Thorium – Nuclear Reactors And Energy

Thorium is a metal with radioactive properties that has the symbol Th and the atomic number 90 in the periodic table, and can be used as nuclear fuel in power plants to generate clean electricity.

What is Thorium?

Thorium is a silvery metal with radioactive properties that was discovered in 1828 by Jöns Jakob Berzelius (a Swedish chemist) and was named after Thor (the famous Norse god of thunder).

In terms of radioactivity, thorium is a nuclear fuel that can be used in reactors to generate low-carbon electricity, but operates in the Thorium-Uranium (Th-U) fuel cycle.

Thorium (Th-232) cannot split like uranium to generate energy (even if it’s bombarded with neutrons) and needs to pass through a series of nuclear reactions to eventually emerge as an isotope of uranium (U-233).

Thorium is called this way fertile, while the emerging isotope of uranium (U-233) is called fissile because it can be used as nuclear fuel in the nuclear fission reaction to generate low-carbon energy.

Today, most of the nuclear reactors available on the planet are using enriched uranium (U-235) or the reprocessed plutonium (Pu-239) as nuclear fuel in the Uranium-Plutonium (U-Pu) fuel cycle, and only a small number of nuclear reactors use thorium and the Thorium-Uranium (Th-U) fuel cycle.

Nuclear Reactor Types that Can Use Thorium as Nuclear Fuel

In theory, any modern nuclear reactor with exotic design can accommodate thorium as nuclear fuel.

However, there are seven types of nuclear reactors that can use the Thorium-Uranium (Th-U) fuel cycle.

1. Molten Salt Reactors (MSRs)

This type of nuclear reactor was first built in 1965 and was tested as the “Molten-Salt Reactor Experiment”.

The molten salt reactor is not using solid fuel pellets of uranium (U-235), instead it uses a mixture that can turn thorium (Th-232) into uranium (U-233).

Uranium (U-233) will be used to generate carbon-free energy during the nuclear fission reaction and also to turn even more thorium into uranium.

There are several benefits of using a Molten Salt Reactor (MSR) such as:

• is 16x more fuel efficient than the current reactors using fuel rods (uranium);
• the nuclear waste resulted decays much faster and is 200x less radioactive compared to the nuclear waste produced by the current nuclear reactors;
• the design of a molten salt reactor is almost immune to meltdowns and explosions, so is much safer to use;
• is extremely difficult to use the nuclear fuel from a thorium reactor to build a nuclear weapon.

2. Advanced Heavy Water Reactors (AHWRs)

The Advanced Heavy Water Reactor (AHWR) represents the latest Indian design in terms of next-generation nuclear reactors.

In this design of nuclear reactor, thorium forms a blanket around the reactor core where it absorbs neutrons and becomes uranium (U-233), which is used in the nuclear fission reaction for clean energy generation.

AHWR has several advantages over the current nuclear reactors such as:

• natural circulation of core heat removal;
• thorium as the nuclear fuel;
• passive decay heat removal;
• severe accident management;
• an operating term of 100 years.

3. High-Temperature Gas-Cooled Reactors (HTRs)

Such a nuclear reactor is not using fuel rods (uranium), it uses nuclear pebbles.

Each pebble contains enriched uranium (U-233) obtained through the Thorium-Uranium (Th-U) fuel cycle.

The pebbles with enriched uranium are coated with layers of graphite.

Each pebble will work like a mini-reactor inside the reactor and all pebbles will be recycled around and through the core of the reactor.

The high temperature achieved in the nuclear fission reaction generates electricity in a more efficient way.

Because it uses helium gas for cooling, it is safer to operate than other nuclear reactors that use pressurized water.

Such a reactor is also considered a meltdown free reactor because even without the cooling system, a meltdown is not possible (like in the Tree Mile Island case, at Cernobyl or at Fukushima) because the nuclear physics inside the reactor are slowing down the chain reaction if the temperature increases.

4. Pressurized (Light) Water Reactors (PWRs)

Thorium can be used as nuclear fuel in a PWR, but it needs to be in certain arrangements in order to reach a satisfactory nuclear reaction.

However, this type of nuclear reactor is not very suited for thorium because there is no possibility to convert significant amounts of U-233 into energy.

These reactors are widely used around the world, and they can only become early platforms for thorium as a source of clean power.

5. Boiling (Light) Water Reactors (BWRs)

This type of nuclear reactor can use shorter (half length) fuel rods that contain an arrangement of thorium-plutonium.

The heat produced by the nuclear fission reaction is used to turn the water that surrounds the reactor into steam.

The steam is then transferred through pipes and is used to spin the turbines that will produce clean energy.

6. Fast Neutron Reactors (FNRs)

There is no advantage in using thorium as nuclear fuel in a fast neutron reactor instead of using depleted uranium (DU) as fertile.

U-238 has good potential in fast neutron reactors, so the nuclear fuel obtained through the Thorium-Uranium (Th-U) fuel cycle cannot compete with it.

7. Accelerator Driven Reactors (ADS)

The ADS system is a different nuclear fission energy concept that can accommodate thorium as nuclear fuel.

Spallation neutrons used by the ADS system are produced when high-energy protons released by an accelerator strike a heavy target such as lead.

The neutrons will be directed to a region containing thorium fuel (thorium-plutonium), which will produce heat due to the nuclear fission reaction like in a conventional reactor.

Without the proton beam, the ADS system cannot sustain the chain reaction.

ADS systems are creating concerns related to their reliability and their high energy consumption.

Thorium Advantages Over Uranium

• Thorium is three to four times more abundant on planet Earth than uranium.
• Mineral and isotopic processing, which means that almost 100% of thorium that naturally occurs can be used as fuel compared to 0.72% of uranium (U-235).
• Thorium-based power plants require less investment because there is no need of pressure rated vessels, a pressure rated containment dome, piping, no redundant safety systems, etc.
• Fuel and power plant efficiency, because a large part of the thorium fuel burned is turned into energy compared to 0.5% of uranium.
  1 ton of thorium generates the same amount of energy like 90 to 100 tons of uranium.
  Thorium is also more thermally conductive than uranium (by 29%).
• Improved safety features, meaning that a nuclear reactor based on liquid fuel design has low operating pressures, so it requires passive safety features.
  Thorium dioxide is also more chemically stable than uranium dioxide.
• Using thorium as nuclear fuel generates less nuclear waste, and also the radioactive waste decays faster than the conventional nuclear waste.
• Non-Proliferation, thorium fuels produce 3,000 times less plutonium than uranium fuels, and also the plutonium (Pu-238) produced using thorium is way too hot for making materials for a bomb.
• A few current reactor designs can accommodate thorium as nuclear fuel such as: Pressurized (Light) Water Reactors (PWRs), Boiling (Light) Water Reactors (BWRs), CANDU Reactors, Very High Temperature Reactors (VHTRs) and Molten Salt Reactors (MSRs).
• Supply of medical isotopes, which means that thorium can be used as a supplier of rare isotopes such as: Thorium-229, Actinium-225, Astatine-211, Lead-212 and Radium-223.

Conclusion

It seems that we need to wait a few more years, maybe even a decade until we can say that thorium has become a new source of cleaner power better than uranium.

However, in the meantime, many teams of scientists around the world are working hard to make nuclear fusion as a source of unlimited clean power for humanity.

If they will succeed to make nuclear fusion commercially viable quicker than making thorium the next nuclear fuel used in the nuclear fission reaction instead of uranium, we can say that thorium has missed its chance to show how much clean energy can produce compared to uranium, or maybe we have realized too late that thorium would have been a better option for us.