D.E. Burk ,
"Forward model calculations for determining isotopic compositions of materials used in a radiological dispersal device,"
M.S. Thesis, Texas A&M University, May 2005.
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 irradiated nuclear fuel is used as the dispersion
material for the RDD, it will be beneficial for law enforcement
officials to quickly identify where the irradiated nuclear fuel
originated. One signature which may lead to the identification of
the spent fuel origin is the isotopic composition of the RDD
debris. 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 3,000 MWd/MTU per step. To ensure a conservative answer,
2,500 MWd/MTU 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 uncertainty between it and the 2D pin
cell (0.07% for 235U) did not warrant the additional computational
time required. The computational time for the multiple
radial-region pin cell was 7 times that of the 2D pin cell. For
this reason, the 2D pin cell was used to benchmark the isotopics
with data from other reactors. The reactors from which the
methodology was benchmarked were Calvert Cliffs Unit #1, Takahama
Unit #3, and Trino Vercelles. Calvert Cliffs is a pressurized water
reactor (PWR) using Combustion Engineering 14×14 assemblies.
Takahama is a PWR using Mitsubishi Heavy Industries 17×17
assemblies. Trino Vercelles is a PWR using non-standard lattice
assemblies. The measured isotopic concentrations from all three of
the reactors showed good agreement with the calculated values.