Nonlinear Optimization and the Inverse Analysis of Irradiated Nuclear Material Gamma Spectra

North Carolina State University is continuing research on the application of nonlinear optimization to the inverse analysis of gamma spectra measured from irradiated nuclear materials. The ongoing work has extended inverse depletion analysis to arbitrary irradiation environments and configurations of irra- diated nuclear material. The inverse analysis under development at NCSU employs the SCALE module ORIGEN to predict the gamma source term for an irradiated material given the original isotopic com- position and irradiation history (burnup and cooling time). Currently, the inverse solver for irradiation history is being tested against gamma spectroscopy measurements of thin SNM samples irradiated in the Washington State University TRIGA reactor. The computed gamma line intensities are compared to the measured photopeak areas, and a combined genetic algorithm and Levenberg-Marquardt nonlinear optimization is used to estimate the irradiation history. This paper presents inverse analyses of the irradiated samples for differing parameter combinations to demonstrate the viability of this method.