Application of the 226Ra–230Th–234U and 227Ac–231Pa–235U radiochronometers to UF6 cylinders

230Th-234U and 231Pa-235U radiochronometry of hydrolyzed uranium hexafluoride gas

“…These solutions were dried down to form solid uranyl compounds and transferred to long-term storage containers for shipment to LLNL (t = 6). After all the UF 6 gas was extracted from the canisters, they were sawed open and the solid "loose heel" material was extracted and analyzed in a parallel study [3] 1 3…”

“…After extraction of the UF 6 gas, the cylinders were cut open to extract solid hydrolysis uranium materials (i.e., the "loose heel" material) that could be poured out of the cut cylinders, and wall deposit material that was scraped from the sides of the cylinders. These materials were investigated in Rolison and Williams [3].…”

“…The degree to which these assumptions are met in diverse materials produced during different stages in the nuclear fuel cycle is an area of active research; different materials that are both consistent and inconsistent with model age assumptions have been recognized [1][2][3][4]. Furthermore, simply because a material is discovered to have discordant model ages does not preclude gaining useful information from the radiochronometric investigation.…”

“…However, U-series daughter and granddaughter progeny, including Th, Pa, Ac and Ra, theoretically do not form volatile fluorides at the pressure-temperature conditions under which UF 6 is in the gas phase [5], so any conversion of a uranium hexafluoride gas to a different phase is predicted to reset the model age clock to the time the gas was converted into a form that was capable of retaining progeny nuclides, usually when converted to a solid fluoride or oxide form. Previous analysis of solid uranium hydrolysis products ("heels") extracted from emptied UF 6 gas cylinders found that these materials contain the bulk of daughter and granddaughter progeny produced from radioactive decay of the uranium, so that the model ages of the heels are well in excess of the known times of cylinder filling [3]. However, it is unknown via direct measurements of real-world nuclear fuel cycle materials whether the UF 6 gas in equilibrium with the heels will exhibit complete radiochronometric resetting to the date when it was extracted from the cylinder and converted to a liquid or solid form.…”

“…Here we investigate this process through radiochronometric analysis of a set of solid hydrolyzed uranium samples produced from highly enriched UF 6 gas extracted from cylinders at a known time. The model ages of the cylinder heels from one of the same cylinders were determined in a previous study [3]. The full complimentary nature of all of these analyses allow for a comprehensive understanding of the behavior of parent and progeny isotopes in UF 6 gas cylinders and provide the foundational knowledge vital towards interpretation of these signatures in similar, yet unknown, materials.…”