On the design page they describe this as a molten salt reactor with dissolved nuclear fuel. One of the principle difficulties designing this type of reactor is

On the design page they describe this as a molten salt reactor with dissolved nuclear fuel. One of the principle difficulties designing this type of reactor is the choice of materials used to conduct the liquid fuel, which is very hot, caustic, and radioactive. Steel and aluminum are out, as they are both chemically reactive at those temperatures. Other materials such as nickel alloys or certain ceramics may be possible, but the radioactivity of the fuel tends to embrittle materials over time as atoms in the containing material are converted by exposure to the fuel's neutron emissions. The affected components include not just the reactor vessel, but also pipes, and in some MSR designs also pumps and heat exchangers. If those are subject to regular replacement, the used components are also low-grade nuclear waste and must be properly contained indefinitely. https://en.m.wikipedia.org/wiki/Molten_salt_reactor https://en.m.wikipedia.org/wiki/Molten_salt_reactor The other main impediment is regulatory: since we've never had commercial reactors of this type we don't have a legal framework to license these types of reactors to be run (and eventually decommissioned) safely. At least, these are my novice understandings from some skeptical background reading. If someone with actual nuclear industry and/or research experience has a more detailed understanding, feel free to weigh in and correct me.