What are thorium reactors and why is China interested in them?

An experimental thorium reactor has been running stably for several months. Moreover, it has been supplied with new fuel for the first time during operation, Chinese researchers recently reported. The news caused a stir worldwide because, at least in theory, thorium reactors offer a number of advantages over nuclear power plants that run on uranium or plutonium:

• Thorium is more readily available and more abundant than uranium.
• Less highly radioactive waste is produced.
• and the reactors should shut down automatically in the event of a loss of coolant.

India and China in particular, both of which have large thorium deposits, are working on the development of this technology worldwide.

What is thorium?

Thorium is a weakly radioactive heavy metal that is estimated to occur three to four times more frequently in the earth's crust than uranium. The largest deposits of thorium-bearing ores are thought to be in India, Brazil and Australia.

Can thorium be used as nuclear fuel?

Not directly, but indirectly. In thorium reactors, uranium 233 is brewed from thorium by neutron bombardment, which is then used as the actual nuclear fuel. In order to start the chain reaction in the reactor, fissile, enriched uranium is also needed, which is added to the nuclear fuel. The Chinese reactor TMSR-LF1 uses a mixture of lithium beryllium fluoride with added fluorides of enriched uranium-235 and thorium as fuel.

Sounds great, what's the problem?

There are only a small handful of thorium reactors worldwide, but several hundred nuclear power plants that run on uranium. This is despite the fact that the basic concepts for thorium reactors were developed back in the 1960s. There are several reasons for this:

• Some thorium reactors allow captured uranium 233 to be diverted and used for military purposes.
• The operation of the reactors also produces highly volatile tritium. Tritium is also a sensitive material that is used in nuclear weapons.
• Thirdly, highly radioactive uranium-232 is also produced during the breeding process. Although it only has a half-life of around 68 years, it is a powerful gamma emitter. This means that the reactor chamber is exposed to strong gamma radiation, which activates the materials themselves.
• Depending on the design and layout, not all thorium reactors are inherently safe. Thorium reactors designed as breeder reactors in particular can have a so-called positive temperature coefficient, which means that the nuclear reaction becomes increasingly violent as the temperature rises. Such reactors can fail in the worst-case scenario.
• One of the biggest challenges with molten salt reactors in particular is the corrosiveness of the molten salts, which can attack pipes and containers. Fluoride salts can react with alloying elements such as chromium, iron and nickel, which are present in many structural materials. This leads to the dissolution of these elements in the molten salt and thus to material erosion.
• Unforeseen problems can also occur. In the Molten Salt Reactor experiment in the USA, which ran from 1965 to 1969, it was only when the reactor was dismantled in 1994 that it was discovered that large quantities of uranium-233 had leaked from the salt into the reactor's exhaust system. These accumulated in the activated carbon filters. Such fissile material volatilization had previously been ruled out as chemically impossible. However, the release process is said to have only taken place after the reactor was in operation, when the molten salt cooled down.

Why is molten salt needed at all?

The molten salt solution serves both as a fuel carrier and as a coolant. Unlike in conventional boiling and pressurized water reactors – with water as a coolant –, the coolant is not used here to slow down the fast neutrons released during nuclear fission. Graphite is responsible for this.

If the reactor gets too hot, a fuse plug under the reactor core melts and the molten salt flows into a collecting basin. The molten salt is distributed in such a way that there is no longer a critical mass – the chain reaction is extinguished.

Can the Chinese divert fissile material?

According to the information available so far, no. According to this, the Chinese are using a so-called single-fluid design: in these reactors, both the thorium and the U-233 are dissolved in the same salt mixture. Here, the brewed uranium-233 remains directly in the reactor circuit and is used to generate energy without the need for separate separation.

In the so-called two-fluid design, the incubated uranium-233 is separated from the incubation area and fed to the fission area. This separation enables more efficient use of the fuel and better control over reactor operation. However, such a design also increases the risk of weapons-grade uranium falling into the wrong hands.

What was weapons-grade uranium again?

Natural uranium consists mainly of uranium-238. The atomic nuclei of this isotope are comparatively stable and therefore not suitable for nuclear weapons. For conventional nuclear power plants, as well as nuclear bombs, the easily fissile uranium-235, which also occurs in small quantities in uranium ore, is therefore enriched (for example with the help of ultracentrifuges). Alternatively, plutonium can be incubated – or the easily fissile uranium-233 can be used.

Is thorium technology ready for application?

Not really. The Chinese TMSR-LF1 is a pure research reactor – designed to operate for ten years. This operating time corresponds to around 300 full load days over the entire period, with a maximum operating time of 60 days per year at full power. Compared to conventional nuclear reactors with a service life of 30 years, this is not very much.

During this time, the operators want to test various operating modes, materials and fuel cycles under real conditions. The results will serve as the basis for a planned 373 MW reactor, which is scheduled for construction by 2030.

This article first appeared on t3n.de .

(mho)