How will the nuclear power of the future work? The third generation of reactors is under construction, but what’s next? This is the question answered by the “Generation IV” International Forum, which presents the nuclear fission processes being studied for the 4th generation of reactors.
The criteria that these reactors are expected to meet are: sustainability (economical use of natural resources and lower waste production), safety, economic competitiveness and not facilitating the development of military nuclear power. There are currently 6 types of reactor, which have the particularity of having several operating modes.
- sodium-cooled fast reactor
- sodium-cooled reactor (SFR) ;
- gas-cooled Fast Reactor (GFR) ;
- lead coolant reactor (LFR).
- very-high-temperature reactor (VHTR);
- molten-salt reactor (MSR);
- supercritical water reactor (SCWR);
They hold extraordinary promise for fast neutron reactors. They could use uranium 238 to operate, which would multiply the available supply 100-fold and make it possible to use the many tons of this component that are stored in the meantime. The next step would be nuclear fusion, but this is still at a highly experimental stage.
It should be noted that, globally speaking, the 4th generation has been studied since the 1950s, even if it was only formalized with the international forum set up in 2001. Between the 1970s and 2000, however, there was a period of inactivity, as funding dried up.
Fast-neutron reactors
Today’s reactors are known as slow neutron or “thermal” reactors. The fuel is surrounded by a moderator that slows or “thermalizes” the neutrons. The fuel must be easily fissionable, such as uranium 235. Fast neutron reactors use no moderator. This means they can fission all heavy nuclei, rather than just fissile ones. In particular, this would make it possible to recycle MOx and URE (Uranium Ré-Enrichi) stocks: leftover spent fuel that can no longer be used in conventional power plants.
The disadvantage is that more neutrons tend to escape from the reactor. To avoid this, and at the same time take advantage of it, fertile materials can be placed around the periphery of the core: they will collect the outgoing neutrons and become fissile. This is known as overgeneration. This can enable a reactor to produce as much fuel as it consumes. It can also transmute waste into shorter-lived compounds.
There are three types of fast-neutron reactor (FNR), each using a different coolant: gas(Gas-cooled fast reactor system, GFR), molten sodium(Sodium-cooled fast reactor system, SFR) or lead alloy(Lead-cooled fast reactor system, LFR).
The sodium-cooled fast reactor
This is the most advanced 4th-generation technology. It is preferred to water, notably because it slows down electrons less. There are currently two Russian fast-neutron reactors in operation: BN-600 (560MWe) and BN-800 (820MWe) have been in service since 1980 and 2015 respectively. In China, a 600MWe reactor is under construction (Xiapu). In India, a 470 MWe reactor (PFBR) is under construction.
Research reactors have been built in Kazakhstan, the UK (Dounreay Fast Reactor; Dounreau Prototype Fast Reactor), the USA (Enrico Fermi 1), Germany (KNK II) and Japan (Monju).
In France, research reactors (Rapsodie 1967-1983, Phénix 1973-2010 and Superphénix 1986-1996, two prototypes). The Astrid project was due to enter service at the end of 2020, but was abandoned in 2019.
The gas-cooled fast reactor
There are currently several technologies that use gas as a coolant. These include the older French UNGG and British Magnox reactors. Advancedgas-cooled reactors (AGRs) have been developed on the basis of the latter. Cooled with CO2, they can reach a higher temperature than PWRs (640°C vs. 325°C) and therefore achieve higher yields (41% vs. ~34%).
I have no information on their fast-neutron version.
Fast-neutron reactor with lead-alloy coolant
RNRs could use lead or an alloy with bistmuth (known as a lead-bistmuth eutectic cooler). As with sodium cooling, the advantage lies in the very high temperatures that can be reached: in excess of 500°C. Theoretically, it would be possible to reach 800°C, but this has not yet been achieved in practice.
In Belgium, there is currently an RNR project using lead as a coolant: MYRRHA. Supported by SCK CEN, this is a highly unusual research reactor, since its load is controlled by a particle gas pedal (“linac”). If the linac stops, the nuclear reaction stops immediately. It is cooled by lead-bismuth eutectic (LBE), which has a low melting point (125°C) and boiling point (1670°C). Unlike water reactors, it can therefore operate at normal pressure. It could be used to produce therapeutic radioisotopes by 2027.
There is also a 300MW prototype in Russia: Brest-300 at Seversk. Other projects include the dual fluid reactive reactor in Germany, the Swedish Advanced Lead Reactor in Sweden and the SSTAR in the United States.
The Newcleo start-up is developing small lead-fired RNRs, from 30 to 200MWe.
Very-high-temperature reactors
The Very High Temperature Reactor system (VHTR) uses graphite as moderator and helium as coolant.
A demonstrator of this type, using ahigh-temperature gas-cooled modular pebble bed (HTR-PM), was commissioned in China at the end of December 2021.
Some use molten salt as a coolant (liquid-salt very-high-temperature reactor, LS-VHTR)
Supercritical water reactors
At 374°C and 221 bar, water enters a supercritical state, i.e. it is both gaseous and liquid. This is the form in which it is used in supercritical water-cooled reactors (SCWRs). This enables a thermal efficiency 30% higher than current “light water” efficiencies. Like our current reactors, these can use uranium oxide (235) or MOX.
Molten-salt reactors
Molten salt reactors(MSRs) are very popular. This is because molten salts are very dense and can be used in a wide range of applications
Their advantage is that molten salts can rise much higher in temperature than water: 700°C. This opens up extensive cogeneration possibilities. Above all, these temperatures are reached under normal pressure conditions, which limits the risk of leakage and dispenses with the very thick, robust enclosures used by pressurized water reactors. They are particularly safe, as the fuel can be rapidly cooled and solidified, thus stopping any untimely chain reaction.
One of the benefits is often the ability to use thorium as fuel. What’s more, even with uranium, the process would produce fewer actinides, the most problematic radioactive waste. The challenges are salt corrosion and transmutation by neutron flux.
There are currently two approaches:
- A fast-neutron reactor, as we have seen, in which the fissile material is dissolved in a molten salt, which serves as a coolant.
- A reactor in which the fuel is enclosed in graphite and the salt serves as coolant.
In both cases, you have three circuits rather than two: a primary circuit with the fuel, a heat exchanger with another molten salt, and then a steam generator that turns the turbines.
Primarily initiated by the Oak Ridge experimental reactor, which ran from 1965 to 1969, this is now an avenue being explored by several companies, including several designers of small modular reactors (SMRs).
China is soon to launch a small, experimental 2MW thorium reactor: the TMSR-LF1. This would be a slow reactor using as fuel salt a mixture of FLiBe (= mixture of lithium fluoride and beryllium fluoride), zirconium fluoride, uranium and thorium. The cooling salt would be FLiBe. As this type of reactor requires no cooling water, it is planned for western China, where water is in short supply.
Several start-ups are developing small, modular 4th generation molten-salt reactors:
- Terrestrial Energy presented a 390MWe Integral Molten Salt Reactor concept at the end of 2021.
- TerraPower is also developing a Molten Chloride Fluoride Reactor.
- French start-up Naarea, with reactors ranging from 10 to 40MW.
- Moltex is developing 40MWth fast neutron reactors.
Apart: reactors coupled to a particle gas pedal
Another type of reactor, which does not strictly speaking belong to the 4th generation (because it has not been identified by the international forum), but could very well do so, is a system based on a particle gas pedal. It is this accelerator that will produce the fast neutrons and control the chain reaction. This is extremely safe, as the chain reaction stops as soon as the gas pedal stops. It would also be a solution that uses fast neutrons to promote the transmutation of radioactive atoms into less problematic ones.
The Transmutex project is developing this technology.
To find out more:
- https://www.world-nuclear-news.org/Articles/US-companies-look-to-expand-Natrium-reactor-deploy
- On molten salts: https://fissionliquide.fr/
- The Twitter feed of a nuclear engineer, Tristan Kamin, with many resources, extremely comprehensive: https://twitter.com/TristanKamin/status/1402011017685647361