Progress On High Energy Delayed Gamma Spectroscopy for Direct Assay of Pu in Spent Fuel

The direct, nondestructive measurement of fissile and fissionable isotopes in spent fuel is not yet possible. Current methods which infer plutonium content through proxy measurements and confirmatory burnup calculations have relatively large uncertainty and do not satisfy the desire for a measurement that is independent of operator declarations. We are currently exploring the High Energy Delayed Gamma Spectroscopy (HEDGS) technique for direct, independent Pu measurement in light-water reactor fuels. HEDGS exploits the unique distribution of fission-product nuclei from each of the fissile isotopes. Fission is stimulated in the sample with a source of interrogating neutrons, and delayed gamma rays from the decay of the short-lived fission-product nuclei are measured. The measured gamma spectrum from the unknown sample is then fit with a linear combination of gamma spectra from pure U-235, Pu-239, and Pu-241, as deduced from the known fission-product yield curves and decay properties of the fission- product nuclei, to determine the proportions of these fissile isotopes. In previous work, we performed preliminary modeling studies of HEDGS on idealized single fuel pins of various burnups. Here, we report progress on extending our GEANT-based modeling tools to efficiently model full pressurized water reactor (PWR) fuel assemblies using variance reduction techniques specific to the background emissions and induced signal, as appropriate. Modeling and analysis methods are demonstrated using a notional HEDGS instrument design and a small library of spent fuel assemblies with burnup ranging from fresh to 60 GWd/MTU. Estimates of the statistical uncertainty (sources of systematic error are not addressed in this paper) that could be realized for the assay of U-235, Pu-239 and Pu-241 are presented, along with results from empirical benchmarking of predicted HEDGS signatures. Also discussed is the additional work needed to address key systematic uncertainties and to support a more definitive study of HEDGS viability in future work.