Investigation of the impurities and fission products in the AVR coolant gas at an average hot-gas temperature of 950°C

Distribution of fission products palladium, silver, cerium and cesium in the un-corroded areas of the locally corroded SiC layer of a neutron irradiated TRISO fuel particle

“…Furthermore, Ag and Cd usually do not coexist with U or Ce. The present study indicates that Ce-containing precipitates with low Ce concentrations (usually <1 at.%; e.g., Precipitates 7 and 12 in Table 1, and Precipitates 2,6,13,20,23,and 34 in Table 2) are present in the unreacted SiC areas outside the carbon "channel" . These precipitates are anticipated to be from the carbon "channel."…”

“…Ag partitions to palladium silicides and Pd in the carbon areas, and the palladium silicides and Pd may assist Ag transport through the carbon areas and across the SiC layer; in addition, Ag and Pd migrate through nanocracks in the carbon areas [28] . The present study indicates that Ag containing precipitates are present in the that Ag also migrates through the unreacted SiC areas; the mechanisms for this migration are anticipated to be similar to those for Ag transport in the SiC layer in other TRISO particles without localized SiC corrosion, for example, grain boundary diffusion [15-17, 19, 45-48] , neutronenhanced diffusion [13,41,49] , Pd-assisted transport [17,24,29,30] , and vapor-phase migration [6,14,21] .…”

“…The results of the present study indicate that Pd-U, Pd-Si, and Pd-Si-U precipitates are all located at SiC GBs, and that pure Pd precipitates are present at SiC GBs and inside SiC grain interiors, as shown in Figures 2-6 and associated Tables 1-5. The migration of Pd in SiC is well known [6,8,13,29,37,38] , and the diffusion coefficient of Pd in SiC is relatively high [39,40] . It is likely that both the lattice diffusion and GB diffusion coefficients are significant; accordingly, both lattice diffusion and GB diffusion are becoming significant for Pd transport in SiC around 1,000 o C. In addition, neutron irradiation is anticipated to have significantly enhanced the diffusion, owing to high concentrations of defects generated in far excess of thermal equilibrium values [13,41] .…”

“…Both historical in-reactor experiments [6][7][8] and the first Advanced Gas Reactor experiment (AGR-1) at Idaho National Laboratory (INL) [4,5] indicated release of certain metallic FPs (e.g., Ag and Pd) across intact TRISO coatings. It is very important to understand the transport mechanisms of metallic FPs across the TRISO coating layers, since the release of radioactive FPs (especially 110m Ag) is a potential worker safety concern.…”

Influence of SiC grain boundary character on fission product transport in irradiated TRISO fuel

“…The silicon carbide (SiC) layer of tristructural isotropically (TRISO) coated nuclear fuel particles has been carefully engineered to act as the primary containment for fission products in high-temperature gas reactors; however, significant release of certain fission product elements through seemingly intact fuel particles has been reported in a number of irradiation experiments [1][2][3][4][5].The body of research over the last 40 years strongly suggests that grain boundaries are the primary pathway for fission product migration through the SiC layer. This theory is supported by results of recent electron microscopy examinations of neutronirradiated silicon carbide (SiC) from TRISO-coated particles from the US Department of Energy's Advanced Gas Reactor (AGR)-1 experiment, which found strong evidence of fission products at SiC grain boundaries [6][7][8].…”