The big worry about nuclear reactors is that the solid fuel rods are going to melt down.

If the core of the reactor loses its cooling water – as it did both at Three Mile Island and Fukushima – then the fuel rods overheat. Even though the nuclear reaction may stop, the decay heat is enough to melt the zirconium fuel rods so that the uranium pellets inside get exposed. If there is some water remaining, the heat may be enough to split off hydrogen, which can cause a hydrogen explosion, as occurred at Fukushima and was feared at Three Mile Island.

In the old days it was argued that the overheated core would melt right through the steel reactor vessel and the concrete containment structure and be on its way to China – “the China syndrome.” Then it would probably hit groundwater and cause a steam explosion that would make “an area the size of Pennsylvania uninhabitable” and so on and so on. All that proved to be fanciful although a good plot for a Hollywood thriller.

But what if the fuel is already liquid so it can’t melt down? Instead it can just be harmlessly drained off into a different container where the fuel will be diluted enough to end the reaction.

This is the principle of the “molten salt reactor,” a design first conceived by the great Dr. Alvin Weinberg in the 1950s and experimented with for twenty years before being relegated to the bookshelves when the nation decided not to do anything more with nuclear energy.

The principal of an MSR is that the nuclear fuel is dissolved in a bath of molten salts. The proximity of the fuel molecules (either uranium or thorium) is enough to heat the salt mixture to around 600 degrees Centigrade – still far below the boiling point of the salts around 1430 degrees C. The heat is then transferred to a turbine to produce electricity.

But what if the fuel solution starts to overheat? Then something very good happens. The salt mixture starts to expand, which moves the fuel molecules further apart from each other. This slows the nuclear reaction and brings the temperature down again. The reactor is thus self-regulating and can’t overheat.

Then there’s something even better. At the bottom of the reactor is a “freeze plug,” a stopper that holds the fuel mixture in the reactor vessel. If for some reason things start to get really hot, the plug is made of a material that will melt at 700 degrees. The molten fuel mix then drains into a large bathtub beneath the reactor, where it spreads out and cools until the nuclear reaction stops. Eventually the molten salt will solidify into a solid block that is relatively easy to handle. The reactor is “a walkaway safe,” meaning that if it spins out of control the operators can just walk away and the reactor will shut itself down. The mistakes and misreadings of gauges that caused both Three Mile Island and the Chernobyl accident can’t occur.

In addition, an MSR operates at normal atmospheric pressure. Light water uranium reactors must be brought to high pressure (the “pressurized water reactor”) because the cooling and moderating water will evaporate at high temperatures. Being under pressure, however, makes conventional reactors vulnerable to leaks and explosions that can scatter radioactive water into the atmosphere. With MSRs, there is no such danger.

Believe it or not, the MSR project actually began as an effort to power a large Air Force bomber with an on-board nuclear engine. The NB-36 bomber actually made a number of flights in the 1950s with a reactor on board before the idea was eventually dismissed as impractical. But Oak Ridge quickly switched to experimenting with the molten salt reactor as an alternative to solid fuel reactors in power plants. The result was a 7.4 megawatt thorium-powered reactor that went critical in 1965 and ran for four years. This was followed by a larger MSR that ran for 1.5 years in the 1970s. At this point, however, research was closed down in favor of the fast breeder reactor, which in turn was close down in the 1990s.

So things sat on the shelf at Oak Ridge for 40 years until Kirk Sorensen, a nuclear engineer at NASA and an enthusiast of thorium, began the heroic task of posting hundreds of Oak Ridge research papers his website, www.energyfromthorium.com. (See “Kirk Sorensen: Thorium’s One-Man Band, RCE, 8/21/15) Interest started to grow so that there are now six small companies exploring the possibility of licensing a molten salt reactor.

• Transatomic Power. Founded in 2011 by Leslie Dewan and Mark Massie, two 2010 MIT graduates, who have raised $2.5 million from the Founders Fund to begin experiments on a prototype MSR.

• ThorCon Power. Also founded in 2011 by Jack Davanney, another MIT graduate and veteran of the ship-building industry, ThorCon is trying to build a 250-watt modular molten-salt reactor that can be barged to the site.

• Flibe Energy. Sorensen’s company, Flibe is named after the fluoride, lithium, beryllium salt mixture that will dissolve the fuel. Flibe is trying to raise money to do the engineering work on a liquid fluoride thorium reactor.

• Terrestrial Energy. Named after this author’s 2008 book on nuclear energy, the Canadian company has signed a contract with Oak Ridge National Laboratory to build a demonstration molten salt reactor in the next five years.

• Moltex Energy. A British company that is also working on a molten salt reactor. John Durham, one of the co-founders, is also co-founder of the Weinberg Foundation, which is trying to promote molten salt energy.

• Seaborg Technologies. Named after Glenn Seaborg, a pioneer in nuclear technology, this Danish firm of young physicists and chemists is attempting to make the waste-consuming molten salt reactor a reality.

Meanwhile, the Chinese are moving ahead rapidly with molten salt as one of the nuclear technologies they have targeted for development. The Shanghai Institute of Applied Physics is planning to build a prototype within the next few years. The Shanghai program is collaborating with – wouldn’t you know it – the Oak Ridge National Laboratory, where the molten salt reactor was born sixty years ago.