Aspects Of Crystal Growth And Atomic-Scale Characterization Of U/Th Age-Dating Particles

Trace analysis of particulate material collected on environmental swipe samples continues to be one of the cornerstones of the IAEA’s process of verifying member state compliance as part of the Non-Proliferation Treaty (NPT). While complementary to traditional bulk analysis, the ability to measure individual particles can be far more impactful from a radiochronometry perspective. In radiochronometry, the “age” of the material ideally records the time when the sample was manufactured or produced in the final form that is analyzed in the laboratory. Capturing this information from individual particles provides a much more detailed record of a nuclear facility’s operational history. Consequently, the 234U/230Th radiochronometer is being evaluated as a means of establishing model ages for discrete particles with sufficient confidence. To support the extension of current capabilities, reference materials with a suitable particle form factor and well-documented purification date are actively sought for development with large geometry secondary ion mass spectroscopy (LG-SIMS).To meet this need, PNNL is formulating a range of fit-for-purpose uranium particles standards that address the changing criteria encountered with the incremental development of an LG-SIMS age-dating methodology. PNNL’s current inventory of UO2 particle standards have been designed and tailored for interlaboratory comparisons between the IAEA Environmental Sample Laboratory (IAEA-ESL) and the expansive Network of Analytical Laboratories (NWAL). Building on this initial foundation, a modified chemical strategy for the hydrothermal synthesis and growth of single-crystal Th1-xUxO2 particles is being explored. In this presentation, considerations surrounding initial chemical separation, precursor formulation, and finely controlled synthesis of homogeneous particles will be outlined. Mechanistic details of PNNL’s particle synthesis strategy and the thorough characterization of elemental composition, crystal structure, and isotopic profile will be discussed at length.