Transport Mechanism and Microstructural Effects on Ag and Cs Diffusion in beta-SiC
EMSL Project ID
34303
Abstract
The Very High Temperature Reactor (VHTR) has been identified by the Generation IV initiative and by the US Department of Energy's Next Generation Nuclear Plant (NGNP) as a promising future reactor technology. The VHTR utilizes TRistructural-ISOtropic (TRISO) fuel as the fuel of choice. TRISO fuel is comprised of an oxide fuel kernel, surrounded by a graphite buffer layer, followed by successive isotropic layers of dense inner-pyrocarbon (IPyC), Silicon Carbide (SiC), and an additional outer pyrocarbon (OPyC) layer. Each layer serves a specific function during processing and operation. The SiC layer provides structural integrity and serves as the main fission product barrier during operation. The SiC layer must prevent fission products from being released from the particles to ensure safe and efficient operation. Yet fission products like Ag (at normal operating temperatures) and Cs (at accident conditions) have been observed to release through intact particles.Diffusion coefficients from particle fractional release plate-out experiments have been reported for Ag and Cs in SiC with variability. 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. It has been reported that the retention of Ag and Cs is affected by SiC microstructure, while the dominate transport mechanism is hypothesized to be a short circuit mechanism such as grain boundary diffusion. An understanding the transport mechanism and microstructural contributions is the first step in developing an engineering solution to the problem of fission product release in the TRISO fuel system. This work aims to provide insight on the transport mechanism, diffusion kinetics, and microstructural effects of Ag and Cs transport in SiC, ultimately helping provide safe, reliable, and more efficient nuclear energy.
In order to understand the release of Ag in SiC we have designed an experiment to understand the role of microstructure on fission product transport. We have encapsulated SiC/Ag and SiC/CS diffusion couples in a molybdenum canister to ensure contact between the two diffusion couple elements and no loss to the surrounding environment. The diffusion couples will be exposed to temperatures spanning 800 to 1500oC for up to 1000 hrs to simulate normal and the onset of accident conditions. Analysis of the Ag and Cs chemical interactions will require techniques with high spatial resolution and high chemical sensitivity not available at the University of Wisconsin because of the low diffusion coefficients associated with SiC and the small volume associated with short circuit paths such as grain boundary diffusion. We propose to obtain concentration profiles with RBS and SIMS in order to obtain information on diffusion kinetics and the dominate transport mechanism. We also propose to conduct elemental mapping of surfaces normal to the diffusion couple interface using AES and SIMS and conduct AES on in-situ fracture surfaces to understand the contributions from specific microstructural features.
Project Details
Project type
Exploratory Research
Start Date
2009-06-01
End Date
2010-06-06
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
T. Gerczak, T.R. Allen, and Z. Zhu, Fission Product Transport of Cesium and Silver in CVD-SiC, American Nuclear Society Transactions, 2010 Annual Meeting, San Diego, Ca June 13-17, v. 102 pp 822-823