Around the world, most nuclear power reactors work by splitting uranium to make heat, and using water to carry the heat away so it can be used to make electricity. The uranium is a solid, sometimes in metal form and sometimes ceramic with a metal support system. The design works well, but it dates from the 1950s, and some engineers are re-thinking the whole package.
Enter Terrestrial Energy, of Mississauga, Ontario. Its engineers say that water works fine, but they point out that at reactor temperatures, the water has to be kept under very high pressure to keep it from boiling away. That means heavy, expensive pipes and vessels, and a lot of safety systems designed to kick in if a pipe breaks. A reactor builder could avoid most of that by replacing the water with salt, melted into a liquid, to move the heat. Salt can carry far more heat per unit volume than water, at atmospheric pressure.
And why use solid fuel, which could melt if it gets overheated? Terrestrial Energy starts with uranium in liquid form, mixed into the molten salt. That makes it easy to add a bit more fuel from time to time, and thus to control the system partly by managing the amount of material in the reactor that can be split to sustain the nuclear chain reaction.
Terrestrial calls the result an Integral Molten Salt Reactor. Building on pioneering work done by the Oak Ridge National Laboratory in Tennessee, Terrestrial Energy’s design is for a plant that shuts for refueling just once in seven years. Like all uranium-based reactors, the IMSR produces plutonium as it runs. But the fuel is in the reactor for so long that much of the plutonium is consumed as the reactor runs, aiding energy production and making the design unattractive for anyone who wanted fuel for a bomb. Burning off more of the plutonium also makes the wastes easier to handle.
And while water is essential to most reactors, the water molecules have a tendency to trap neutrons, the sub-atomic particles that sustain the chain reaction. Build a reactor with fewer materials that absorb neutrons, and it takes far less uranium fuel to produce a given amount of energy.
Most current reactors are very large, because if the machine is full of parts and structures designed to handle high pressures, there are economies of scale to building them big. But that is far less true for equipment that runs at atmospheric pressures, so Terrestrial’s reactor is intended to be built in a factory, which is good for cost and quality control, and shipped by truck. Small has a variety of advantages, including ease of financing. “There’s less sticker shock,’’ said David LeBlanc, the company’s chief technical officer.
So far the work is preliminary, and the company is still months from having a design it can submit to Canadian regulators for approval. But Terrestrial, which recently joined the Nuclear Energy Institute, is another demonstration that as with so many other technologies, from aviation to medicine to computing, innovative thinking is going strong and for nuclear energy, more good things lay ahead.