Molten Salt Reactors

Liquid salt reactors could be part of the future of power production

---

How Molten Salt Reactors Work

The uranium-233 turns more thorium-232 into uranium (over about a month), keeping the cycle going. To start the cycle and to keep it going a fissile material must be used. This is uranium-233, is rare in nature.[1][2][8]

There are different varieties of molten salt reactors. All of them feature molten salt as a coolant, but the fuel comes in different shapes. The most common one is where the fuel is dissolved within the salt, but designs for reactors using solid fuel and pebble fuel exist.[1]

Thorium is not the only option, uranium, and plutonium can also be used. Thorium has benefits over the others and is where most research is today.

---

The Benefits over "Conventional" Nuclear Reactors

Conventional nuclear reactors are Pressurised water reactors, which use a nuclear fuel covered in water and kept pressurised so that the water can stay liquid and transfer heat away from the core. The core is typically solid fuel moderated by graphite.

Molten salt reactors have many benefits over the more conventional uranium reactors. They use thorium, which is more common, produce less waste, and is safer. This helps reduce both the cost and size of the nuclear reactor

---

Efficiency and Radioactive Waste
Molten salt reactors are more efficient for a variety of reasons and produce less waste.

The most obvious reason that they are more efficient is the operating temperature. Higher temperature leads to more efficient generation of electricity. These reactors also need no solid fuel, since making and disposing of solid fuel is complicated this saves time and resources. (Uranium enrichment is a 16 step process.)[3][6]

Since thorium molten salt reactors require a fissile material to start, they can use up waste plutonium or uranium. This can help us clean up the massive amount of extra nuclear weapons we have. Thorium is also far more common and easier to mine than uranium. The concentration of uranium in the ore it is commonly mined in is 0.1-1%, while the concentration of thorium is anywhere between 2-10%. This makes it much cheaper and more environmentally friendly to mine thorium instead of uranium. (There are large amounts of uranium in salt water, enough to sustain humanity's current power consumption for billions of years. Methods and efficiency of extraction are unknown, not making this an option currently.) There is still an estimated 3 times as much thorium as uranium in Earth’s crust. [6]

To keep running the reactor must create roughly the same amount of fuel as it uses, or be refuelled often. This can either be done by adding fissile material (refuelling) or by having the reactor turn more fertile material into fissile material than it consumes. Achieving this state, called breeding, is complicated and in many places research has stopped due to safety concerns. (They were using liquid fluorine and that is very chemically active, which isn’t good if it hits air.) These breeder reactors have the added bonus of having even less waste. In fact, when they produce more uranium-233 than they consume they are producing fuel that can start other reactors. [4][5][7]

Either way only around 3% of the original mass of thorium must be added every 18 months to keep the reactor running, while an entire 33% of the original uranium must be added in the same time period for PWR. This is partly due to the massive inefficiency in PWR. Much of the uranium is turned into elements that are stable for long periods of time (but still radioactive) and are not fissile, like Pu-240. [6][7][8]

---

Safety
More conventional pressurised water reactors (PWR) require the water to be at 350C and under 150 times Earth’s gravity to keep it liquid. Molten salt on the other hand is liquid under Earth’s pressure between 500°C up to about 1400°C. As seen with reactors in the past, mistakes and natural disasters could cause explosions, which send radioactive particles into the air and water. Molten salt reactors on the other hand would not explode, and would leak less in the case of an accident. As the salt spread out the nuclear reaction would slow and then stop, making cleanup far less costly. This can manually be done by pushing it into holding tanks. If a salt that could catch fire is used, that could shoot radiation into the air. That would have much the same effect as a typical PWR exploding, but there would be less radioactive elements so it wouldn’t have as big an impact. [1][3]

On top of that, thorium is extremely hard to make into weapons (and they wouldn’t be that good). That means terrorists and governments can only really use it for power, which is a benefit to everybody involved. [6]

---

Cost and Size

The Molten salt reactor is inherently safer. That gives more benefits than just peace of mind, it means there need be less safety features. That cuts down on the cost and size of the reactor immensely. The core of the nuclear reactor doesn’t have to be as big, however, that is partially balanced out by higher neutron leak. More neutrons means more shielding is needed to protect the workers. Molten salt reactors made with thorium use far less fuel and have far less waste than uranium. [6][7]

Molten salt reactors are also far more efficient, by over a factor of 10 in the worst case. That combined with the fact the price of uranium is increasing will lead to a far lower operation cost. A company claims a thorium reactor design they have is production ready. They claim they can build a 500 megawatt thorium reactor for 1.7 billion. Other companies can build 1000 megawatt PWR for around 7 billion.[9]

---

Thorium has been long overlooked. This is likely due to the fact governments can’t make weapons out of it. Now it is being overshadowed by fusion. Fusion has many benefits over thorium, but research is slow and we don’t have much of the knowledge and tools needed to carry it out.

---

Want to see more science posts? Subscribe and Upvote!

[1] [2] [3] [4] [5] [6] [7] [8] [9]