Hastelloy-N, it's expensive stuff but it works. Corrosion is one of the difficulties of molten salt reactor designs, but it's manageable with the right materials. And the tradeoff is that you don't need an enormous high-strength containment vessel (because everything radiological is at 1 atm), you don't need to pump water into the thing constantly to avoid "really bad things" happening (Fukushima), etc, etc.
Specifically you should read the findings of the MSRE (Molten Salt Reactor Experiment) at ORNL.
Wikipedia has a relatively laymans explanation of the state of the Hastelloy-N when the experiment concluded.[1]
It's worth mentioning Hastelloy-N isn't that expensive all things considered. It would probably come out to a small percentage of the reactors total cost factoring in R&D expenditure etc.
Additionally, while using a lot of Hastelloy-N would be expensive, in the context of a full reactor it's a modest expense. Especially when you compare it against the extremely enormous forgings and gigantic reinforced concrete structures that traditional lightwater reactors use. Overall you end up with a much smaller and much safer reactor. And one that can burn existing high-level transuranic waste as well. It's such a huge win it's criminal we're not spending billions in R&D right now.
Well, thanks for the citation. I'm now much more confused about why this is what I was told in class by an expert in the field. Maybe he meant specifically the difficulty of finding a steel up to the challenge, it was a decade ago so some fidelity may have been lost!
