Criticality measurements for control and accountability of fissile materials on critical assemblies Page: 1 of 6
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Criticality Measurements for Control and Accountability of Fissile Materials
on Critical Assemblies
R. Paternoster
Los Alamos National Laboratory, Los Alamos, NM 87545
G Kompaniets and D. Polyakov
Kurchatov Institute, Reactor Research Center, Moscow, Russia
Introduction
Operational critical assemblies are subject to the same standards of materials control and
accountability (MC&A) as other fissile materials. Typically these assemblies are not easily
dismantled and the fuel components cannot be inventoried by traditional methods of
destructive or non-destructive materials assay. Because of the large mass or unusual
geometry of the fuel components these parts are often categorized as difficult-to-measure
(DTM) items. The use of criticality to infer fissile inventory, in conjunction with physical
security and two-person control procedures, provides accurate and cost-effective MC&A
for these large critical assembly components. This method is being implemented at the
Los Alamos Critical Experiments Facility (LACEF) and at several national nuclear
research centers in Russia.
Description
The use of criticality to infer fissile inventory for MC&A verification measurement of large
critical assembly fuel components is based on the documented sensitivity of critical
assemblies to changes in the configuration and the demonstrated reproducibility of the
assemblies from operation to operation. This is illustrated with reference to two applicable
critical assemblies at Los Alamos.
Critical assemblies are very sensitive to changes in fuel loading. The Flattop fast critical
assembly at Los Alamos is currently fueled with a uranium core (16.2 kg (~ 93.2% 235U
metal). In the core support pedestal there are cylindrical voids machined for the placement
of mass adjustment buttons. A 44 gram U(93) button is typically worth 0.084 $. Three
buttons were recently added to the core adding 0.241 $, where 1 $ is the interval between
delayed and prompt critical. The control rod positions changed approximately 6.70 in.
between the two configurations. The sensitivity of the system was 0.127 cm (0.05 in.) of
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Paternoster, R.; Kompaniets, G. & Polyakov, D. Criticality measurements for control and accountability of fissile materials on critical assemblies, article, May 1, 1997; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc680485/m1/1/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.