Molten salt reactors, a type of nuclear reactor first explored in the 1950s, could be the future of clean energy — if we can overcome the problems that have held them back for more than half a century.Part of the reason why is that it takes approximately 7 years and $10 billion to build a new nuclear plant like the ones we already have, and some would-be operators are wary of making such a big investment, especially when electricity from natural gas and renewables is getting cheaper.In the 1950s, US researchers started exploring the concept of molten salt reactors, which use molten salt — salt that’s solid at room temperature, but liquid at high temperatures — in place of water as the material transferring the heat and keeping the fuel at a stable temperature.Molten salt reactor designs also include a safety feature called a “freeze valve” or “freeze plug.” This plug separates the molten salt mixture above from a holding tank below it.But over the course of the next four years, it was shut down unexpectedly 167 times — mainly due to technical problems involving various components — and in 1969, it was shut down permanently."