Neutron Assay System for Confinement Vessel Disposition

Conference · Fri Jul 13 00:00:00 EDT 2012

OSTI ID:1046510

Frame, Katherine C ^[1]; Bourne, Mark M ^[1]; Crooks, William J ^[1]; Evans, Louise ^[1]; Mayo, Douglas R ^[1]; Miko, David K ^[1]; Salazar, William R ^[1]; Stange, Sy ^[1]; Valdez, Jose I ^[1]; Vigil, Georgiana M ^[1]

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1. Los Alamos National Laboratory

Waste will be removed from confinement vessels remaining from 1970s-era experiments. Los Alamos has 9+ spherical confinement vessels remaining from experiments. Each vessel contains {approx} 500 lbs of radioactive debris such as actinide metals and oxides, metals, powdered silica, graphite, and wires and hardware. In order to dispose of the vessels, debris and contamination must be removed. Neutron assay system was designed to assay vessels before and after cleanout. System requirements are: (1) Modular and moveable; (2) Capable of detecting {approx}100g {sup 239}Pu equivalent in a 2-inch thick steel sphere with 6 foot diameter; and (3) Capable of safeguards-quality assays. Initial design parameters arethe use of 4-atm {sup 3}He tubes with length of 6 feet, and {sup 3}He tubes embedded in polyethelene for moderation. This paper describes the calibration of the Confinement Vessel Assay System (CVAS) and quantification of its uncertainties. Assay uncertainty depends on five factors: (1) Statistical uncertainty in the assay measurement; (2) Statistical uncertainty in the background measurement; (3) Statistical uncertainty in the isotopics determination - This should be much smaller than the other uncertainties; (4) Systematic uncertainty due to position bias; and (5) Systematic uncertainty due to fluctuations in cosmic ray spallation. This one can be virtually eliminated by performing the background measurement with an empty vessel - but that may not be possible. We used modeling and experiments to quantify the systematic uncertainties. The calibration assumes a uniform distribution of material, but reality will be different. MCNPX modeling was used to quantify the positional bias. The model was benchmarked to build confidence in its results. Material at top of vessel is 44% greater than amount assayed, according to singles. Material near 19-tube detector is 38% less than amount assayed, according to singles. Cosmic ray spallation contributes significantly to the background. Comparing rates with and without a vessel showed that spallation adds an average of 27.27 singles/s and 5.45 doubles/s to background. Errors in the background rates were estimated at 20%.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: DOE/LANL
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1046510
• Report Number(s): LA-UR-12-22981; TRN: US1203780
• Resource Relation: Conference: Institute of Nuclear Materials Management Annual Meeting 2012 ; 2012-07-16 - 2012-07-16 ; Orlando, Florida, United States
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
36 MATERIALS SCIENCE
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
42 ENGINEERING
ACTINIDES
CALIBRATION
CONFINEMENT
CONTAMINATION
DESIGN
DISTRIBUTION
FLUCTUATIONS
GRAPHITE
NEUTRONS
NUCLEAR MATERIALS MANAGEMENT
OXIDES
SILICA
SPALLATION
STEELS
WASTES