An old nuclear technology is getting another look, and it could mean clean, emission-free electricity, while at the same solving the problem of nuclear waste. It's called a molten salt reactor, and it's an idea that dates to the late 1950s. A start-up called Transatomic Power, based in Cambridge, Mass., is working on a newer version that uses nuclear waste as fuel. Transatomic's founders are Russ Wilcox, formerly the CEO of E-Ink, and Leslie Dewan and Mark Massie, two MIT students.
The reactor still makes waste, but what comes out is radioactive for only 300 years, as opposed to millennia. Transatomic calls it a Waste Annihilating Molten Salt Reactor, or WAMSR.
"It's stuff that is in the middle of the periodic table," said Dewan, Transatomic's chief science officer. "It's a lot easier to isolate."
A big selling point of this design is that it would help deal with the nuclear waste problem. The Nuclear Energy Institute says there are some 67,000 metric tons of uranium from fuel in the United States alone. It can also be built smaller at lower costs out of modular parts.
The WAMSR can do this because unlike current reactors, it doesn't need to use enriched uranium as fuel, and the fuel itself doubles as a coolant. No need to build this near a water source like an ocean or river that can ultimately flood and cause damage.
But first a bit about how nuclear plants work: In commercial nuclear reactors, a core houses fuel rods that contain uranium oxide pellets. The radioactivity from the uranium produces heat. To make electricity, water is piped into the core and turned into steam, which is then used to drive generators. This is called the "light water" design.
To produce enough heat for the steam, the uranium needs to be highly radioactive. That requires purified or enriched uranium. For a power plant that means that up to about 20 percent of it has to be U-235, an isotope that makes up less than 1 percent of the naturally occurring uranium. (Most of the time the enrichment percentage is lower, on the order of five percent -- the higher levels are used in research reactors and naval vessels).
As the fuel is "burned," the uranium decays into other elements, including plutonium, zirconium, cesium, xenon and iodine. Eventually, enough other elements mix in with the uranium that the nuclear reactions slow down, reducing the efficiency of boiling the water. The fuel is then called "subcritical," or spent, and is put into a waste storage facility.
That's where the WAMSR comes in. Spent fuel from other reactors is dissolved in fluorine to make a molten "salt," a chemical whose elements are bound together by their positive and negative charges. The molten salt is pumped out of the core and into a heat exchanger, where the water is boiled to drive a turbine. Since the heat energy is being transferred to the water, the fuel cools down.
This type of reactor was first proposed as a way of powering a bomber; in the 1960s and 70s there was one operated at the Oak Ridge National Laboratory. But the nuclear industry had settled on the light water reactors, and that became the industry standard.
Because the molten salt behaves like a liquid, it's easier to get it into a shape that allows for self-sustaining reactions, Dewan said. The shape matters because to make sure that neutrons and nuclei hit others (and sustain the fission) it's necessary to reduce the surface area of the fuel as much as possible. A sphere is the perfect shape, but a cylinder works as well. The fission reactions in the molten salt "burn" more of the uranium in it, so eventually much more turns into other elements that don't stay radioactive for so long.
The radioactivity dies down more quickly, too, so it's easier to build containment facilities. Designing a structure to last a few centuries has been done (think of the average cathedral). But to build one that will last ten or a hundred times as long is much harder; let alone figuring out how to warn future people of the danger when it isn't likely anyone will speak English -- or even remember that English existed.
The fuel is also safer. When accidents have happened in power plants, such as at Fukushima in Japan, it was because the cooling systems failed. At Fukushima the generators that powered the water pumps were flooded by a tsunami. The heat built up in the reactor core until there was a meltdown. The result was a release of hydrogen, which exploded.
The WAMSR's core is "plugged" with a chunk of solid material that is actively cooled. If that cooling fails, then the plug melts and the molten salt drains out into a pool Once outside of the reactor vessel it will simply cool off and eventually solidify – and since it isn't in the right shape anymore, the fission reactions won't be self-sustaining. A failure of the cooling system power would stop the reactor, rather than leading to a meltdown.
With all these pluses, outside experts say there are still some problems. Transatomic isn't releasing the details of its design, though Dewan and Massie outlined the basics in a TEDx talk on Nov. 1. She said some of these issues have been addressed.
First is the process of taking the fuel out of the reactor vessel, and replacing it. A number of the fission products in nuclear waste are gases -- notably radioactive iodine, xenon, and cesium. "It's one thing to deal with the solids, it's another to deal with the gaseous fission products," said Jim Malone, chief nuclear fuel development officer at Lightbridge, a company that is working on a thorium reactor design. He said there has to be some method of containment.
Dewan said the fuel would be processed by draining off some of it and removing the "poisons," or elements that slow the nuclear reactions down. But that requires a lot of heavy-duty chemical processing.
As one of the selling points of this kind of reactor is that it would be cheaper to build, having that type of processing plant on-site might alter that calculation, said Tanju Sofu, department manager of the nuclear engineering division at Argonne National Laboratory. "You'd probably need a fuel cycle facility attached to the plant," he said.
Sofu also noted that molten salt needs to be pumped around, and any moving parts have to function for a long time. Many metals when exposed to radiation -- especially the neutrons produced in a reactor -- become brittle over time. Replacing parts inside the core raises the same containment problems that processing the fuel does.
Metallurgy and materials science have come a long way since 1959, and Dewan says there is a lot of experience in industry with pumping molten fluoride salts and doing maintenance on complex systems Perhaps the biggest issue will be getting the nuclear fuel itself. Technically, nuclear waste is all property of the Department of Energy, said Mike Mayfield, director of the division of advanced reactors and rulemaking at the Nuclear Regulatory Commission. So there would need to be some discussion of how the fuel is getting processed and working out an agreement to get it.
Even with those obstacles, the WAMSR is still a worthwhile innovation, Sofu said. "In the 1990s the DoE did a study of next generation designs," he said. "The molten salt reactor was one of the four or five most promising concepts. It has a lot of advantages in fuel cycle consideration and resource utilization."