After fuel kernel size and shape are measured by optical shadow imaging, the TRISO coatings are deposited via fluidized bed chemical vapor deposition in a 50-mm diameter conical chamber within the coating furnace. Computer control of temperature and gas composition ensures reproducibility and tight deposition control with typical batch sizes of 60 to 100 g.
CFP physical properties are measured by a suite of instruments. Mercury porosimetry, helium pycnometry, and liquid gradient density columns measure coating densities. Metallographic polishing is followed by computer-automated optical microscope imaging and analysis of coating thicknesses, pyrocarbon layer anisotropy is measured by the ORNL-developed, R&D 100 Award-winning two-modulator generalized ellipsometry microscope (2-MGEM), and coating microstructures are examined by scanning electron and transmission electron microscopy. X-ray tomography provides complimentary non-destructive examination of CFPs with up to 1.4 µm resolution.
The CFPs are prepared for irradiation testing via fabrication of small cylindrical fuel compacts. The compacting process applies a protective overcoat of resinated graphite matrix precursor to the CFPs. A computer-controlled servo press forms the overcoated particles into the final fuel shape, with a series of heat treatments up to 1800°C to carbonize and purify the graphite matrix. Compact metrology, characterization and determinations of maximum defect populations are performed to NQA-1 standards prior to reactor insertion. Uranium carbide/uranium oxide (UCO) fuel compacts fabricated by ORNL for the DOE-NE Advanced Gas Reactor Fuel Development and Qualification Program have exceeding historical burnup levels for LEU CFPs with zero particle failures and are the first successful demonstration by the US of this fuel technology.