How Molten Salt Reactors Could Lead to the Next Energy Production Boom

Molten salt nuclear reactors could be the carbon-free energy producers of the future.
Christopher McFadden

If you've never heard of molten salt reactors before, prepare to have your mind blown. These cutting-edge pieces of technology might well be the answer to freeing our species from its addiction to fossil fuels. 

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, but they all, in essence, primarily utilize molten fluoride salts kept under low pressure as the primary coolant for the reactor.

While molten salt reactors were initially developed around 60 years ago, they have since fallen out of favor, due to problems with corrosion in the early examples. This led to investment in other types of energy which seemed more lucrative and promising at the time.

However, the molten salt reactor's best days might well be ahead of it. Today, interest in molten salt reactors is being revived as an alternative to standard nuclear and carbon-based energy sources.

Read on to find out why.

What are molten salt reactors?

A molten salt reactor (often abbreviated to MSR), is a special class of nuclear reactor in which the coolant and/or the fuel comprises a molten salt mixture. 

early molten salt reactors 1960s
Photo from the control room of an early molten salt reactor experiment at the Oak Ridge National Laboratory, circa 1965. Source: ORNL/Flickr

Molten salt reactors come in many shapes and sizes, and varying designs and generally use a molten fuel often made up of enriched uranium, thorium, or plutonium fluoride salts that are dissolved in a mixture of lithium and beryllium fluoride salts  -- as opposed to solid fuel used in most other types of nuclear reactors.

The core of the reactor then uses graphite to direct the flow of the molten salt at around 1,292°Fahrenheit (700°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). This type of MSR  uses thorium and uranium dissolved in a fluoride salt. 

While not technically a molten salt reactor, another more recent example is currently under development by a company called TerraPower. Founded by Bill Gates, TerraPower is developing a sodium-cooled fast reactor called Natrium

This reactor will use liquid salt as the coolant for the reactor, rather than the water used in more conventional nuclear power plants. Any heat not used to generate steam is then stored in the molten salt, in giant tanks. 

molten salt reactor natrium
While not technically an MSR, TerraPower's Natrium is another interesting type of nuclear reactor. Source: TerraPower

The interesting part of this is that the tanks of molten salt can be used as sort of stored heat batteries for future energy use if demand for electricity is not high at the time of generation. This could mean that Natrium plants could be used in conjunction with solar or wind generation and then plug the gaps in energy demand when the sun isn't shining or the wind isn't blowing. 

The best part is that a prototype reactor has already been announced on the site of an old coal power plant in Wyoming. This fully functioning plant will serve as the first demonstration project for the startup's tech.

It will cost around $4 billion to build and should be ready by the end of the decade. 

To understand MSRs in greater detail and why the use of fuel salts is beneficial, let's examine MSR's energy production capabilities. 

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How does a molten salt reactor work?

One way the energy production capabilities of a given process can be measured is by using something called its "energy returned on energy invested" (ERoEI), otherwise known as energy return on investment. In short, 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. 

molten salt reactors scales
Molten salt reactors have a very high potential return on investment with regards to energy. Source: winnifredxoxo/Flickr

To be of any real use, more energy needs to be extracted from the process than consumed to run the system -- whatever it may be. This is important, as the whole point of something like a power station is to produce energy for export. 

Incidentally, this is one of the current major roadblocks for other new forms of nuclear energy like nuclear fusion

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 stack up?

You may, or may not, be surprised to hear that the estimated ERoEI of molten salt reactors comes in at about 1200. That is frankly enormous, and definitely worthy of further research and development. 

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 hold your horses. While ERoEI is a useful way to estimate the energy return from a process, it isn't the only way in which we can look at the benefit of a particular power source.

How Molten Salt Reactors Could Lead to the Next Energy Production Boom
This schematic describes the different possible heat applications for the Integral Molten Salt Reactor, a new design from Terrestrial Energy. Source: Terrestrial Energy/Wikimedia Commons

You can also, for example, 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 trains, whereas a molten salt reactor would only require just 2,205 lbs (1000 kg) of fuel (usually thorium or uranium) to get the same results.

That is an order of magnitude difference. 

Another benefit of molten salt reactors is their potential life span and relatively simpler maintenance requirements. Currently, most traditional nuclear power plants last for around 35 years, or so. In the U.S. most of these were built in the 1970s and 1990s, meaning many of them will soon need to be decommissioned (or overhauled).

Very few new reactors are being built today to replace them -- they are very costly to build and very hard to get approved. This will lead to a very serious energy crisis in the U.S. if renewable technologies can't handle the pressure. 

This will likely mean most regions returning to fossil fuel power plants to plug the gaps. Many experts believe nuclear is the only real option, but with each new plant taking around a decade to build, this could become a very serious problem, very soon. 

Some molten salt reactors, like Seaborg Technologies' compact molten salt reactor (CMSR), have a modular design. This means that future maintenance, repair, and core replacement could be relatively simple affairs -- theoretically massively extending their serviceable life. 

molten salt reactor cmsr
Molten salt reactors can use spent fuel from other reactors as a fuel source. Source:  Seaborg Technologies

Given this, one commonly-asked question is, "why aren't we funding this, already?". One reason is that the problem with any new nuclear technology is the public's perception of what nuclear energy is and its perceived risk -- especially the problem of nuclear waste.

But, here, too is another reason that molten salt reactors really do need to be seriously considered -- they don't create much if any, waste. 

The waste from molten salt reactors

Molten salt reactors are actually one of the better power plant designs in terms of the production of waste, especially when compared to traditional coal-fired power plants. When coal is burned for energy production, a significant amount of ash and other particulate matter is produced as a result. Coal plants also 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 also 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. 

How Molten Salt Reactors Could Lead to the Next Energy Production Boom
A diagram of the core of a molten salt reactor. Source: Terrestrial Energy Inc./Wikimedia Commons

Molten salt reactors are just one highly efficient variant of traditional nuclear power plants which, lately, have 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 1,000 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 to this as well. They can, for example, actually use the waste from other nuclear reactors as a source of fuel.

This is also done relatively efficiently, as one year's waste 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 a relatively safer alternative to traditional nuclear power. 

How safe are molten salt reactors? 

When most people think of nuclear reactors, another term usually springs to mind -- "Nuclear meltdown". The two most dreaded words around nuclear energy.

While nuclear reactor malfunctions are rare, when they go wrong, they really go wrong. 

molten salt reactor chernobyl
Source: Tim Porter/Wikimedia Commons

Conventional solid-fuel nuclear reactors can be at risk of meltdown if the heat from the core isn't managed properly. If left to run riot, the results can be absolutely devastating.

However, molten salt reactors have another ace up their sleeve as a potential power source -- they are relatively safe. 

For starters, since molten salt reactors have a core that is already melted, there's basically no risk of meltdown. But, there is an even more important reason why molten salt reactors are very safe. 

Unlike more conventional nuclear reactors, the cores of the MSR are not under pressure (in fact they are usually at atmospheric pressure). This means that a catastrophic 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 and would prevent events like Fukushima. This also means that it doesn't require an expensive containment area for such scenarios because any leak would be localized to the area immediately around the reactor.

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 pot, 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 begins in the molten salt reactor, the harmful fission products bond with the molten salt, and these dangerous byproducts are thus disposed of safely. Conventional nuclear power doesn't handle things this way. 

But wait, there is more. 

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 an electric fan cools and solidifies the salt to create a solid salt plug, keeping the rest of the salt from flowing down the pipe.

molten salt reactors 101
Source: ANSTO

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 in the tanks is then dissipated throughout the earth 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. This stockpile of fuel is what is at risk.

Molten salt reactors require refueling very sporadically, meaning in most cases these reactors don't require any excess fuel to be stored on-site. This, therefore, decreases 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 (regrettably), 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. 

With enough heavy investment, and public interest, molten salt reactors might be the solution we've been looking for to meet our ever-growing energy needs. 

Be sure to watch this space. 

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