Rachel Seibert

The structure and speciation of fission products within the SiC barrier layer of tristructural-isotropic (TRISO) fuel particles irradiated to 19.6% fissions per initial metal atom (FIMA) burnup in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL) was investigated. As-irradiated fuel particles, as well as those subjected to simulated accident scenarios, were examined. The TRISO particles were characterized using synchrotron X-ray absorption fine-structure spectroscopy (XAFS) at the Materials Research Collaborative Access Team (MRCAT) beamline at the Advanced Photon Source. The TRISO particles were produced at Oak Ridge National Laboratory under the Advanced Gas Reactor Fuel Development and Qualification Program and sent to the ATR for irradiation. XAFS measurements on the palladium and silver K-edges were collected using the MRCAT undulator beamline. Analysis of the Pd edge indicated the formation of palladium silicides of the form PdxSi (2?=?x?=?3). In contrast, Ag was found to be metallic within the SiC shell safety tested to 1700?°C. To the best of our knowledge, this is the first result demonstrating metallic bonding of silver from fissioned samples. Knowledge of these reaction pathways will allow for better simulations of radionuclide transport in the various coating layers of TRISO fuels for next generation nuclear reactors. They may also suggest different ways to modify TRISO particles to improve their fuel performance and to mitigate potential fission product release under both normal operation and accident conditions.

Technetium-99 (99Tc) is important to the nuclear fuel cycle as a long-lived radionuclide produced in ∼6% fission yield from 235U or 239Pu. In its most common chemical form, namely, pertechnetate (99TcO4 −), it is environmentally mobile. In situ hydrogen sulfide reduction of pertechnetate has been proposed as a potential method to immobilize environmental 99TcO4 − that has entered the environment. Reactions of 99TcO4 − with sulfide in solution result in the precipitation of Tc2S7 except when olefinic acids, specifically fumaric or maleic acid, are present; a water-soluble 99Tc species forms. NMR (1 H, 13C, and 2D methods) and X-ray absorption spectroscopy [XAS; near-edge (XANES) and extended fine structure (EXAFS)] studies indicate that sulfide adds across the olefinic bond to generate mercaptosuccinic acid (H3MSA) and/or dimercaptosuccinic acid (H4DMSA), which then chelate(s) the 99Tc to form [99TcO(MSA)2] 3−, [ 99TcO(DMSA)2] 5−, or potentially [99TcO(MSA)(DMSA)]4−. 2D NMR methods allowed identification of the products by comparison to 99Tc and nonradioactive rhenium standards. The rhenium standards allowed further identification by electrospray ionization mass spectrometry. 99TcO4 − is essential to the reaction because no sulfide addition occurs in its absence, as determined by NMR. Computational studies were performed to investigate the structures and stabilities of the potential products. Because olefinic acid is a component of the naturally occurring humic and fulvic acids found in soils and groundwater, the viability of in situ hydrogen sulfide reduction of environmental 99TcO4 − as an immobilization method is evaluated.