U.S. Department of Energy

Pacific Northwest National Laboratory

Monitoring diffusion of actinide daughters and granddaughters in metals for chronometer applications

Please join us on Tuesday, March 15, 2016, from 11:00 a.m. to Noon in the RPL Plutonium Room, when the NPSI will host a monthly Seminar Series featuring LDRD PI Dallas Reilly presenting on the project Monitoring Diffusion of Actinide Daughters and Granddaughters in Metals for Chronometer Applications.

Research Team: Dallas Reilly, David Abrecht, L Yulan, Shenyang Hu, Jon Schwantes, and Janet Cloutier

Tuesday, March 15, 2016
Dr. Dallas Reilly
NPSI Project Manager
PNNL
Actinide chronometers serve to age-date various activities associated with the production and processing of nuclear materials, both for raw materials and for substances originating from used nuclear fuel. Competent understanding of chronometer accuracy and precision are vital to applying this information to national security applications, but other factors such as chemical and physical interactions of the parent/daughter/granddaughter nuclides, namely due to often drastically varying chemical properties, should also be considered. As part of the third collaborative materials exercise (CMX-3) of the Nuclear Forensics International Technical Working Group, participants analyzed samples from two highly enriched uranium “logs”, generated at the Y-12 National Security Complex. Previously unknown to the communities interested in chronometers, participants discovered that the uranium/thorium (U/Th) chronometer is reset during the casting process. Thus, chronometers may provide a useful measure of the time since casting. However, the theoretical basis to describe why and how these chronometers are reset (and why others are not) is missing and the analyses facilitated by CMX-3 alone are not sufficient to provide that basis. This project seeks to perform controlled experiments and modeling efforts to provide this theoretical basis in order to appropriately apply the use of chronometers for national security applications. Initial experiments will involve the heating of U metal samples to melting or near melting temperatures, followed by a slow cooling to allow the Th to migrate. The Th concentration as a function of depth on focused ion beam “lift-outs” (10 µm deep x 2 µm x 20 µm wide sections that are machined out of the surface of a material) will be probed with secondary ion mass spectrometry, a tool that is excellent for spatial analysis of chronometers on the lift-out scale. The gradients of Th formed will allow for a competent modeling effort, which will be applied to other parent/daughter/granddaughter series of interest.
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