This technology is not as new as they make it out to be. They've been used molten sodium in breeder reactors since the early 50's. That being said, things haven't always gone well. Take Fermi I as an example.
Fermi I was a so-called 'fast breeder' reactor built just South of Detroit, near Monroe, Michigan. 'Breeder' reactors were supposed to, in addition to producing electricity, they would also transmutate lower grade Uranium into a more radioactive version which could then be harvested from the expended fuel rods and reprocessed into fuel for a conventional reactor. The idea was sold to the public as something that would produce more fuel that it would use, getting the electricity as basically a 'free' byproduct. I know all of this as I was a teenage when this plant was being built and my cousin worked for one of the electrical contractors that helped build the plant. He got us a private tour of the plant while it was still under construction. We were even allowed into the containment building, right up next to the reactor itself (that was a year or so before the reactor was 'fueled'). As the plant was nearing completion the PR people promoted the idea that this plant was going to revolutionize the nuclear power generating industry and that Fermi I was going to be the first of dozens of plants like it. In the end, it was the first and ONLY commercial 'fast breeder' reactor ever built. Note that the only other ones that were in operation in the US were those used to produce Plutonium for use in nuclear weapons. Obviously these were run by the government and were not used to produce power.
For more information about Fermi I, go to:
Note that the term 'China Syndrome' was coined as a result of descriptions by the engineers and scientists of what might have happened had the entire core melted in the Fermi I plant in 1966. The accident that happened in 1966 caused a control rod to stick and the reactor overheated causing a partial melted-down of the fuel-rods. They were able to shut the reactor down and cool the core avoiding a total meltdown, but it was close. There was a book written in 1975, titled 'We Almost Lost Detroit', which detailed what went wrong and how close we came to a full blown nuclear disaster.
For the record, I was living in Michigan at time of the accident, but I was in engineering school in the Upper Peninsula, several hundred miles away, but I had several family members, including aunts, uncles and cousins, living within only a few miles of the plant, some of them could even see the containment building from their homes.
People talk a lot about 'Three Mile Island', but in reality, the Fermi I meltdown, because of the reactor design using molten Sodium as the liquid that transferred the heat from the reactor to the steam generation system, if it had ruptured the resulting explosion would have been catastrophic, and the potential to do much more damage than 'Three Mile Island' could ever have done.