In the case of liquid fluoride thorium reactors, I don't think the xenon-135 is what you need to be worrying about; worry about the fluorine gas instead! It's v

In the case of liquid fluoride thorium reactors, I don't think the xenon-135 is what you need to be worrying about; worry about the fluorine gas instead! It's very reactive. Xenon, on the other hand, is chemically stable because it's a noble gas. And it's larger than helium, so it's easier to contain. Also, I'd like to point out that pebble-bed reactors, while usually cooled with helium, could also be cooled with nitrogen that we just pull out of the air. It's more reactive than helium and it forms carbon-14 when you expose it to neutron flux, but those issues are both fairly minor. I think they're going with helium now because they want to get something working as quickly as they can, and they don't want to go mucking with what works.