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Shuffler bias corrections using calculated count rates

Description: Los Alamos National Laboratory has two identical shufflers that have been calibrated with a dozen U{sub 3}O{sub 8} certified standards from 10 g {sup 235}U to 3600 g {sup 235}U. The shufflers are used to assay a wide variety of material types for their {sup 235}U contents. When the items differ greatly in chemical composition or shape from the U{sub 3}O{sub 8} standards a bias is introduced because the calibration is not appropriate. Recently a new tool has been created to calculate shuffler count rates accurately, and this has been applied to generate bias correction factors. The tool has also been used to verify the masses and count rates of some uncertified U{sub 3}O{sub 8} standards up to 8.0 kg of {sup 235}U which were used to provisionally extend the calibration beyond the 3.6 kg of {sup 235}U mass when a special need arose. Metallic uranium has significantly different neutronic properties from the U{sub 3}O{sub 8} standards and measured count rates from metals are biased low when the U{sub 3}O{sub 8} calibration is applied. The application of the calculational tool to generate bias corrrections for assorted metals will be described. The accuracy of the calculational tool was verified using highly enriched metal disk standards that could be stacked to form cylinders or put into spread arrays.
Date: April 1, 2001
Creator: Rinard, Phillip M.; Hurd, J. R. (Jon R.) & Hsue, F. (Faye)
Partner: UNT Libraries Government Documents Department

WASTE CRATE ASSAY SYSTEM (WCAS) : ASSAY SOLUTIONS FOR VERY LARGE REMOTE HANDLED CRATES

Description: An advanced passive neutron counter has been designed and fabricated to measure the plutonium content in large remote handled (RH) waste crates. The waste crate assay system (WCAS) was developed under an agreement between Los Alamos National Laboratory, Japan Nuclear Fuel Limited (JNFL), and BNFL Instruments Inc. (BII) to measure the plutonium content in the waste generated in the Rokkasho reprocessing facility. The primary goal of the design was to produce an assay system for large waste containers. The system also includes 200-L drum pallet assay capability. The measurements are based on neutron-time correlation counting of the passive neutron emissions from the 240Pu, and the plutonium isotopic ratios are used to calculate the total plutonium. The system is designed for both RH waste and low-activity plutonium waste. The system permits the measurement of the singles, doubles, and triples rates and the multiplicity mode analysis is used together with the 'add-a-source' method to correct for the matrix materials in the crates. In the multiplicity analysis, the efficiency for counting the neutrons emitted from the crate is directly calculated from the three measured rates. For improved detectability limits, advanced methods have been incorporated in the WCAS-A to reduce the cosmic-ray neutron backgrounds. These methods include statistical filters and truncation of high-multiplicity events. The paper describes the WCAS-A design, performance, and calibration.
Date: January 1, 2001
Creator: Menlove, Howard O.; Rinard, Phillip M.; Li, T. K. (Tien K.); Romero, M.; Hiruta, K. & Nasuno, S.
Partner: UNT Libraries Government Documents Department

Comparison of Shuffler and Segmented Gamma Scanner Measurements of 55-Gallon Drums Containing HEU Embedded in Varied Matrices.

Description: Many forms of nuclear waste materials are constantly generated at Los Alamos Chemical and Metallurgical Research (CMR) Facility. For disposal, accounting of the nuclear material must be done. We were recently tasked with measuring sixteen 55-gallon drums, each containing about 150 to 250 grams of HEU. The lower two-thirds of the drums consisted mostly of paper, plastics, and other combustibles. The HEU in this bulk waste had been previously assayed using a segmented gamma scanner (SGS) and ranged from 1 to 20 grams per drum. The drums were later re-opened and specific packages of HEU waste solids, mostly polyethylene bottles filled with uranium oxide and/or fluoride, were placed on top of the bulk waste. The number of bottles ranged from one to seven with an additional {sup 235}U mass between 150 to 230 grams per drum. Although many of these additional mass values were based on results from an old chemical analysis, they are expected to be reasonably accurate. We counted the drums on the SGS and shuffler instruments. The results were biased low on the SGS which was calibrated for normal combustible waste (not bottles embedded in waste) and biased high on the shuffler calibrated for small cans of oxide. The biases were caused by the differences among the calibration standards and the drums. This paper is a study of the nature of these biases, providing corrections and comparing the resulting performance characteristics of the instruments. This effort was greatly enhanced by having photographic as well as real-time radiographic information on many of the drums contents. A new software tool, recently developed at Los Alamos to calculate shuffler count rates accurately and generate bias correction factors, was applied to these drums. The application of the calculational tool to generate bias corrections for these types of matrices is described. The ...
Date: January 1, 2002
Creator: Hurd, J. R. (Jon R.); Grimes, L. V. (Linda V.); Gomez, C. D. (Cipriano D.); FitzPatrick, J. R. (John R.); Vigil, G. M. (Georgiana M.); Tobin, S. J. (Stephen J.) et al.
Partner: UNT Libraries Government Documents Department