A composite position independent monitor of reactor fuel irradiation using Pu, Cs, and Ba isotope ratios

Martin Robel, Brett Isselhardt, Erick Ramon, Anna Hayes, Amy Gaffney, Lars Borg, Rachel Lindvall, Anna Erickson, Kevin Carney, Terry Battisti, Andrew Conant, Brian Ade, Holly Trellue, Charles Weber

Research output: Contribution to journalArticlepeer-review

10 Scopus citations

Abstract

When post-irradiation materials from the nuclear fuel cycle are released to the environment, certain isotopes of actinides and fission products carry signatures of irradiation history that can potentially aid a nuclear forensic investigation into the material's provenance. In this study, combinations of Pu, Cs, and Ba isotope ratios that produce position (in the reactor core) independent monitors of irradiation history in spent light water reactor fuel are identified and explored. These position independent monitors (PIMs) are modeled for various irradiation scenarios using automated depletion codes as well as ordinary differential equation solutions to approximate nuclear physics models. Experimental validation was performed using irradiated low enriched uranium oxide fuel from a light water reactor, which was sampled at 8 axial positions from a single rod. Plutonium, barium and cesium were chemically separated and isotope ratio measurements of the separated solutions were made by quadrupole and multi-collector inductively coupled mass spectrometry (Cs and Pu, respectively) and thermal ionization mass spectrometry (Ba). The effect of axial variations in neutron fluence and energy spectrum are evident in the measured isotope ratios. Two versions of a combined Pu and Cs based PIM are developed. A linear PIM model, which can be used to solve for irradiation time is found to work well for natural U fuel with <10% 240Pu and known or short cooling times. A non-linear PIM model, which cannot be solved explicitly for irradiation time without additional information, can nonetheless still group samples by irradiation history, including high burnup LEU fuel with unknown cooling time. 137Ba/138Ba is also observed to act as a position independent monitor; it is nearly single valued across the sampled fuel rod, indicating that samples sharing an irradiation history (same irradiation time and cooling time) in a reactor despite experiencing different neutron fluxes will have a common 137Ba/138Ba ratio. Modeling of this Ba PIM shows it increases monotonically with irradiation and cooling time, and a confirmatory first order analytical solution is also presented.

Original languageEnglish
Pages (from-to)9-19
Number of pages11
JournalJournal of Environmental Radioactivity
Volume195
DOIs
StatePublished - Dec 2018

Funding

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 .

FundersFunder number
U.S. Department of Energy
Lawrence Livermore National LaboratoryDE-AC52-07NA27344

    Keywords

    • Barium isotope ratio
    • Cesium isotope ratio
    • Cesium-133
    • Cesium-135
    • Cesium-137
    • Environmental release
    • Fission products
    • Nuclear forensics
    • Nuclear nonproliferation
    • Nuclear safeguards
    • Nuclear terrorism
    • Plutonium isotope ratio
    • Reactor modeling
    • Spent fuel analysis
    • Stable isotopes

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