Using Irradiated Polymers to Forensically Reconstruct Uranium Enrichment

"Trust, but verify" is the operative phrase when it comes to verifying nuclear non-proliferation treaties (NPTs). Trust is easier accomplished than verification. Fortunately, polymers are ubiquitous in uranium enrichment systems, finding use as gaskets, containers, and resins and epoxies to name a few, and they are particularly sensitive to ionizing radiation damage. While the majority of research about radiation effects concerns metals and ceramics, the ubiquity and sensitivity of polymers renders them both useful and under-utilized to act as retrospective dosimeters in any process involving ionizing radiation.<br/><br/>Here we show that multiple polymers, ranging from PTFE [1] to PCTFE [2] to fluorinated epoxy and beyond, exhibit sufficient sensitivity to function as retrospective dosimeters in forensically reconstructing uranium enrichment activities. Differential scanning calorimetry (DSC) is used to establish the relationship between a polymer's irradiation dose and changes to its material properties. Nanoscale flash scanning calorimetry (nano-FSC) further extends this ability to nanoscopically sized specimens, rendering the technique effectively non-destructive and adding additional elements of isotopic selectivity and spoof-proofing. Raman spectroscopy and synchrotron-based small-angle x-ray spectroscopy help to confirm the physical nature of the radiation damage resulting in DSC/FSC-measurable property changes. These reveal a core-shell damage geometry which explains how parts per billion direct radiation damage leads to far higher signal to noise ratios than would be expected. Finally, machine learning is used to improve upon physics-first signal feature identification to help separate nearby dose-response curves, significantly improving the ability to discern between closer levels of radiation damage.<br/><br/>Challenges to facile exploitation of this technique outside of the laboratory remain, including mapping which polymers require which measurement statistics to establish confidence intervals for NPT verification activities, accounting for the uranium-bearing corrosion deposits which form on the insides of enrichment equipment, and further linking the precise structural changes from radiation damage to changes in material properties as measured. The idea of <i>in situ </i>ion irradiation flash scanning calorimetry (I³FSC) will be presented here for the first time, which should allow researchers to improve the data throughput of this technique by a thousand fold, analogous to our group's 1000-fold data throughput enhancement using <i>in situ </i>ion irradiation transient grating spectroscopy (I³TGS) [2].<br/><br/>References:<br/><br/>[1] Connick, R. C., Hirst, C. A., Woller, K. B., Logan, J. V., Kemp, R. S., Short, M. P. "Measuring Very Low Radiation Doses in PTFE for Nuclear Forensic Enrichment Reconstruction." <i>arXiv preprint arXiv:2107.08313</i> (2021).<br/>[2] Nguyen, A., Connick, R. C., Reinfurt, D., Hirst, C. A., Kemp, R. S., Short, M. P. "Measuring Forensic Signatures of Historical Uranium Enrichment in PCTFE." <i>Proc. Amer. Nucl. Soc.</i> 28 (2023).<br/>[3] Dennett, C. A., Buller, D., Hattar, K., Short, M. P. "Real-time thermomechanical property monitoring during ion beam irradiation using in situ transient grating spectroscopy." <i>Nucl. Instrum. Meth. B</i> <b>440</b>:126-138 (2019).