Today, the main nuclear fuel is enriched uranium. Extracted in many countries, it must undergo several processes before it can be used as fuel: first, it is transported in the form of “yellow cake”, then fluorinated and, in the form of gas, passed through a centrifuge that separates uranium 238 and 238. The second isotope represents only 0.7% of natural uranium, whereas 3.5% is required. Once enriched, the gas is defluorinated and transformed into uranium oxide (UOx), then used in long zirconium alloy tubes. Once used, part of the gas can be recycled by recovering plutonium (MOx) or re-enriching uranium (URE).
Uranium mining
Uranium quarries around the world
Uranium is extracted by physical or chemical processes(in situ leaching). Once extracted, the ore is concentrated and prepared in the form of “yellow cake” (a yellow paste reminiscent of semolina) or triuranium octaoxide (U3O8), a gray powder.
At present, according to the World Nuclear Association, the main producing countries are (in 2021):
- Kazakhstan (45.14%, 21.82Kt)
- Namibia (11.9%, 5.75Kt)
- Canada (9.71%, 4.69Kt)
- Australia (8.67%, 4.2Kt)
- Uzbekistan (7.24%, 3.5Kt)
- Russia (5.45%, 2.6Kt)
- Niger (4.65%, 2.25Kt)
- China (3.9%, 1.88Kt)
Total production is 48,332 tonnes.
On the ecological impact, see:
- https://twitter.com/maxcordiez/status/1456224558021849090
Uranium resources: how much is there?
Currently, identified uranium reserves stand at 6,147,800 tonnes, most of which are located in Australia (28%), Kazakhstan (15%), Canada (9%), Russia (8%), Namibia (7%), South Africa (5%) and Brazil (5%). (source: WNA) This corresponds to ~98 years of current demand (~63Kt). Nevertheless, this only describes the identified quantities that can be recovered for a reasonable price (if I understand correctly, $130/kg). This would rise to 8Gt including those that could be recovered for $260/kg. If we looked, we’d probably find a lot more.
Finally, it should be borne in mind that, as this ore is fairly abundant (there’s even some in France) and relatively inexpensive, it hasn’t been searched for much. It is likely that much larger reserves exist.
In particular, it would be possible to extract uranium from seawater: there is around 3.3mg/L. Estimates of the cost of the process vary, but it seems clear that it is possible. The reserve would be virtually limitless. Uranium supply warnings therefore seem highly questionable.
Finally, it should be borne in mind that very little uranium (<1%) is actually used today. When fast-neutron reactors become operational, usable fuel reserves will be multiplied by 100, and all the depleted uranium in storage will have to be added.
Uranium enrichment
Once mined, uranium contains only 0.7% of its isotope U235. This is insufficient for use as fuel in light-water power plants (= the vast majority of today’s power plants): enriched uranium has 3.5% U235.
Initially, the powder (yellowcake, mainly composed of U3O8) is transformed first into uranium tetrafluoride (UF4), then into uranium hexafluoride (UF6). Uranium has to be gaseous, and while the evaporation temperature of the former is over 1000°C, that of the latter is barely 60°C. Once enriched, it is centrifuged to separate the UF6 containing uranium 235 from that containing uranium 238.
In France, it is transformed into UF4 at Malvési, then transported to the Philippe Coste plant on the Tricastin nuclear site, where it is transformed into UF6, then transported to the enrichment plant on the same site. The centrifuge consumes the equivalent of 2-3 reactors of electricity and would produce fuel for the equivalent of 100 reactors. (article on uranium enrichment by T.Kamin)
Fuel finalization
At this stage, uranium is still gaseous. The fluorine still needs to be removed and, to make it easy to use, oxidized to make the famous uranium dioxide (UO2) in powder form (UOx). It is then handled and compressed into 7g pellets smaller than a finger.
For pressurized water reactors, these are stacked in zirconium-alloy tubes known as “rods”. They can be up to 5 meters long, and are used in assemblies of 264 rods held in place by grids (these were the problem for the Okiluoto3 EPR in 2022, I believe) and 25 tubes (making a 17×17 pack).
One pellet will be able to deliver 4800 kWh of heat, transformed into 1600 kWh of electricity.(source)
Fuel use
In the reactor, the fuel will be altered for 3-4 years. In the end, 95% of the fuel will be U238, 1% U235, 1% plutonium and 4% fission products. As its properties change over its lifetime, the fuel is partially renewed every year or year and a half. Once used and removed from the reactor, the fuel assemblies are stored in deactivation pools, then transported to the reprocessing plant. This applies to France. The USA, for example, does not recycle its spent fuel at all. The reprocessing plant in France is Orano’s La Hague facility.
First, the rods are cut up and dipped in nitric acid, which separates the fuel from the cladding. The latter is recovered and compacted, forming “long-lived intermediate-level waste”. The fuel itself:
- Uranium is extracted for storage or recycling (known as “reprocessed uranium”)
- Plutonium for recycling
- Fission products and minor actinides are vitrified. This is long-lived high-level waste.
Tristan Kamin’s article has many more details and images: Transport and treatment of spent fuel
Uranium mono-recycling
One of France’s unique features (and that of very few other countries) is that it only recycles its fuel once. As a result, most of the fuel is not nuclear waste.
MOX fuel
Plutonium from spent fuel is fissile, so it can still be used. It is mixed in oxide form (PuO2) with depleted uranium (UO2) to make MOx (Mixed Oxides). This fuel has its own particular constraints. The process is fairly expensive and, while it has practical advantages other than saving enriched uranium (the final waste is 5 times smaller), it is only moderately profitable.
In France, only 24 of the 58 reactors are authorized to use it. In France, it is produced by Orano’s Melox plant. Its use saves 10% of natural uranium requirements.
Used MOx could theoretically be reprocessed, but there are too many fission-inhibiting atoms. Americium 241 or 242 and curium 244 are particularly problematic.
Reprocessed uranium (TRU)
Reprocessed uranium (TRU) can be re-enriched (REU). Its U235 content is slightly higher than that of natural uranium, but it is “polluted” by other isotopes, such as U236, which will absorb neutrons and thus limit the chain reaction, or U232.
EDF used ERU from 1994 to 2013 in 4 reactors at the Cruas power plant. Only these reactors can use it. However, as uranium prices collapsed, the practice was no longer competitive, and due to an “unsatisfactory effluent treatment process”(Lesechos). It is due to resume in 2023. Recycling TRU would increase the recycling rate from 10% to 18 or even 25%.
Fast-breeder reactors
The concept of fast-breeder reactors was developed at the very beginning of the history of civil nuclear power, in 1945 by Enrico Fermi, and rapidly implemented by several prototypes, such as Rapsodie in France in 1959. Breeder reactors produce more fissile material than they consume. For example, U238 is simply fertile: it can become fissile. In a fast breeder reactor, it could be completely consumed. France’s accumulated reserves alone, on the order of 300,000 tonnes, represent a potential 6,000 years’ supply for the current French fleet.
- Threads by Tristan Kamin
- Summary of fuel reuse: https://twitter.com/TristanKamin/status/1402011017685647361
- Discussion of a Japanese reactor: https://twitter.com/TristanKamin/status/1541759865537847296
- Series of articles by Tristan Kamin: Fuel cycle #Summary
- Wikipedia, Uranium mining, extremely rich https://en.wikipedia.org/wiki/Uranium_mining
- CEA: https://www.cea.fr/comprendre/Pages/energies/nucleaire/essentiel-sur-cycle-du-combustible-nucleaire.aspx