficiency becomes another important variable. To account for the variations in both the cask and the detectors, a mathematical model is introduced that uses neutron source multiplicity information and a master equation to determine the probability to obtain detectable neutrons on a per source event basis. This model allows for the probabilities of absorption, fission, and detection of a neutron to be modified without requiring subsequent runs in a Monte Carlo code. This significantly reduces the total number of runs that are required for parameter sensitivity and uncertainty surveys. Additionally, the model predicts the probability of detecting n number of neutrons from a single fission chain event. Using a sum of the probabilities for n > 1, the probability of detecting correlated neutrons from a single neutron event can be found. The multiple neutrons detected from a single source event are of particular interest for the capability of using them to provide source location information, which can be used for neutron imaging of a dry storage cask (self-emission tomography). From the analysis for a Tn-32 Cask, it is found that although the attenuation of correlated events is very significant, there are a sufficient number of correlated neutrons that are detected per second that can be used for future neutron imaging approaches. (Author)