Pd, a high yield fission product, has been shown to migrate and cluster throughout the kernel, OPyC, SiC, and IPyC layers in TRISO particles especially at rapid diffusion sites such as grain boundaries and layer interfaces. Collection of Pd in and around the SiC layer leads to degradation through the formation of Pd silicides and free carbon. As such, Pd corrosion of the SiC layer is expected to have deleterious effects on the mechanical performance of this layer. However, little work has been done to evaluate the effects of Pd corrosion on the mechanical performance of the SiC or the PyC-SiC interlayers. The existing investigations are typically conducted on post irradiated specimens, but neutron irradiation is challenging because of irradiation time requirements, hazards associated with handling radioactive materials, and limited access to neutron irradiation sources. The mechanical investigation of surrogates is an attractive alternative because challenges associated with neutron irradiated samples do not exist. A necessary first step to evaluating the mechanical behavior of neutron irradiated specimens through the investigation of surrogate materials, is determining whether surrogate mechanical data is representative of neutron irradiated sample behavior. This study aims to determine the relevance of surrogate mechanical data to actual neutron irradiated specimen behavior through SEM in situ tensile experiments and the comparison of as received, surrogate, and neutron irradiated SiC-PyC interlayer mechanical results. The as received and surrogate samples to be used in this study are composed of ZrO2, graphite, and SiC while the MNT64X sample irradiated in AGR 2 is composed of UO2, graphite, and SiC. Surrogate sample Pd attack was driven by Pd sputtering and heat treatment while Pd attack in the irradiated samples were driven by neutron irradiation. The study will require 52 hours which includes approximately 45 hours to prepare samples and approximately 7 hours to carry out tensile tests. This work will begin when the grant is awarded, and completion will depend on the availability of equipment. However, sample preparation and experimentation should require no more than two normal working weeks. These experiments will provide valuable information regarding the use of surrogate specimens in place of neutron irradiated specimens and may lead to more efficient, less costly experiments in the future.

The DOE's Office of Nuclear Energy aims to advance nuclear technology to meet the nation's energy, environmental, and economic needs. Developing robust, high-performance nuclear fuels like TRISO is essential for next-generation reactors to provide safe, clean, and abundant energy. This study directly supports DOE-NE's R&D efforts by evaluating the mechanical performance of irradiated TRISO fuel. In particular, we will investigate how fission product palladium affects the SiC-PyC interlayers in TRISO particles. Understanding Pd-induced degradation and the resulting mechanical performance in these structures will enable the design of more durable TRISO fuels with extended lifetimes. Additionally, this study will pioneer the use of unirradiated surrogate TRISO samples for mechanical testing. This approach does not require time consuming neutron irradiation or the handling of radioactive materials thus greatly reducing the cost and accelerating the pace of these studies. Despite the importance of SiC-PyC interlayers to TRISO integrity, few studies have examined how Pd corrosion impacts their mechanical performance and no work has validated surrogate sample data as an alternative for irradiated particle data. UTSA's Extreme Environments Materials Lab is uniquely equipped to fill these knowledge gaps. By combining their existing Pd diffusion research with the proposed mechanical testing, we can holistically evaluate how Pd degrades TRISO layers. The surrogate sample approach we pioneer here will also provide an invaluable new capability for cost-effective irradiated fuel studies. In summary, this timely project aligns with DOE-NE's mission by elucidating Pd attack and failure modes in TRISO particles. The knowledge and testing techniques developed will accelerate the qualification of robust, high-performance TRISO fuels needed for next-generation nuclear reactors.