Radiation hot spot formation on UF₆ 30B cylinders

The accurate material accountancy of a country's enriched uranium hexafluoride (UF₆) stockpiles is imperative to the International Atomic Energy Agency (IAEA) for determining appropriate material safeguards. Uranium hexafluoride is typically stored in large cylinders with enrichment verification typically performed with a NaI gamma detector using the enrichment meter method. This approach for calculating enrichment assumes that the UF₆ is homogeneous inside the cylinder. The measurements and tests presented in this manuscript show that the UF₆ in cylinders left outside in the elements can experience fractionation, leading to inhomogeneity and subsequent development of radiation hot spots on the cylinders. These hot spot locations can produce errant enrichment measurements for both the enrichment meter and NaIGEM methods. In dark-colored cylinders that experience significant surface heating from the sun, UF₆ sublimates off the cylinder walls, leaving only the uranium daughter products in a horizontal patch (stripe) along the cylinder's midline. This daughter patch is hypothesized to be the source of the radiation hot spots. The patch forms during the summer and tends to decay away during the winter, when solar intensity and temperatures are reduced.