SiC layer microstructure in AGR-1 and AGR-2 TRISO fuel particles and the influence of its variation on the effective diffusion of key fission products

Abstract

Tristructural isotropic (TRISO) coated particle fuel is a promising fuel form for advanced reactor concepts such as high temperature gas-cooled reactors (HTGR) and is being developed domestically under the US Department of Energy's Nuclear Reactor Technologies Initiative in support of Advanced Reactor Technologies. The fuel development and qualification plan includes a series of fuel irradiations to demonstrate fuel performance from the laboratory to commercial scale. The first irradiation campaign, AGR-1, included four separate TRISO fuel variants composed of multiple, laboratory-scale coater batches. The second irradiation campaign, AGR-2, included TRISO fuel particles fabricated by BWX Technologies with a larger coater representative of an industrial-scale system. The SiC layers of as-fabricated particles from the AGR-1 and AGR-2 irradiation campaigns have been investigated by electron backscatter diffraction (EBSD) to provide key information about the microstructural features relevant to fuel performance. The results of a comprehensive study of multiple particles from all constituent batches are reported. The observations indicate that there were microstructural differences between variants and among constituent batches in a single variant. Insights on the influence of microstructure on the effective diffusivity of key fission products in the SiC layer are also discussed.

Grain boundary complexions in silicon carbide

“…However, EBSD results, such as those recently published by Lillo and Gerczak,3,19 could provide a route to select conditions to produce more stable coatings with a lower probability of complexion transitions. However, EBSD results, such as those recently published by Lillo and Gerczak,3,19 could provide a route to select conditions to produce more stable coatings with a lower probability of complexion transitions.…”

Estimation of Routine Discharge of Radionuclides on Power Reactor Experimental Rde

“…There is a significant retention of the fission products in the kernel itself. The fuel kernel and the SiC coating are the major barriers for fission products release from the fuel particle [6,[13][14]16] . The graphitic materials do not retain fission gases and iodine, but operation experience indicates that fission metals like strontium and cesium are strongly retained in these materials.…”

“…The graphitic materials do not retain fission gases and iodine, but operation experience indicates that fission metals like strontium and cesium are strongly retained in these materials. Retention in the graphitic materials are a strong function of temperature and fast fluence [6][7][8][9]12,14] . Figure 3 is the basis for calculation of the process of fission products release from pebble fuel to the environment.…”

“…The primary circuit inventory of long-lived condensable radionuclides such as Cs-137 and Sr-90 continues to increase during the lifetime of the facility. The amount of each nuclide present in the coolant depends on the release of that nuclide from the fuel, radionuclide half-life, adsorption of certain species of radionuclides in the fuel, in core graphite and on graphite dust, plate out of certain species on primary circuit components, drop out of dust in areas of low velocity, and performance of the helium purification system (HPS) [8][9][13][14] . Table 3 shows that the dominant fission products in source terms on routine operating appear to be noble gases because they are inert, which means that they do not interact with matters.…”

“…As upholstery, high density OPyC shrinks during irradiation time, which also produces compressive stress in SiC dimension, and is partly compensating for the components of the tensile stress caused by the internal gas pressure. OPyC coatings are also effective in maintaining the fission gas in the fuel particles in case of SiC fabrication defect that fails to work up to about 1800 °C [2][3][4]7,[12][13][14] .…”