This report describes a computer-based gamma spectrometer system that was developed for measuring isotopic and total plutonium concentrations in nitric acid solutions. The system was installed at the Tokai reprocessing plant where it is undergoing testing and evaluation as part of the Tokai Advanced Safeguards Exercise (TASTEX). Objectives of TASTEX Task H, High-Resolution Gamma Spectrometer for Plutonium Isotopic Analysis, the methods and equipment used, the installation and calibration of the system, and the measurements obtained from several reprocessing campaigns are discussed and described. In general, we find that measurements for gamma spectroscopy agree well with those of mass spectrometry and of other chemical analysis. The system measures both freshly processed plutonium from the product accountability tank and aged plutonium solutions from storage tanks. 14 figures, 15 tables.

MGA calculates relative isotopic abundances of plutonium and other actinides in a sample. The code performs its analysis using data from a gamma-ray spectrum of the sample taken with a germanium detector. This volume describes the structure of the program and the procedures used for measuring samples and analyzing the spectra. It is assumed that the user is familiar with standard practices and equipment used in gamma-ray spectrometry.

The isotopic composition of plutonium and uranium is needed for purposes of sample confirmation, or for interpreting results from calorimeters or neutron-coincidence measurement instruments to determine nuclear material mass. The authors have developed measurement methods and computer codes utilizing high-resolution gamma-ray spectrometry to measure the relative isotopic abundances of plutonium and uranium in various forms nondestructively. The computer codes, known as MGA and MGAU, have unique analysis methodologies that the authors briefly describe in this paper.

A field test has been carried out for more than 2 years for determination of plutonium concentration by K-edge absorption densitometry and for determination of plutonium isotopic abundance by transmission-corrected passive gamma-ray spectrometry. This system was designed and built at Los Alamos National Laboratory and installed at the Tokai reprocessing plant of the Power Reactor and Nuclear Fuel Development Corporation as a part of the Tokai Advanced Safeguards Technology Exercise (TASTEX). For K-edge measurement of plutonium concentration, the transmissions at two discrete gamma-ray energies are measured using the 121.1- and 122.1-keV gamma rays from 75Se and 57Co. Intensities of the plutonium passive gamma rays in the energy regions between 38 and 51 keV and between 129 and 153 keV are used for determination of the isotopic abundances. More than 200 product solution samples have been measured in a timely fashion during these 2 years. The relative precisions and accuracies of the plutonium concentration measurement are shown to be within 0.6% (1 sigma) in these applications, and those for plutonium isotopic abundances are within 3% for 238Pu, 0.4% for 239Pu, 1.2% for 24°Pu, 1.3% for 241Pu, and 7% for 242Pu. The time required is 10 min for the concentration assay, 10 min for the isotopics assay, and about 15 min for handling procedures in the laboratory.

Gamma-ray spectroscopy measurements have been made on mixed uranium and plutonium oxides in sealed containers to determine the uranium and plutonium enrichment and isotopics. Experimental results obtained using two different methods were in good agreement with the known contents. The first method is applicable to thick samples of freshly reprocessed mixed oxide and determines isotopic abundances from measured absolute gamma-ray intensities. Measurement times depend on plutonium enrichment, but for mixed oxide enriched to 12% in plutonium, the fissionable content can be determined to better than 0.5% in 2 h. The second approach utilizes intensity ratios of selected pairs of gamma-rays to determine plutonium enrichment and uranium and plutonium isotopes. This method requires at least 12 h to determine the plutonium enrichment to an accuracy of 0.5%. However, it cannot be applied until the 238U daughter activities in the mixed oxide reach equilibrium, which requires at least 5 months after separation. Preliminary conclusions drawn from these two noninvasive and nondestructive measurement techniques, and recommendations for future experiments are discussed.