Molten-Salt Reactors

Written by Philippe Werrie
Edited for Oralgrooves.com by Evie

The Other Nuclear Reactor

While proponents tout the carbon free nature of the nuclear reactor, ecologists condemn them for their risk and the nuclear waste they produce. Did you know that while there are hundreds of active nuclear power plants in the world, most of them use the same basic design? There’s actually a lot of potential ways to design a nuclear reactor. In this post, I want to introduce you to the molten-salt reactor.

A diagram of a molten salt reactor, courtesy of Wikipedia

Components

An actual nuclear power plant is a complicated machine but its basic mechanism is easy to conceptualize : the nuclear fuel inside the reactor core undergoes nuclear fission. In short nuclear fission is when a neutron is sent flying through the nuclear fuel and hits one of the atom constituting this fuel. When the atom is hit, it splits into two smaller atoms (it undergoes nuclear fission). This generates a lot of heat. A coolant (liquid or gas) then passes through the reactor to absorb the heat and cool the reactor down. The heat extracted by the coolant is then used to generate steam that will hit steam turbines, thus generating electricity. A nuclear power plant is nothing more than a (big) steam engine that generates electricity! To make the neutrons more likely to hit the fuel and therefore generating heat, we can slow down the neutrons. This is done by putting a neutron moderator in the reactor core close to the nuclear fuel.

Building One

To build a nuclear reactor you’ll therefore have to choose three elements: fuel, coolant, and neutron moderator.

  • For fuel you can use uranium, plutonium or thorium.
  • For coolant you can use water, heavy water, gas, molten salt or liquid metal.
  • The neutron moderator (which slows down the neutrons so they have a better chance of hitting the fuel and creating fission) could be water, graphite, or beryllium.

Almost all of the reactors built in the past 70 years all over the world use the same elements: uranium (sometimes mixed with some plutonium) for fuel, water for coolant and water plus graphite as a neutron moderator. So why do we all use water/uranium reactors? It’s because that’s what was needed to build nuclear bombs and nuclear submarines. Hyman Rickover was the U.S. Navy admiral in charge of building nuclear subs and he was also the guy who oversaw the creation of the first electrical power plants. It’s no wonder that nuclear power plants used the same design! What if we decided to change things up and build another kind of reactor? Could we build safer reactors that generate less nuclear waste? Some researchers are taking on the challenge. They want to use thorium as fuel and liquid molten-salt as coolant.

Molten-salt reactors have many theoretical advantages:

1. They work at low pressures (current reactors are at above 2000 psi).

2. There is no risk of hydrogen explosions (like Fukushima) or steam explosions (like Chernobyl).

3. They tend to be failsafe. When a molten-salt reactor overheats, the nuclear reaction slows down automatically. It’s because the liquid expands and fissile material spreads apart, slowing down the reaction.

4. If there is a power failure, molten-salt reactors can be shut down without needing any electricity. There is a frozen plug at the bottom of the vessel. If there is a power failure, the plug ceases to be kept cool. It then melts and the molten salt drains into a reservoir below the reactor.

5. Molten-salt becomes solid when it cools down. If there were to be a leak it wouldn’t spread into the environment and it might even self-plug.

6. While there will still be waste products, there will be much less than existing reactors.

So quite a few potential benefits…

The Next Question…

If they are (potentially) so much better why isn’t anyone operating molten-salt based nuclear plants?

That’s where money and politics comes into play. There have been several test reactors: the US built one in the 1960’s and they even linked it to the grid; but as we know this was not the design picked for nuclear power plants and the project was shut down. China built its own test molten-salt reactor in the 70’s. Nobody knew about it outside of China. While those projects worked at small scale, more work was needed to design a full scale commercial design that would also work for a long time at the temperature required (the molten-salt reaches temperatures of 1100°F to 1600°F!).

So why are there still no molten-salt reactors out there?

One big problem is that a lot of money has been poured into our current reactors and we have hundreds of power plants. With new designs, regulations, fuel manufacturing, fuel recycling, we would have to start from scratch and it would cost billions. The existing players have no interest in throwing away their current investment which is totally understandable. Plus, with the existing uranium based technology, companies are not only selling the plant but also the fuel assembly and recycling. It’s a very lucrative business that wouldn’t be needed with thorium.

So will we ever see molten-salt reactors?

There are only three things missing: the technology, a big player (a government or private company) with big pockets, and the will to make it happen.
Currently, the Netherlands and Norway are doing experiments with thorium. France is also conducting research. India is building a new test reactor. Japan is building one with the help of U.S. and Russian technology. A Canadian startup is working on a small-medium size reactor.

So there you have it, a brief intro on molten-salt reactors.