Physical Mechanisms for Fission Product Concentration Differences from Variations in Reactor Power History

A method for uniquely determining power history characteristics of spent fuel assemblies based on the concentration of various fission products in the fuel assembly has been developed. It is envisioned that this method could be used as a transparency aid at reprocessing facilities to verify the identity of spent fuel assemblies. Specific fission product concentrations measurably vary as a result of differences in reactor power history. A discussion of the physical properties of various fission products responsible for concentration differences along with the mechanisms by which these properties produce concentration differences is presented. Several different mechanisms based on variations in the physical properties are illustrated through simplified models. These models led to the identification of an extensive list of possible monitor ratios which is presented. A case study was also performed to assess the distinguishing capabilities of the given monitors. The variations between cases include modifications to specific power, the number of shutdowns, and the duration of the shutdowns while maintaining a constant final burnup. For all but one of the cases a monitor ratio is shown to vary by at least 20% with the base case while some ratio differences for cases with simple modifications reached 300%. This paper concludes with a proposed technique using the presented monitor ratios to verify the identity of spent fuel assemblies based on differences in reactor power history.