Last week I wrote a little bit about how fertile thorium can be turned into reactor fuel. A discussion of how a thorium reactor might differ from a uranium reactor seems a reasonable follow-up topic.
The reactors I worked with in the Navy were light-water pressurized water reactors. Like virtually every reactor we have on Earth our reactor core was filled with fuel elements – uranium fuel was sealed into a zirconium cladding and these metal fuel elements were arrayed in the reactor vessel in a precise pattern. As the uranium fissions it produces highly radioactive fission products – these are trapped within the metal fuel elements and if the metal melts (as happened at Fukushima) all of this radioactivity is released.
The reactor vessel was filled with light water – normal water (heavy water consists of water in which some of the hydrogen atoms are replaced with a heavier isotope, deuterium). But it’s not enough to simply sustain a fission reaction, to generate power it’s also necessary to find a way to use the energy produced by that fission. In the case of a reactor plant that means heating water to boiling and using the steam to spin a turbine – since this happens most efficiently at high temperatures it helps to have the coolant as hot as possible. And if we are going to try to heat water to high temperatures then that water will have to be at high pressures to keep it liquid. In turn, this requires building components that are not only able to withstand temperatures of several hundred degrees, but also pressures of up to a few thousand pounds per square inch. And at these extreme pressures even a small flaw can cause problems. Not only that, but water at such high pressures will be forced from the reactor plant at tremendous speeds, eroding the pipe and making the hole larger, as well as emptying the plant all too quickly. Not all water-cooled reactors are pressurized water plants – boiling-water reactors are another “flavor” of light-water reactor – but all water-cooled reactors operate at high pressures and temperatures and all suffer from the stresses and worries that go along with those conditions.
Back in the 1950s an alternate reactor design was tested – one in which the high temperatures didn’t require concomitant high pressures. Not only that, but this reactor also eliminated the need for high-precision fuel elements and many of the problems associated with them. And to top it all off, it ran on thorium. Part of the novelty comes from using molten salt and the rest comes from using homogeneous liquid fuel. With the government’s love of initials it was called an LFTR – liquid fluoride thorium reactor – and here’s how it worked – and how it could work today.
One of the interesting things is that the reactor uses liquid fuel – instead of putting an array of metal fuel elements into a tub of water a liquid fuel reactor has fuel particles suspended in the liquid coolant. When the coolant is circulating through relatively narrow pipes the geometry prevents a fission chain reaction from taking place; the fission chain reaction occurs in the more bulbous reactor vessel, where enough of the liquid can collect to sustain criticality (remember that, in a nuclear reactor, “criticality” simply means that the rate of fissions – the reactor power – is remaining constant over time; ALL reactors are critical when they are operating). Anyhow – returning to the main point, a liquid fuel reactor dispenses with fuel elements, control rods and their associated drive mechanisms, and so forth because reactor criticality is controlled solely by the shape of the pipes and other components through which the fuel passes.
Another advantage of liquid fuels is that they can be continually processed to remove fission products as well as to adjust the amount of uranium or thorium it contains. So rather than fission products building up in fuel rods they can be scrubbed from the fuel as it circulates through the reactor system – doing this can help to reduce radiation levels in the reactor plant and buildings as well. So – in theory – a liquid fuel plant should be radiologically “cleaner” than an ordinary reactor, reducing radiation dose to the workers. Not only that, but any reactor accident would likely release far less radioactivity than did the Chernobyl and Fukushima accidents.
The other novel part of the LFTR is the liquid fluoride – the thorium is in the form of a thorium fluoride salt (“salt” is a general chemical term that refers to a specific category of chemical compounds of which table salt is only one); the salt is heated to its melting temperature of several hundred degrees by the fission reaction and the hot salt circulates through a heat exchanger to boil water, forming the steam to drive turbines. What’s nice about molten salt is that it doesn’t have to be at high pressure to be at a high temperature, so the system is under less stress and leaks are less quickly catastrophic. All of these features – lower pressure, less in-core machinery, simpler emergency cooling systems, and so forth – mean that LFTRs are also likely to be much less expensive to build than our current light-water designs.
Another nice feature comes when you combine both of these factors – the inherent safety of the LFTR design. Consider – energy is only produced when the liquid fuel is in a critical configuration in the reactor vessel, and that heat is given up when the liquid salt is pumped through the heat exchanger to create steam. So what happens if the pumps fail? In a typical water-cooled reactor the coolant stops flowing through the core, the fuel heats up to the melting point, and havoc ensues. In a liquid-fuel reactor the fission reaction continues, the liquid heats up, and it melts a drain plug in the bottom of the reactor vessel – the liquid fuel then drains into a shallow tank where it loses its critical geometry and the reactor shuts down. This should – in theory – prevent the sort of meltdown that destroyed the reactor cores in Fukushima and Three Mile Island and that caused the massive releases of radioactivity last year in Japan. I know that a number of more recent reactors are designed to be “inherently safe” but the LFTR comes closer to this ideal than does any water-cooled reactor if only because the fuel is already molten and, when it loses its critical configuration, the laws of physics dictate that it will shut down.
So with all of these advantages it’s natural to wonder why our reactors are still the “conventional” designs loaded with solid fuel rods and cooled with high-pressure and high-temperature water. Martin (the author of Superfuel) contends that the liquid fuel designs were torpedoed in part by the established water-cooled reactor designs that were being pushed by Hyman Rickover, the architect of the US Navy’s nuclear power program. That, plus the nuclear establishment’s level of comfort with a proven technology (the light-water reactors) might have sealed the fate of the liquid-fuel thorium reactor design, in spite of its advantages. I suspect that the reality is a combination of these factors. Regardless of the reasons why these reactors have not been built in the nuclear nations, it seems likely that thorium will become part of the Indian and Chinese nuclear programs – more on that in a future posting!