Forward Model Calculations for Determining Isotopic Compositions of Materials Used in a Radiological Dispersal Device

In the event that a radiological dispersal device (RDD) is detonated in the U.S. or near U.S. interests overseas, it will be crucial that the actors involved in the event can be identified quickly. If spent nuclear fuel is used as the material for the RDD, law enforcement officials will need information on the origin of the spent fuel. One signature which may lead to the identification of the spent fuel origin is the isotopic composition of the RDD debris. In order to use this signature, it is necessary to have a well developed understanding of the uncertainties in predicting the isotopic composition of spent nuclear fuel from fundamental reactor physics calculations. The objective of this research was to benchmark a Forward Model methodology for predicting isotopic composition of spent nuclear fuel used in an RDD while at the same time optimizing the fidelity of the model to reduce computational time. The code used in this study was Monteburns-2.0. Monteburns is a Monte Carlo based neutronic code utilizing both MCNP and ORIGEN. The size of the burnup step used in Monteburns was tested and found to converge at a value of 3,160MWd/MT per step. To ensure a conservative answer, 2,500MWd/MT per step was used for the benchmarking process. The model fidelity ranged from the following: 2-dimensional pin-cell, multiple radial-region pin-cell, modified pin-cell, 2D assembly, and 3D assembly. The results showed that while the multi-region pin-cell gave the highest level of accuracy, the difference in accuracy between it and the 2D pin-cell (0.07% for 235U) did not warrant the additional computational time required (7 times that of 2D pin-cell). For this reason, the 2D pin-cell at normal operating temperature and pressure was used to benchmark the isotopics with data from three other reactors. The isotopic concentrations from all three of the reactors showed good agreement with each other. The SENTRY database at Los Alamos National Laboratory contains reactor data from around the world. Using the Forward Model methodology developed in this research, each of these reactors could be simulated and isotopics of spent fuel can be determined. If an RDD event occurs, material can be collected and compared to the data from the Forward Model calculations to determine the reactor of origin of the spent fuel.