Article

The uranium cylinder assay system for enrichment plant safeguards

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Abstract

Safeguarding sensitive fuel cycle technology such as uranium enrichment is a critical component in preventing the spread of nuclear weapons. A useful tool for the nuclear materials accountancy of such a plant would be an instrument that measured the uranium content of UF cylinders. The Uranium Cylinder Assay System (UCAS) was designed for Japan Nuclear Fuel Limited (JNFL) for use in the Rokkasho Enrichment Plant in Japan for this purpose. It uses total neutron counting to determine uranium mass in UF cylinders given a known enrichment. This paper describes the design of UCAS, which includes features to allow for unattended operation. It can be used on 30B and 48Y cylinders to measure depleted, natural, and enriched uranium. It can also be used to assess the amount of uranium in decommissioned equipment and waste containers. Experimental measurements have been carried out in the laboratory and these are in good agreement with the Monte Carlo modeling results.

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... The alpha, n neutron source would increase as the enrichment of the fuel increased, and in UF 6 it is the dominant source of neutrons because of the high alpha, n reaction rate in the fluorine. Indeed, alpha, n neutrons are the primary signature for measuring the 235 U content in UF 6 for measurement systems such as the Uranium Canister Assay System (UCAS) [11]. However, the alpha, n reaction cross section in oxygen is two orders of magnitude smaller than for fluorine, so this neutron source is very small for the LEU fuel assemblies. ...
... The 234 U alpha, n fraction can be estimated from the 234 U mass at the 55 g 235 U/cm loading. The ratio of 234 U/ 235 U is ∼0.0086 at this enrichment level [11], so the 234 U concentration is 0.473 g/cm. Our singles rate is sensitive to approximately 60 cm of the fuel assembly that is centered inside the collar, so the corresponding 234 U mass is ∼28 g. ...
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