The process for licensing a LWR is well established. Commercial LMFBR (Fermi 1) and HTGR (Peach Bottom 1) reactors have been approved in the US but those were a very long time ago. Anybody who wants to license a new reactor type is going to work through an extensive process with the NRC to determine how exactly it is done.
They built an MSR in Wuwei, China and just got approval to start it.
AFAICT materials research is a big part of it. At 800°C even water is pretty reactive; the salt is much more so. Even though an MSR does not need the high pressure of water-based reactors, you can't make all the piping from graphite or platinum.
