Research Programs

Advanced Nuclear Power

-- Select program -- A Very Long Cycle Boiling Water Reactor Solid Hydride Fuel for Long-Lived LWR Core Design Investigation of Nanofluids for Nuclear Applications Supercritical Water Reactors Advanced Gas Cooled Modular Pebble Bed Reactors Fission Product Barrier Integrity Fuel Performance Modeling High Performance Alloy Development for Liquid Metal Service Supercritical CO2 Power Conversion System Methods for Risk-Informing Future Reactor Regulation Gas Cooled Fast Reactor Evaluation

• Further info at Uhlig Corrosion Lab

Publications:

1. H. J. Maclean, and R.G. Ballinger, “Silver Migration in SiC: A New Perspective,” Global 2003, New Orleans, LA, November 16-20, 2003.
2. H. J. MacLean, R.G. Ballinger, “Silver Ion Implantation and Annealing in CVD Silicon Carbide: The Effect of Temperature on Silver Migration,” Second Topical Meeting on High Temperature Reactors 2004 (HTR-2004), Beijing, China, September 22-24, 2004.

Investigators

• Ronald Ballinger
• Heather MacLean
• Michael Short

Fission Product Barrier Integrity

Coated particle fuel is the fuel form of choice for high temperature gas reactor systems. In particular, this fuel form is under consideration for the Very High Temperature Reactor (VHTR) that has been identified as a key technology in the US Department of Energy’s “Nuclear 2020” program. Additionally, the pebble bed reactor, the subject of a recent announcement by the PBMR Corporation of South Africa of approval for construction, will use coated particle fuel. The basic geometry of the coated particle fuel kernel consists of a fuel particle, either UO₂ or UCO, surrounded by successive layers of: (1) a low density graphite buffer (2) an inner dense pyrocarbon (3) CVD SiC (or possibly ZrC), and (4) an outer dense pyrocarbon. From the standpoint of pressure containment the SiC or ZrC layer is the most critical. This layer is also critical to the containment of fission products. In spite of the general robustness of the carbide coating, certain fission products, particularly silver, are released, at least from SiC, at temperatures above approximately 1000°C, apparently by grain boundary diffusion through the SiC layer. The presumed diffusive release mechanism has been supported by values of diffusion coefficients that have been back-calculated from thermally induced release measurements from previously irradiated coated particle fuel. However, to date there has been no direct measurement of the diffusion coefficient from concentration profiles of silver in CVD SiC. In this program experiments have been performed to determine the mechanism for migration of silver in CVD SiC.

Based on the results of this research we draw the following conclusions:

• For the CVD SiC studied Silver does not migrate by a diffusion mechanism.
• The most likely release mechanism is via the gradual evolution of a network of “nano-cracks” during thermal cycling and radiation exposure to the point where a vapor transport release mechanism occurs.
• Given that this is the case and that the SiC studied is similar to the SiC used in coated particle fuel, one must question if Silver release in coated particle fuels is controlled by a diffusion mechanism.

The results of this investigation overturns 40 years of assuming that silver release is by diffusion and opens the door to the development of methodologies to mitigate release through improvements of processing techniques.