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

摘要

In the event that a radiological dispersal device (RDD) is detonated in the U.S. or nearrnU.S. interests overseas, it will be crucial that the actors involved in the event can be identifiedrnquickly. If spent nuclear fuel is used as the material for the RDD, law enforcement officialsrnwill need information on the origin of the spent fuel. One signature which may lead to thernidentification of the spent fuel origin is the isotopic composition of the RDD debris. In orderrnto use this signature, it is necessary to have a well developed understanding of thernuncertainties in predicting the isotopic composition of spent nuclear fuel from fundamentalrnreactor physics calculations.rnThe objective of this research was to benchmark a Forward Model methodology forrnpredicting isotopic composition of spent nuclear fuel used in an RDD while at the same timernoptimizing the fidelity of the model to reduce computational time. The code used in thisrnstudy was Monteburns-2.0. Monteburns is a Monte Carlo based neutronic code utilizing bothrnMCNP and ORIGEN. The size of the burnup step used in Monteburns was tested and foundrnto converge at a value of 3,160MWd/MT per step. To ensure a conservative answer,rn2,500MWd/MT per step was used for the benchmarking process. The model fidelity rangedrnfrom the following: 2-dimensional pin-cell, multiple radial-region pin-cell, modified pin-cell,rn2D assembly, and 3D assembly.rnThe results showed that while the multi-region pin-cell gave the highest level ofrnaccuracy, the difference in accuracy between it and the 2D pin-cell (0.07% for 235U) did notrnwarrant the additional computational time required (7 times that of 2D pin-cell). For thisrnreason, the 2D pin-cell at normal operating temperature and pressure was used to benchmarkrnthe isotopics with data from three other reactors. The isotopic concentrations from all three ofrnthe reactors showed good agreement with each other.rnThe SENTRY database at Los Alamos National Laboratory contains reactor data fromrnaround the world. Using the Forward Model methodology developed in this research, each ofrnthese reactors could be simulated and isotopics of spent fuel can be determined. If an RDDrnevent occurs, material can be collected and compared to the data from the Forward Modelrncalculations to determine the reactor of origin of the spent fuel.