North Carolina State University is conducting research on the application of nonlinear optimization to the inverse analysis of gamma spectra measured from irradiated nuclear materials. Oak Ridge National Laboratory (principally Weber, Romano, et al.) has conducted extensive research on the inverse depletion analysis of spent nuclear fuel. The ongoing work at NCSU is intended to extend inverse depletion analysis to arbitrary configurations of irradiated 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 composition and irradiation history (burnup and cooling time). Deterministic and stochastic gamma ray-tracing methods have been developed and coupled to the Sandia code GADRAS to predict the gamma leakage spectrum, including uncollided photon intensities and Compton continuum from collided gammas. Currently, the inverse solver for irradiation history is being tested against gamma spectroscopy measurements of SNM foils irradiated in the Washington State University TRIGA reactor. The computed gamma line intensities are compared to the measured photopeak areas, and Levenberg-Marquardt nonlinear optimization is used to estimate the irradiation history. This paper presents inverse analyses of the irradiated foils to demonstrate the viability of this method.
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