Wuwei City, Gansu, China. On the edge of the Gobi desert, the production of safe, inexpensive nuclear energy is soon to be underway. The technology will not use uranium, and it will not require water for its cooling process.
Think China and perhaps you are thinking of the capital Beijing and its ever grimy overcast skies.
But China is fighting back the smog. The superpower has reviewed its coal-based power station construction programme from 25 years to 10 years.
A “war on pollution” was declared in 2014, with restrictions on cars imposed in major cities, and the construction of new coal plants banned in certain areas.
The Gobi desert reactor is designed to generate 2 Megawatts of thermal energy. Enough power to supply up to 1,000 households.
This experimental nuclear reactor uses thorium as a fuel.
Thorium (Th) is a weakly radioactive metallic chemical element with atomic number 90. It also has a high melting point. It is silvery and turns dark when exposed to the air, forming thorium dioxide ThO2.
Currently, thorium is rarely used industrially.
Natural thorium can be chemically purified to extract useful daughter nuclides, such as 212Pb, which is used in nuclear medicine for targeted cancer therapies.
However, it offers an attractive alternative to uranium.
Thorium nuclei are prone to alpha decay because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons. All of its isotopes are unstable.
The most stable isotope of thorium is 232Th, one of only two nuclides beyond bismuth that have half-lives measured in billions of years.
It has a half-life of 14.05 billion years. Longer than the age of the Universe!
Thorium-232 decays very slowly via alpha decay, starting a radioactive decay chain that terminates as stable lead 208Pb:
- bismuth, and
This is called the thorium series or cascade.
Four-fifths of the thorium present at Earth’s formation has survived to the present. 232Th is the only isotope of thorium occurring in quantity in nature.
All intermediate elements in the series are present, at least transiently, in any natural thorium-containing sample, whether it is in metal form, as a compound or as a mineral.
Although naturally-occurring 232Th isotopes cannot be fissioned, thorium forms 233U when neutron-enriched in a reactor.
Fission occurs when a neutron slams into a larger atom, forcing it to excite and split into two smaller atoms, also known as fission products. Additional neutrons are also released that can initiate a chain reaction.
When each atom splits, a tremendous amount of energy is released.
The advantage? Thorium is more abundant than uranium and it could satisfy the World’s energy demands for longer.
Abundance of Thorium
Thorium occurs along with uranium and rare-earth elements in diverse rock types. It occurs as veins of thorite, thonanite, uranothonte and monazite in granites, syenites and pegmatites.
Monazite also occurs in quartz-pebble conglomerates, sandstones, and fluvial and beach placers. Thorium occurs along with rare earth elements in bastnäsite, in the carbonatites.
It is estimated to be over 3 times as abundant as uranium in the Earth’s crust.
Thorium is mainly refined from monazite sands in a cracking process, or by hydrometallurgy, as a by-product of the rare-earth metals extraction industry.
By using thorium, China’s reactor has the potential to produce energy that is relatively safe and inexpensive, while at the same time generating a much smaller amount of very long-lived radioactive waste than conventional nuclear reactors.
The disadvantage? The used fuel is difficult and even dangerous to reprocess because many of the daughter isotopes of 232Th and 233U are strong gamma emitters.
The process of burning thorium does not create plutonium, unlike the radioactive decay of the uranium fuel that is currently being used in more conventional nuclear power stations.
The reactor itself is unusual.
An Introduction to Molten Salt Reactors
Recently, the technology of molten salt reactors (MSRs) have been gaining support as many countries look for ways to increase power generation and replace ageing nuclear energy production facilities.
MSRs have different designs:
- Thorium Molten Salt Reactor Liquid Fuel 1 (TMSR-LF1)
- Molten Salt Breeder Reactor (MSBR)
They can store excess heat in thermal reservoirs for water desalination, and they can even be used to produce medical isotopes as part of the real-time liquid fuel recycling process.
Xenon-135 is the most significant neutron-absorbing isotope generated by nuclear fission, and it impacts all phases of reactor operation: startup, power operations and shutdown.
One of the key benefits of MSRs is their ability to remove 135Xe (9.14 h half-life) in real time.
The design and operation of an MSR allows for the continual removal of 135Xe and other fission-produced gases (xenon, krypton, and tritium).
This simplifies reactor operations and makes higher power levels and more efficient operations possible.
Radioactive gases are removed from the fuel by sparging them with an inert gas: carbon-dioxide, argon or helium.
The sparging gas provides an inert cover gas that keeps the salt-based nuclear fuel and fission products from oxidizing.
The sparging occurs while the liquid fuel is recirculated from the inner reactor core through the heat exchangers.
The cover gas and the radioactive gases are then sent through a cleanup process that typically consists of a cryogenic trap, room temperature charcoal beds, and a series of holdup tanks to let the 135Xe and other radioactive noble gases decay enough to meet national regulatory release limits.
The reactor is also unusual in that it has molten salts circulating inside it instead of water.
China’s reactor uses fluoride-based salts, which melt into a transparent, colourless liquid when heated to about 450 ºC.
Rather than solid fuel rods, molten-salt reactors use the liquid salt as a substrate for the fuel, such as thorium, to be directly dissolved into the core.
Unlike conventional uranium-powered plants, MSRs do not need huge amounts of water to cool their reactors. The liquid salt also acts as a coolant to transport heat from the reactor core.
So there is no need for them to be built along watercourses or by the sea. This particularity makes this type of nuclear power plant suitable for operation even in arid landlocked areas.
Molten-salt reactors are considered to be safer than conventional nuclear reactors, because the fuel is already dissolved in liquid form and they operate at lower pressures, which reduces the risk of an explosive meltdown.
Compared to light water reactors, MSRs work at significantly higher temperatures, making them more fuel-efficient.
Feasability of Thorium-based Nuclear Power
All this sounds exciting. However the critics have pointed to the fact that there remains technical problems to be addressed.
At very high temperatures, the salt can corrode the reactor’s structures, which have to be protected.
So the feasibility of MSRs remains in question.
Until now, this technology has not proven to be cost-effective because the extraction of thorium is more expensive than that of uranium. And unlike natural isotopes of uranium, it has to be converted first into a fissile material for it to work.
But MSRs are just one of many advanced nuclear technologies.
Thorium has been tested as a fuel in other types of nuclear reactors in countries such as the US, Canada, Germany, India and the UK.
The Perfect Technology?
By 2019, two of the reactors were under construction in the Gobi desert, with completion expected around 2025.
China expects to put thorium reactors into commercial use by 2030 and it has plans for more…
The Future of Energy
The Chinese experimental reactor got the go-ahead earlier this year.
China’s National Nuclear Safety Administration issued a license to the Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences to operate China’s first TMSR-LF1, which was under construction between 2018 and 2021 in Wuwei.
It will be the first molten salt reactor to be operating since 1969, and the first liquid salt reactor to run on thorium.
Experts believe that China will also be the first country to have a chance to commercialize this technology.
Looking further forward, if the experiments are successful, China hopes to build a reactor with a capacity of 373 MW by 2030, which could supply hundreds of thousands of households with electricity.