Microstructural and Irradiation Effects on Ag and Cs Diffusion in CVD-SiC
EMSL Project ID
39948
Abstract
The DOE has selected the Very High Temperature Gas Cooled Reactor (VHTR) as its next generation nuclear plant. The VHTR is designed to operate at higher temperatures than conventional light water reactors allowing nuclear energy to process heat for industrial applications which have traditionally burned fossil fuels to provide process heat. Replacing fossil fuel process heat with nuclear process heat will alleviate the green house gas emissions and mitigate supply concerns from foreign fuel supplies by reducing demand. At the core of the VHTR technology is fuel performance. For current VHTR designs TRistructural-ISOtropic (TRISO) fuel has been identified as the fuel of choice. The SiC layer in TRISO fuel provides structural integrity and serves as the main fission product (FP) barrier during operation, however, during operation release of 110mAg and 137Cs has been observed at both normal and accident conditions. The release of 110mAg and 137Cs leads to immediate maintenance issues and long term contamination. The SiC layer must prevent FPs from being released from the particle core to ensure safe and efficient operation. Accurate diffusion coefficients are required to predict fuel lifetimes and efficient operation, reducing safety and maintenance concerns. The transport mechanism must be understood to develop an engineering solution to improve the retention of FPs by the SiC layer. Diffusion coefficients from TRISO particle fractional release plate-out experiments have been reported for Ag and Cs in SiC with orders of magnitude variation in reported diffusion coefficients. It has also been shown that fractional release is affected by microstructure with particles with large columnar grains releasing more of their inventory than particles with lamellar grains[1] suggesting a microstructurally dependent diffusion mechanism. Although fractional release measurements provide a rough transport result on the permeability of SiC, the effects of the detailed microstructure on the transport has not been illuminated. The aim of this work is to measure diffusion coefficients for volume (Dv) and grain boundary diffusion (Dgb) for Ag and Cs in SiC at normal operating and accident temperatures, determine solubility limits, understand the effect of grain boundary character distribution, and investigate the effect of irradiation on diffusion. This will be accomplished through Ag and Cs implantation studies on SiC carried out on a Pelletron 3.4 MeV Tandem Accelerator at the ion beam user facility at the University of Wisconsin-Madison. Following implantation, samples will be exposed to elevated temperatures (1200oC-1600oC) to induce thermal diffusion. Single crystal 6H-SiC samples are included to investigate volume diffusion while diffusion in 3C CVD-SiC along different microstructures to understand the effect of grain boundary character distribution and grain shape and size on diffusion. Irradiation effects on CVD-SiC will be investigated via proton irradiations of implanted samples at elevated temperature at the ion beam user facility at the University of Wisconsin-Madison. Diffusion coefficients and solubility limits will be calculated from concentration profiles past the implantation peak measured by Secondary Ion Mass Spectroscopy at Pacific Northwest National Laboratory's Environmental and Molecular Science Laboratory.[1] D. A. Petti, J. Buongiorno, J. T. Maki, R. R. Hobbins and G. K. Miller, Nuclear Engineering and Design, 222, 2-3 (2003)
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members