Nuclear fuels: uranium, cesium, thorium, MOX ..

Nuclear fuels must be capable of triggering a nuclear reaction. To do this, they must, in principle, be “fissile”. Today, uranium is the main fuel used for this purpose. Another avenue being explored, though still experimental, is the use of thorium. Fuel reserves are considerable and do not appear to present any risk of supply shortages. This is all the more clear since advanced reactor technologies could even multiply usable fuel by a factor of 100, using all the uranium 238 available.

I. Uranium, the main nuclear fuel

Today, most reactors run essentially on uranium in the form of UOX fuel, i.e. uranium dioxide.

Enriched uranium: the UOX fuel (Uranium diOXyde)

The fuel used for fission is called “fissile”. Today, everything starts with uranium. It is extracted from rock, dissolved and processed until a yellow powder (“yellow cake”) is obtained, made up of 99.3% non-fissile uranium 238 and only 0.7% fissile uranium 235. However, we want uranium 235 to account for between 3% and 5% of the total. To achieve this, the “yellow cake” is transformed into gas and passed through a centrifuge, which separates the “depleted” uranium, which has lost all its uranium 235, from the “enriched” uranium, which will be used as fuel. Finally, the latter is transformed into “UOX” fuel.

This comes in the form of small 7g pellets, lined up in zirconium tubes. These are called “fuel rods”. In France, they are 4 meters long. Their lifespan is 3 to 4 years.

After initial use, 96% of the material is reusable: uranium 235, 238 and 1% plutonium, which can be reused in the form of MOX fuel. MOX stands for “Mixed uranium and plutonium Oxide”. Its main advantage is ” to potentially recycle 96% of used nuclear fuel, while reducing the volume of high-level waste by a factor of 5 and the radiotoxicity of the waste by a factor of 10, compared with untreated storage of used fuel“. It is used in 22 French reactors.(SFEN) For the time being, however, it cannot be recycled. This is known as monorecycling. Used uranium (“reprocessed uranium”, TRU), can also potentially be re-enriched. This is known as URE fuel. This is not widely practised. The life cycle of enriched uranium is described in a separate article.

Uranium can also be further enriched, as in HALEU fuel (High-Assay Low-Enriched Uranium), which can contain up to 20% uranium 235.

II. Thorium: an alternative fuel?

Thorium is being considered as a fuel because, although it is not fissile in itself, it can be transformed into fissile uranium 233 by capturing a neutron in the reactor. What’s more, the waste is easier to manage: the residues produced contain fewer minor actinides and do not produce plutonium (rq: plutonium is not really waste, since it can be recycled into MOx). This approach will take more than 20 years to reach industrial scale.(SFEN)

According to the CEA, there are several obstacles to the development of a thorium-fired power plant:

  • It would require the development of an entire extraction, fuel processing, reactor and waste reprocessing chain.
  • The thorex process for reprocessing thorium waste has only been tested at pre-industrial level.

In itself, this would be of little interest, as the uranium sector is already very mature. Only India could be interested, as it has large reserves of thorium and had problems with uranium supplies. The latter has been less of a problem since 2008. Its prospects could be more interesting in the very long term, as part of the development of 4th generation reactors. For example, the CNRS is studying molten-salt reactors using thorium.

The World Nuclear Association article contains many details.

III. Fuel reserves: for how long?

Identified uranium reserves are sufficient for around a hundred years. However, this estimate only includes commercially viable deposits. What’s more, as the price of this resource is relatively low, little research is being carried out on it.

As for thorium, there are 6,355 thousand tonnes of identified reserves, a large proportion of which (846,000t) are in India. It doesn’t seem to me that this subject has been given much thought, given that there is currently no market for it.

IV. The fuel of the future: the case of fast-neutron reactors

Today’s reactors use only a small fraction of uranium, because they use “slow neutron” fission. So, above all, they need material that is directly fissile (i.e. capable of generating a chain reaction). As we have seen, only a small proportion of natural uranium is fissile (uranium 235, representing 0.7% of natural uranium). After use, 95% of the uranium remains …

Fast-neutron reactors could change all that. They could consume all the fuel. The development of fast breeder reactors would make fuel reserves virtually unlimited, multiplying present and past fuel efficiency by 100.