Silver diffusion in silicon carbide

The diffusion of silver through SiC has been probably one of the main topics of discussion within the HTR community for the past 40 years. Among all the elements that are produced in the kernel, silver apparently is capable of diffusing through SiC even in what were considered intact fuel. Several ideas have been suggested to explain this phenomena but none has been able to fully explain all the differences observed in actual fuel.

Probably one of the first, if not the first, theory was proposed by Heinz Nabielek (a German colleague fortunately still very active within the HTR community) in 1977. He suggested that the presence of excess silicon was the reason for this diffusion. In 1984 it was observed that 90% of silver was released from large columnar grained SiC coatings, in contrast to the 30% for small grained microstructures. This result suggested that the diffusion of silver was microstructure related. More recently, in 2006 a group in the US proposed that silver diffused due to the presence of nano-cracks in SiC. They reached this conclusion after not observing any diffusion of silver in Ag implanted SiC wafers. It was until 2010 during the HTR conference in Prague that E. López-Honorato and co-workers showed that silver can diffuse through SiC by grain boundary diffusion. Their results also questioned the ideas that silver diffused due to the presence of excess Si or long columnar grains. Instead, they suggested that the characteristics of the grain boundaries and their distribution are what dominate the diffusion of this element. Similarly, Neethling and co-workers also included another piece to the puzzle by observing that the presence of other elements, such as Pd, can enhance Ag diffusion. These latest experiments have prompted the use of computer models to help us understand this issue. Simulations by molecular dynamics at Wisconsin University in the US have also supported a process governed by grain boundary diffusion due to the differences existing between low- energy and high-energy grain boundaries.

Several groups in the US, Europe, South Africa and now Mexico are working towards understanding this challenge that without a doubt is of relevance not only for the nuclear industry but materials science itself.

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