There have been some intriguing stories lately about a different kind of reactor – one cooled by molten salt instead of the more typical water-cooled designs comprising the great majority of the world’s nuclear reactors. While molten salt sounds exotic and new, its origins date back to the 1950s.
History of molten salt reactors
During the early stages of the Cold War, the US Air Force was trying to build a nuclear-powered bomber. While this project eventually went nowhere (the Air Force was reluctant to fly a bomber that left a trail of radioactivity wherever it flew), the idea of using molten salt never quite went away. Since then, the idea has been broached multiple times in various configurations, including the recent interest.
Oak Ridge National Laboratory (ORNL) built a working MSR in 1957 simply to test the idea. Although the reactor achieved criticality, it never produced any usable power. Several years later, ORNL worked on another design to provide the information needed to explore using thorium to fuel a reactor instead of uranium. This experiment ran for over a year before being shut down. After this, ORNL (and the US) continued theoretical studies of MSRs but did not build additional reactors.
Today, various types of MSRs are being investigated by Canada, China, Denmark, France, Germany, India, Indonesia, Japan, Russia, the UK, and the US.
How they work
First, let me define “salt” – table salt is one example of a salt, but only one of many. To a chemist, a salt is produced when an acid is mixed with a base. Mix hydrochloric acid (HCl) with sodium hydroxide (NaOH), and you’ll get water (H2O) and table salt (NaCl).
Other chemical reactions – and types of reactions – will form salts, but they all end up with a molecule that involves an anion (e.g., Na+) and a cation (e.g., Cl-) that are chemically bonded.
One of the nice things about salts is that they are chemically stable at very high temperatures, melt at those high temperatures, allowing them to flow like water, and only boil at very, very high temperatures (a few to several thousand degrees F). Water easily boils at these high temperatures and can only remain liquid under high pressure. Water-cooled reactors typically operate at pressures at least 100 times as much as normal atmospheric pressure (about 14.9 pounds per square inch – psi) or thousands of kPa). MSRs can operate at low pressures, typically at or close to atmospheric pressure, unlike water-cooled reactors
One MSR design simply replaces water with molten salt, circulating the molten salt through the fuel to remove the heat produced by nuclear fission (keeping the reactor cool) and transferring that heat to water, generating steam to turn the turbines that produce electricity. But there are other designs! In one, the molten salt includes salts of uranium and/or plutonium. In these elegant designs, the salt is fuel, moderator, and coolant, and reactor geometry controls power. In the reactor section, the volume is large enough that the uranium and/or plutonium fluoride collects in a large volume and a compact geometry, allowing the fuel to achieve criticality. When passing through narrower pipes enough neutrons “evaporate” from the mixture to keep it from remaining critical so the salt (and fuel) can be pumped and transfer heat without safety concerns.
There are other MSR designs. What they all have in common is that even reactors operating at high temperatures can operate at safer atmospheric pressure.
Advantages and disadvantages
I operated pressurized water reactors in the Navy. We spent a lot of time trying to figure out how to combat accidents in which high-pressure coolants forced their way through the pressure boundaries and into the atmosphere. Because these reactors operated at very high (classified) pressures, the entire reactor system was under high-pressure stress through most of the reactor plant's 25-30-year lifetime. The whole reactor plant was made of steel designed to resist that pressure without giving way. We had leaks due to that pressure differential – not many, but some. Running a reactor plant at atmospheric pressure, without those pressure stresses on the plant and operators, would have made our job much easier and significantly simplified the plant design.
In addition, many salts are better at transferring heat than water. That means MSRs can have a much greater thermal efficiency than water-cooled reactors. In other words, we get more usable power from an MSR than from a water-cooled reactor with the same amount of fuel.
In addition, proponents of a particular MSR design using the thorium cycle note that the reactors produce less radioactive waste and are less prone to producing waste that can be used to make nuclear weapons than conventional water-cooled reactors, which produce plutonium as their “waste” product.
Where they’re used
Currently, MSRs are not producing power anywhere in the world, but that looks about to change. The countries noted earlier are all looking at building MSRs in one form or another, and both China and India are currently constructing MSRs for power production.
While molten salt reactors may sound like the latest sci-fi solution to our energy woes, their roots run deep, and their potential runs hot. As countries race to decarbonize and modernize their energy grids, MSRs could finally get their moment in the sun.
