Molten salt reactors might be the future of carbon-free green energy and they already cost less to run than coal power plants.
First constructed and operated in the 1960s, molten salt reactors are an interesting and promising energy technology. There's a variety of different designs for these reactors, in essence, they all primarily utilize molten fluoride salts kept under low pressure as the coolant for the reactor.
While molten salt reactors were initially developed around 60 years ago, they fell out of the limelight due to their corrosivity, with investment in other types of energy seeming more lucrative at the time. Today, interest in molten salt reactors is being revived as an alternative to standard nuclear and carbon-based energy sources.
What are molten salt reactors?
There are a large number of different designs for molten salt reactors (MSRs). They generally use a molten fuel that is a mixture of lithium and beryllium fluoride salts dissolved in enriched uranium fluorides, rather than the solid fuel used in most reactors. The core of the reactor then uses graphite to direct the flow of the salt at around 1292 degrees Fahrenheit (700 degrees Celsius). The heat produced by the fuel is then used to produce the steam for the reactor which powers turbines that generate electricity.
When wondering what molten salt reactors are, one of the most talked-about MSRs is the Liquid Fluoride Thorium Reactor (LFTR), which uses Thorium and Uranium dissolved in a fluoride salt. To understand this in greater detail and why this layout of fuel dissolved in salts is beneficial, let's examine MSR's energy production capabilities.
How does a molten salt reactor work?
One way the energy production capabilities of a given process can be measured is by using the metric ERoEI, otherwise known as energy returned on energy invested. In essence, this is a ratio of the energy we're able to get out of something to the amount of energy we have to put into the system to get that energy.
Solar panels have an ERoEI of about 10, meaning you get back 10 times the amount of energy that is invested. For fossil fuels like coal, that number is somewhere between 18 and 43. But what about molten salt reactors? How do they work? Their ERoEI is estimated at about 1200.
The energy output from one molten salt reactor is significant and very efficient, making a strong argument for the use of these reactor types.
But ERoEI isn't the only way in which we can look at the benefit of a particular power source, you can also look at how much raw material is needed to produce a given amount of energy. Compared to coal, there's still little competition. In order to produce one gigawatt-year of electricity, a coal plant would need to process over five hundred and seventy kilometers of coal-filled train whereas a molten salt reactor requires just 1000 kilograms of the fuel, which is usually thorium or uranium.
With any offshoot of nuclear energy production, though, there's generally a lot of pushback considering the potential environmental damage and the problem of disposing of nuclear waste that needs to be solved.
The waste from molten salt reactors
Molten salt reactors are actually one of the better power plant designs in terms of production of waste, even when compared to traditional coal-fired power plants. When coal is burned for energy production, a significant amount of ash is produced as a result. Coal plants also happen to produce a significant amount of carbon dioxide as a byproduct too, of course.
Compared to molten salt reactors, coal plants are significantly less efficient. MSRs produce about 1 ton of waste for every gigawatt year of electricity. Compare this to about 9 million tons of carbon dioxide for a coal-powered plant producing the same amount of energy.
However, it should be noted that there is a qualitative difference in the types of waste. The waste from molten salt reactors is radioactive and needs to be stored for a minimum of 300 years before it can be released back into the ground.
Molten salt reactors are just one highly efficient variant of traditional nuclear power plants, and lately, nuclear power has become a safer energy source. The comparison to traditional nuclear reactors actually makes the efficiency of MSRs far more clear. As mentioned before, a typical MSR requires about 1000 kilograms of salt fuel per gigawatt year of electricity produced. A traditional solid fuel nuclear reactor requires around 250 tons of enriched uranium to do the same job, and much of the waste has to be stored for upwards of 100,000 years before it can be released back on the earth.
The significant amount of waste from modern nuclear reactors is a problem, but molten salt reactors pose a potential solution. Many MSR designs also allow for much of the waste from traditional reactors to be utilized as fuel. This is also done efficiently, as the waste from one year from a normal reactor could power an MSR for about 250 years.
The efficiency of these molten salt reactors is clear, what's more, they also happen to be relatively safer alternatives to traditional nuclear power.
MSRs are safe
Nuclear meltdown. The two most dreaded words around nuclear energy. Conventional solid-fuel nuclear reactors can be at risk of meltdown if the heat from the core isn't managed properly. Since molten salt reactors have a core that is already melted, there's basically no risk of meltdown.
Perhaps most importantly, because the cores of the MSR is not under pressure, explosion is not a potential risk. The fact that MSRs can be run under atmospheric pressure means that a leak in a tube doesn’t automatically result in the expulsion of a bunch of fuel and coolant. This is a major safety advantage that enables passive decay heat removal, this would prevent events such as Fukushima. This also means that it doesn't require an expensive containment area for such scenarios.
The best way to think about how an MSR works is to think of it as a pot that holds hot, viscous fluids. Through nuclear reactions in those fluids, the pot is heated, all by itself. If you run water around that pot, it turns into steam, producing electricity, but this also cools the pot. However, as the pot cools, the nuclei of the atoms in the viscous fluid inside, the salts, get closer together, causing the nuclear reaction pace to speed up, producing more heat faster. It's a self-regulating system. This means MSRs are relatively easy to operate and often don't require control rods to guide the nuclear reactions.
After fission onsets in the molten salt reactor, the harmful fission products automatically bond with the molten salt, disposing of these dangerous byproducts safely. Conventional nuclear power doesn't handle things this way.
There's a final safety precaution in these reactors as well. At the bottom of the "pot" that holds the molten salt, there's a drain pipe for the salt. Under normal circumstances there is an electric fan that cools and solidifies the salt to create a solid salt plug, keeping the rest of the salt from flowing down the pipe. If electricity were to fail or something else were to go wrong, the fan automatically shuts off. The plug then melts and the molten salt drains down the pipe into large tanks. The heat from the molten salt then gets dissipated throughout the earth while in those tanks through natural convection, a relatively safe way to deal with the problem.
Along with preventing meltdown, the other main safety metric that has to be considered with nuclear reactors is how likely the fuel is to be stolen and used for making nuclear bombs and other devices by bad actors. Conventional nuclear power sites require a significant amount of solid fuel to remain onsite in order to keep the reactors operational simply due to the burn rate.
Molten salt reactors require refueling very sporadically, meaning in most cases these reactors don't require any excess fuel to be stored on site, decreasing the probability of any of this nuclear fuel finding its way into the wrong hands.
As of today, molten salt reactors are being pushed further and further into the energy production limelight. Several private companies are building their own style of the molten salt reactor to hopefully serve as fossil fuel replacements. While nuclear isn't the golden child of renewables, molten salt reactors provide enough benefit with few compromises to potentially serve as the carbon-free energy production method of the future, in conjunction with traditional renewables as well, of course.