One of the big pushes in the effort to spark a 21st century renaissance in nuclear power is the development of new reactor designs that only a couple of decades ago were experimental with only a limited prospect of ever becoming practical.

One class of these so-called Generation IV or Gen IV power plants is molten salt reactors, which replace enriched uranium or plutonium fuel rods and water moderator/coolant with a mixture of nuclear fuel and molten salt. It's a concept that seems a bit odd at first, but it's one that provides all sorts of advantages over the common pressurized water reactors in use today.

There are several different kinds of molten salt reactors, but they have a number of features in common.

For one thing, they operate at higher temperatures than conventional reactors and at atmospheric pressure. This makes them much more efficient and reduces mechanical stresses, while also eliminating the threat of a runaway meltdown because the nuclear reaction is self-limiting. Also, dangerous or damaging gases like hydrogen and xenon are easily vented off by a simple chemical process.

Because they operate at temperatures of about 600 °C (1,112 °F), molten salt reactors have a 50% greater efficiency. They can continuously recycle their fuel, which reduces nuclear waste – and new fuel can be added and the waste removed by what is essentially plumbing.

They're also quite flexible, capable of handling a variety of fuels, which helps not only with the economics but also with reducing the proliferation of nuclear weapons. Not to mention that the reactor designs can be modular and readily adapted for small-scale plants that can be used for a variety of industrial applications, including petroleum production, hydrogen generation, desalination, floating power plants, and ship propulsion.

This sounds all well and good, but why haven't molten salt reactors been built before? The answer is that such reactors have been used since the very dawn of the nuclear age. In fact, one of the first reactor designs drawn up for the Allied Manhattan Project to build the first atomic bomb would have used a slurry of salt and uranium. However, that one didn't last long because there wasn't enough uranium fuel available and the molten salt design wasn't any good for making plutonium, so Oppenheimer et al went for a graphite reactor instead.

Since then, there have been a number of molten salt projects, including one for submarines and another for (Lord help us) powering aircraft, but they never really caught on. That's because nuclear reactors aren't anywhere near as simple as the illustrations in school textbooks might lead one to believe.