Interaction between metal fission products and TRISO coating materials: A study of chemical bonding and interdiffusion

The goal of this project is to elucidate the chemical bonding and interface formation of metal fission products with the coating materials used in state-of-the-art TRISO fuel particles. Particular emphasis is placed on an analysis of intermediate chemical phases at the interface, the intermixing/diffusion behavior, and the electronic interface structure as a function of material choice (metal and coating materials), temperature, and external stress. Furthermore, we intend to assess the chemical state of some of the metal fission products. The findings are expected to give valuable information about failure mechanisms of TRISO particles and fission product transport. Secondly, through simulating experiments, the project is intended to give indications for optimized irradiation testing and postirradiation examinations within the AFCI effort at ORNL. Third, it is a goal of the project to derive strategies to tailor the interface properties for an optimization of TRISO particles in terms of, e.g., chemical and long-term stability. In detail, we propose to study the interface formation of Pd, Ag, and Cs with SiC and pyrolytic carbon. Using the TRISO coating materials (and single crystal references) as substrates, interfaces will be prepared under controlled conditions in an ultra-high vacuum environment and will be studied with a variety of different spectroscopic and (when applicable) structural methods. In addition, realistic microstructures will be studied. The combination of surface sensitive techniques (e.g., photoelectron spectroscopy) with bulk sensitive methods (e.g., X-ray emission spectroscopy) will probe the chemical properties as well as the diffusion behavior in several complementary ways. In addition, spatially resolving methods will be employed to characterize cross sectional profiles. A variety of surface modification methods will be applied ex-situ (e.g., for stressed coating layers) or in-situ (e.g., by ion bombardment) prior to or after the interface formation to study the dependence of the interface properties on the surface/interface morphology and quality of the coating material.