Lars wrote:If the graphite is exposed to the fuel salt then expansion beyond the original size is a concern. Namely, some of the Xe would tend to get trapped inside the graphite rather than get sparged out and would steal neutrons. Not an issue if the graphite has a metal cladding but then we get back to concerns over damage to the wall - though without the strength requirement so we can be more aggressive.
Yes, I`ve wondered if Xenon trapping is the only thing that ORNL was really worried about in terms of pebbles beyond the tradition fast fluence limit. ORNL would certainly have wanted to minimize xenon since it will lower your breeding ratio. Today though I think we can spare the neutrons so the question of how long a pebble can last is still an interesting one. Perhaps it is of of secondary concern though considering that it should be much easier to replace pebbles than logs. Pebbles do have issues of their own though and might represent a bigger R&D program than we realize. For a DMSR design I`ve really been liking pebbles for awhile now but a good old fashioned huge version of the MSRE that doesn`t need to replace graphite like ORNL proposed has a lot to admire as well. I used to cringe at the thought of how big the DMSR cores might be but when you really compare them to older graphite designs or even just a PWR vessel they really aren`t that big.