Nuclear power stations have serious advantages compared to other forms of generation. There is the low cost of producing electricity once capacity is already built and in place, generation of quite large amounts of electricity In areas where it is lacking or where there is no possibility of effective alternative sources of energy and, lastly, zero CO2 emissions.
Nuclear power, however, does have its share of significant disadvantages. Among these are the high cost and long time-frame for construction as well as inertia and poor competitivity in rapidly-changing market conditions and the complex nature of technology processes. Added to this are the risks of uncontrollable reactions and the resulting widespread destruction and problems with disposing of nuclear waste associated with the operation of power stations.
It was those last factors that have produced a very negative public view of nuclear power. In the public eye, the notion is still that of the image of a metal barrel with a yellow and black symbol of radiation from which some sort of green, glowing substance is dripping. And thanks, in no small part, to 30 years of cartoons featuring The Simpsons, on which much more than a single generation was raised.
The accident at the Fukushima nuclear power station in March 2011 had pitched the nuclear power sector into a deep crisis. Many European countries, citing safety concerns, put an end to their nuclear programmes and abandoned any development of the sector. Europe’s planned energy transition to a carbon-free economy gives the nuclear industry a chance for a renaissance. All the more so that many of the safety problems could be resolved by opting for a metal known as thorium – in place of uranium.
Thunder without bombs
Thorium is a soft, weakly radioactive metal described in 1829 by the Swedish academic Jöns Jacob Berzelius. He proposed naming the unremarkable silvery metal after the Norse god Thor – the god of thunder and lightning.
The metal was truly discovered much later – in 1882 by Lars Fredrik Nilson, a Swedish chemist. As of now, about 30 isotopes of the metal are known, most of which have a half-life ranging from 10 minutes to 30 days. But the most stable isotope is thorium-232, with a half-life of 14 billion years. And it is from this that natural thorium is derived.
Thorium-232 is an even-even isotope – it has an even number of protons and neutrons – it is not fissile by means of thermal neutrons and cannot be used as nuclear fuel. This property had long limited the use of thorium in nuclear reactions only to become later the basis of its success.
As long ago as the 1940s, scientists noted that if thorium-232 was irradiated with neutrons, its atoms would decay with the release of large amounts of energy. As a result, after several interim reactions, the end result was uranium-233 – a good nuclear fuel suitable in all types of modern reactors.
Such technology offered a string of serious advantages. Natural thorium is made up nearly 100 % of thorium-232. This means that, unlike uranium ore, containing 0.7 % uranium, there is no need to enrich it through costly and complex technology. In other words, in terms of energy, one tonne of mined natural thorium is equivalent to 200 tonnes of uranium ore or 3.5 million tonnes of coal.
Natural thorium is contained in a dozen minerals relatively widespread around the world – its reserves are three to four times those of uranium. The largest reserves are found in Australia, India, Norway, the United States, Canada, South Africa, Brazil and Kyrgyzstan. Russia has very little thorium.
As thorium decays, there is no formation of plutonium or other products with a long half-life. That means thorium reactors do not produce dangerous radioactive waste. Strange as it may seem, it was that very property that led to its rejection by the nuclear industry – much of the industry’s activity was financed by the military and for decades a choice was made in favour or uranium reactors that ended up producing weapons-grade plutonium.
The only country to have continuously worked on thorium reactors throughout the last half of the 20th century was India – a country which has not signed the nuclear Non-Proliferation Treaty. Shipments of uranium to India were barred, but the country had considerable reserves of thorium. As a result, an experimental 13MW reactor producing uranium-233 was built at the Kalpakkam nuclear power station.
But that failed to unleash a revolution.
A reactor with an on-off switch
It was Carlo Rubbia, Nobel prize laureate and winner of the Global Energy Prize, that gave thorium reactors a new lease of life. He proposed, within the framework of the European Organization for Nuclear Research (CERN) a thorium reactor using a neutron accelerator. This accelerator pushes protons at around three-quarters of the speed of light.
As the protons strike thorium atoms, the protons cause them to decay with the release of a large number of protons, which are then used to stimulate nuclear reactions. Lead is used as a coolant. As a result of this reaction, large amounts of thermal energy are released which can partly be turned into electricity and brought on stream to a power grid.
A thorium reactor is quite safe, as switching off the proton accelerator leads to a shutdown of the reactor (though without taking into account the decay of interim elements).
But the main advantage is the virtual complete absence of radioactive waste. The uranium that is produced is used as a secondary nuclear fuel. Waste produced by a thorium reactor amounts to less than 1 % of that produced by a uranium reactor. In addition, its half-life rarely exceeds 200 years, while waste from uranium reactors can remain radioactive for thousands of years.
Such reactors do not require construction of deep “tombs” for waste. And a thorium rector releases no harmful emissions into the atmosphere, mainly CO2.
Despite all these advantages, thorium reactors have not undergone widespread development, largely because of the high cost of building the accelerator. And the energy conversion efficiency of the reactor is not very high as the accelerator uses large amounts of energy in its operations.
Carlo Rubbia repeatedly sought financial support from the European Union for thorium reactors . But in 2010, he sold the patent for thorium reactors to the Norwegian company Aker Solutions.
The cost of building an ADTR (Accelerator Driven Thorium Reactor) is estimated at $3 billion – with a capacity of up to 600 MW. Such a project calls for underground construction to avoid the necessity of building a reinforced concrete dome. The presumption is that a single loading of thorium fuel would enable it to work for several years.
Liquid reactors
Another use of thorium involves the creation of a reactor using liquid fuel – the so-called Liquid Fluoride Thorium Reactor (LFTR). Instead of solid fuel elements, these reactors involve the use, as a coolant, of molten salt and fluorides, in which oxides of thorium and uranium dissolve easily. The temperature of such a site is approximately 540 C and atmospheric pressure remains low, thereby reducing the risk of explosions. And the system is subject to self-regulation.
If the molten salt overheats, it expands and fewer thorium atoms enter the area of operation of the neutrons – and the reaction slows. If the mixture cools, conversely, it shrinks and that allows the reaction to speed up. A thorium reactor therefore does not need as complex a regulating system as a standard nuclear reactor.
This reactor allows for constant removal of fission products from the reaction zone and the injection of fresh fuel. The molten mass with a high concentration of fission products can be pumped into a containment basin, where it can be transformed into uranium-233. The uranium can then be separated through a chemical process from thorium that underwent no reaction and used as nuclear fuel in the reactor’s second active zone.
The reactor would be safe as plans call for the installation of a tank under the reactor’s main body plugged with a “cork” made up of the same mixture of fluorides, but this time cooled. In the event of a power cut, as occurred at Fukushima, the cooling ceases, the “cork” melts and the mixture flows into a tank where the reaction stops, owing to the absence of the neutron source. The molten mixture cools.
The processes in such reactors do not require a great capacity and scientists believe they could therefore be used as a power station for a city or even a city district. And the energy conversion efficiency of these reactors is considerably higher than those using solid coolants. Construction costs are considerably lower than those encountered with uranium reactors.
Despite the clear advantages of thorium reactors, their use has not become widespread for the moment. In the early 2000s, the European Union removed their installation and integration from its energy plans – partly as a result of pressure from France, which has its own network of uranium-based nuclear power stations. For the moment, China and India are working on their development and integration.
But European policies aimed at moving towards clean, if expensive energy could give a second chance to the nuclear industry.
