The use of the BIC set in the characterization of used nuclear fuel assemblies by nondestructive assay

This paper explains why the burnup, initial enrichment, and cooling time of a used fuel assembly - collectively called the BIC set of variables - characterize it to first order for the purposes of nuclear-materials safeguards and burnup credit. From an analysis by basic nuclear engineering, it is shown that the physical properties and the isotopic content of a used fuel assembly are basically three-dimensional vector spaces. By extensive referencing of the NDA literature, the paper then shows that the BIC variables are independent variables with respect to the physical properties and the isotopes. Therefore, the knowledge of all three BIC variables is a necessary condition for the accurate characterization of a used low- or high-enriched uranium (LEU or HEU) fuel assembly. For a plutonium mixed-oxide (MOX) fuel assembly, a fourth variable for the BIC set (the curium-producing ability) is also necessary. The paper also discusses other possible variables besides the BIC set, to demonstrate that the knowledge of the BIC set is also a sufficient condition in many cases. Logically, it is therefore necessary to make at least three independent NDA measurements (or four, for MOX) to achieve a unique solution (characterization) if a reliance on information provided by the reactor operator is to be avoided. By this fact, the common question, “What is the accuracy of a particular NDA technique?” is revealed to be a poorly posed one with regard to used fuel assemblies. The result of the paper is a better paradigm for interpreting and improving the NDA practice of both the safeguards community and the burnup-credit community.