Inverse Solutions in Spectroscopic Analysis with Applications to Problems in Global Safeguards C. F. Weber, V. A. Protopopescu, M. H. Ehinger, A. A. Solodov, C. E. Romano Oak Ridge National Laboratory  P.O. Box 2008, Oak Ridge, TN 37831-6170 ABSTRACT This work describes a nondestructive strategy and algorithm designed to evaluate the burnup and plutonium content of light-water-reactor spent fuel and thereby confirm declared values. In contrast with previous methods that focus on only a few photopeaks (e.g., 137 Cs at 662 keV), the present approach involves the entire gamma spectrum up to 2000 keV. Spectra are used as input for the inverse code INDEPTH, which is designed to predict reactor parameters (fuel enrichment, power level, irradiation time, and cooling time) when given either a set of nuclide inventories or the gamma spectrum that they produce. This approach has the advantage of often making possible the determination of parameters other than burnup when they are unknown or in doubt. In addition, error in one photopeak evaluation is mitigated by the inclusion of the entire spectrum. The solution procedure involves multiple runs of the forward code ORIGEN/ARP, each of which produces an extensive list of nuclides formed through depletion/decay processes. The gamma spectrum of these nuclides is compared with the gamma spectrum from a detector through a bin-by-bin sum of squared error. New choices for the reactor parameters that are input to ORIGEN/ARP are determined using a gradient search technique, and the best parameter set is that which minimizes the squared error between calculated and measured gamma spectra. The method is applied to the analysis of gamma data taken from various sections of actual spent reactor fuel and is compared with declared values and other methods of evaluation. The sensitivity of the inverse solution with respect to various parameters is calculated and indicates that the algorithm is stable and robust. One example includes the presence of multiple solutions, each of which can be characterized using additional information. INTRODUCTION This project seeks to develop calculational strategies and algorithms to facilitate the nondestructive evaluation of burnup and Pu content of light water reactor (LWR) spent fuel. The focus is on using inverse problem solutions for estimating burnup and Pu content directly from the whole gamma spectrum of nuclear fuel. A number of actual LWR fuel samples have been measured at ORNL using high-purity germanium (HPGe) detectors to give a detailed gamma spectrum from 0 to about 1.6 MeV. 1 Some of these samples have been or will be destructively assayed to determine the exact Pu content and validate the methods described in this paper. An important tool in evaluating fuel rod inventories is the computer code ORIGEN-S, 2 which is used to model depletion/decay processes. This code uses input parameters that describe reactor operation (enrichment, power, irradiation time, decay time) to calculate the inventories of Pu and U  Oak Ridge National Laboratory, Oak Ridge, TN 37831-6170, is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.