Nuclear Fuel Basics

The energy humans use comes mainly from two different sources, both based on nuclear energy. Less than five percent of our energy is produced from uranium in nuclear power plants based on fission. The rest originates mostly from our solar system’s huge fusion reactor, the Sun .

This article is mainly about uranium as the fuel for nuclear fission reactors. If you want to know more about uranium as a mineral, read my former article on the subject here.

U-235 isotope is the only naturally occurring substance that is fissile. Fissile means that it is capable of starting and sustaining a chain reaction of splitting atoms. As the atom is split, several neutrons are released, which will then hit other atoms, splitting them and releasing new neutrons. In practice, there needs to be enough fissile U-235 atoms close enough to each other to start the chain reaction. We can also help the chain reaction start and maintain itself by slowing the released neutrons down, with for example water.

Only about 0.72 percent of natural uranium is of the isotope U-235. In order to start and maintaining a chain reaction in a light water reactor, which is the most common type of reactors operated around the world, we need to increase the relative amount of fissile U-235 atoms in uranium . This is called enrichment. In today’s light water reactors, the share of U-235 atoms in the nuclear fuel is normally between three to five percent. The rest of the fuel is mainly of the isotope U-238 .

Enriching the uranium is nowadays done mostly with devices called centrifuges . For enrichment, the uranium ore is first transformed into uranium oxide, and after than for example into uranium hexafluoride (UF6). After enrichment, the uranium is transformed into ceramic uranium dioxide, of which the small nuclear fuel pellets are made of. These pellets are put into tubes made of zirconium which are then bundled together to make a nuclear fuel assembly. When these assemblies end up in the reactor and to the right kind of circumstances, a chain reaction of splitting atoms starts. This releases heat, which is used to boil water and make steam , which in turn is used to generate electricity.
The fuel assemblies inside a reactor get switched for fresh ones roughly once a year, but only part of them, usually one quarter or a third, gets changed in any one time. So a given assembly usually spends a few years inside the reactor. During this time, the share of U-235 in the fuel goes down to roughly one per cent, and various fission products start to pile up. Both of these phenomenon start to interfere with the chain reaction more and more, and finally it’s time to change the fuel assembly for a fresh one.

It comes as a surprise for many, that around a third of the nuclear energy actually comes not from uranium, but from plutonium . What, you ask? How did that plutonium get there?

Plutonium is created in the reactor, when a neutron hits a U-238 atom just so that the nucleus captures the neutron. This makes the nucleus unstable, and after a few quick transformations, plutonium (Pu-239) is created. Plutonium-239 is a fissile isotope like U-235, and is therefore able to sustain the chain reaction .

Although U-235 is a somewhat rare isotope, and even though the nuclear fuel manufacturing process sounds complicated (and is complicated), uranium has so much potential nuclear energy in it, that the cost of the fuel is only a few percent of the total cost of nuclear energy. Uranium fuel is more than ten times cheaper than coal on a price per energy content -comparison .

After the fuel is removed from the reactor, it is placed in a cooling pool and it effectively becomes spent fuel, or high-level nuclear waste. The mere words “nuclear waste” instil powerful emotions, even fear, in many of us. Few people know what this spent fuel actually contains, how dangerous it is and for how long, and what can be done with it. These are topics for future articles.


i: The only energy source that is not based on nuclear energy directly or indirectly is tidal energy, which is born from the gravitational forces between the Earth and Moon, and to a lesser extent, between the Earth and Sun. Part of geothermal energy comes from the forming of the earth, but around half of it (estimates vary) comes from the natural decay of various radioactive isotopes inside our planet.

ii: Some less common designs based on heavy water (like CANDU) or the usage of graphite to slow down the neutrons (like RBMK) can use natural uranium as their fuel, without enrichment.

iii: As we found out in a previous article on uranium, there is also small quantities of U-234 in uranium.

iv: Previously a technology called gaseous diffusion was used, but since centrifuges are tens of times more energy efficient, it has replaced gaseous diffusion.

v: Or for example uranium tetrachloride (UCl4)

vi: More about boiling water in my previous article [linkki]

vii: I have used the excellent The World Nuclear Univesity Primer – Nuclear Energy in the 21st Century (third edition, 2012) by Ian Hore-Lacy as the primary source here. Published by World Nuclear Association

viii: Some reactor-designs are able to make (or breed) more nuclear fuel from uranium than they use in the process. These reactors are commonly called breeders. So while current mainstream reactors use mostly the rarer U-235 isotope as their fuel, the uranium and its more common U-238 isotope still holds vast amounts of nuclear energy that we can make use of with breeder reactors.

ix: Assuming it is used in common light water reactors.

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