Oxidation Performance and Microstructural Analysis of High-Temperature Gas-Cooled Reactor Materials under Off-Normal Conditions

Tristructural-isotropic (TRISO) particles are the proposed fuel form for generation IV high-temperature gas-cooled reactor (HTGR). The versatility of TRISO particles allows for use in other advanced reactor designs or concepts, including salt-cooled high-temperature reactors (FHRs), microreactors, nuclear thermal propulsion, or light water reactors (LWRs) as a fully ceramic microencapsulated fuel. TRISO particles are composed central fuel kernel with four coating layers (three graphitic and one silicon carbide (SiC)). Hundreds of thousands of TRISO particles, each 800-1,200 µm in diameter, are coated in a composite graphitic matrix material. The graphite matrix material is a blend of natural and synthetic graphite with a resin binder. The graphitic compact is 60 mm in diameter in the pebble geometry, with approximately 220,000 pebbles used in a pebble bed HTGR. Off-normal reactor conditions for this type of reactor are a break in the pressure boundary between cooling lines, introducing oxidizing contaminates to the helium and fuel compacts. This scenario would first cause the surrounding matrix material to degrade and evolve volatile oxidation products. The oxidized structure of the matrix material may provide additional pathways for the oxidants to be introduced to the SiC layer. This response suggests that the deteriorated matrix material can aid in creating a mixed gas atmosphere by supplying small amounts of CO and H2 to the steam-contaminated coolant causing the SiC layer to be exposed to these corrosive conditions. The SiC layer provides the structural integrity of the particle and retains both gaseous and metallic fission products in the particle. A failure in this layer may lead to the release of fission products into the fuel compact and cause the particle to be considered a failure. The first portion of the investigation explores the oxidation performance of two blends of matrix graphite when exposed to low partial pressures of steam (< 0.2 atm H2O) at high temperatures (1300 °C < T < 1400 °C) in a thermogravimetric analyzer. Characterization techniques include Raman spectroscopy, thermogravimetry, quadrupole mass spectrometry, scanning electron, and optical microscopy. The results from the gas quantification of the matrix gas products are used to replicate TRISO particle exposure after the graphite matrix material was compromised and formed a mixed gas atmosphere. Determining the oxidation response of the particles and the surrounding matrix material under low partial pressures (< 0.20 atm) of steam and other mixed gases, such as carbon monoxide (CO) and hydrogen (H2), at high temperatures (T > 1,100 °C) will provide insight on the impact of oxidants on the microstructure of the material and progression of the accident scenario. Microstructural analysis will be performed utilizing SEM, FIB, and XRD of post-exposed samples to provide insight into the evolution of the features due to oxidation. The project focuses on the microstructural evolution of the matrix graphite and SiC layer under off-normal reactor conditions. Understanding this behavior of both the graphite matrix material and SiC layer under an off-normal event will aid in investigating the limitations of the fuel form. The oxidation and microstructural data provided by the studies presented here will help refine TRISO fuel performance models to better predict the behavior under accident scenarios.