The headspace gas in uranium hexafluoride cylinders contains noble gases from the atmosphere along with helium produced by radioactive decay of U and its daughters. We have developed a portable mass spectrometry system for analyzing the noble gas composition of UF6 cylinder headspace gas, with the aim of constraining a cylinder’s fill and operation history. After a cylinder is filled with UF6, the accumulation of 4He from alpha decay of uranium proceeds at a rate that is determined by the mass and isotopic composition of uranium and any other alpha-producers that are present. To test this approach, U-He age dates were measured on a variety of UF6 sample cylinder sizes (containing 10 g to 2 kg UF6) with enrichments ranging from 3 to 90 atom % 235-U. Cylinder headspace is sampled and purified using chemical and cryogenic trapping. The remaining headspace is then analyzed using an online variable-inlet noble gas mass spectrometer (VIMS). Samples are also collected and analyzed offline using conventional noble gas mass spectrometry. The measured U-He ages correspond well with the known fill ages and access history for UF6 in test cylinders with fill dates that ranged from six months to 47 years.

U-He age dating of uranium hexafluoride

2019 Spring National Meeting

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