Nuclear Fuel and the Nuclear Fuel Cycle

(Transcript of the lesson commentary.)

Nuclear materials and their energy potential

Nuclear fuel can be any material capable of releasing energy through nuclear fission or fusion reactions. In theory, almost all isotopes of elements from the beginning of the periodic table could be used as fuel in nuclear fusion and isotopes of elements from the end of the table for nuclear fission. However, only some of them can be found or produced in sufficient quantities and their nuclear reactions are energetically advantageous. Deuterium and tritium isotopes of hydrogen best suit such conditions for fusion whereas fission has been practically based on uranium and plutonium isotopes for several decades. Thorium isotopes are also being considered for the future.

Uranium is an element that is abundant in the earth’s crust and in the world’s oceans. For nuclear energy, the fissile isotope uranium-235 is important, which unfortunately is only 0.7% in uranium ore and its proportion in the fuel must be increased by complex enrichment processes. The predominant isotope of uranium-238 is also useful because in the reactor, it produces the fissile isotope plutonium-239 by neutron capture. Another fissile isotope, uranium-233, does not occur in nature, but can be produced by a similar nuclear reaction of neutron capture by a thorium-232 nucleus.

In addition to plutonium-239, the plutonium-241 isotope can also be used as nuclear fuel, which is also created artificially by the interaction of a neutron and the atomic nucleus of plutonium-240. The use of plutonium in the role of nuclear fuel is a bit more complicated compared to uranium fuel. The reason is the artificial origin, the high toxicity of plutonium and its compounds, and the lower safety of plutonium-burning reactors.

When comparing the energy potential of nuclear and conventional fuel, we can start from the assumption that the fission of 1 kg of uranium-235 nuclei could theoretically release 1011 kJ of energy. The synthesis of light elements is better by order of magnitude, in which up to 1012 kJ can theoretically be obtained from 1 kg of fuel. In practice, however, the use of nuclear fuel is much lower and modern fission energy reactors are able to obtain approximately 5 × 109 kJ of energy from 1 kg of fuel enriched to 4% uranium-235. Compared to the calorific value of coal of 104 kJ/kg and the calorific value of hydrogen of approximately 105 kJ/kg, it is still clear that a kilogram of nuclear fuel can provide us with several orders of magnitude more energy than conventional fuels.

Basic division of nuclear fuel

The basic division of nuclear fuel must of course be the division into fusion fuel and fuel for fission reactors. Fusion reactors, which are still only in the development phase, generally include isotopes of light elements, such as deuterium, tritium or helium. Fission reactors, which are relatively widespread around the world, on the other hand, can easily handle fuels with nuclei of isotopes of heavy elements, mainly uranium, plutonium and thorium. By enumerating the elements, we arrive at the second type of division of nuclear fuel, according to the predominant element making up the fuel or a mixture of elements, as in the case of MOX fuel.

From a physical point of view, nuclear fuel can be further divided according to different parameters. Probably the most important division is according to the enrichment of the fissile isotope. In special types of heavy water reactors, nuclear uranium fuel can be used without costly enrichment at its natural uranium-235 concentration of 0.72%. Even such a small amount of fissile material is enough to start and maintain a chain fission reaction of uranium, thanks to the excellent moderation capabilities of the heavy water moderator.

The most common enrichment of a fissile isotope in nuclear fuel ranges from 2 to 5%. With such enriched fuel, most nuclear power plants work with one of the thermal types of nuclear reactors. Enrichment to 5% is enough for the nuclear fuel to produce heat through fission during the few years it is in the reactor. Enrichment from 5 to 20% is typical for research and experimental reactors. Some research reactors may require even higher than 20% fuel enrichment, but in most cases high uranium-235 fuel enrichment is typical for fast reactors.

Another parameter for dividing the fuel is its chemical form. Historically, the metal form of fuel is the oldest — it was already used by Enrico Fermi in his first Chicago pile. However, the excellent coefficient of thermal conductivity was accompanied by a low melting point, so later, the fuel production switched to safer oxides. The vast majority of nuclear fuel used in current power plants is in this form. In the phase of experimental tests, nuclear fuels are based on carbides or nitrides. These forms are likely to find use in high-temperature fission reactors in the near future. The last, rather exotic form of nuclear fuel are various alloys of uranium with other metals, for example aluminium. It finds its place exclusively in small research reactors.

Nuclear fuel can still be divided according to the state in which it is found in the reactor, although this division is given more for completeness. Practically all fuels are used in the solid state, but there have been experimental trials in the past with fuels in the form of liquid salts.

The most frequently used type of fuel in nuclear energy today is solid, slightly enriched oxide uranium fuel in the form of small pellets with a proportion of fissile isotopes up to 5%.

The front end of the nuclear fuel cycle: making fresh fuel

When we look at the nuclear fuel cycle as a whole, it can logically be divided into two parts — the part for using the fuel in a nuclear power plant and the part dealing with what to do with the fuel after its energy use. The first part includes the complete production of fuel from ore extraction and primary processing, through enrichment and production of fuel assemblies, to transportation to the nuclear power plant and loading into the reactor’s core. The second part includes temporary storage of spent fuel assemblies, possible reprocessing of fuel and methods of its final storage in deep geological repositories.

Uranium is relatively abundant in the Earth’s crust and uranium ore deposits are found practically all over the world. The ore is mined either in classic surface or deep mines or by the in situ leaching method. The mined ore is crushed and ground and the uranium is then chemically extracted and concentrated into uranium oxide, known as yellow cake because of its colour. This oxide contains 60—70% uranium.

The next step in the production of nuclear fuel in the first part of the cycle is to increase the concentration of the fissile isotope uranium-235. This process, called enrichment, is based on physical principles because chemically, both isotopes of uranium are the same. They differ only in the tiny mass of the three neutrons, which is why their separation is very energy and time-consuming. There are several ways to enrich uranium. The oldest ones use a calutron, working on the principle of a mass spectrometer or a semi-permeable membrane through which uranium hexafluoride gas is repeatedly forced through.

The most widespread method of uranium enrichment uses centrifuges. Uranium hexafluoride gas is spun in them at a relatively high speed, during which the slightly heavier uranium-238 is pushed to the edge of the centrifuge by centrifugal force and the lighter uranium-235 remains closer to the centre. The enriched gas component is then forced out of the centrifuge and used in the next cycle. The latest method of uranium isotope separation is based on the excitation of only uranium-235 with a precisely tuned laser. The positively charged ions of the isotope are then collected on the negatively charged electrode.

Small cylindrical pellets are pressed and baked from the enriched uranium oxide powder in the next stage of the cycle. Several hundred of these ceramic pellets enclosed in a 4—5-metre-long hermetic jacket of zirconium alloy form a fuel rod. The bundle of fuel rods is then used to make a fuel assembly — the basic unit designed to safely handle nuclear fuel in a power plant. The rods are fixed in the assembly in such a way that their displacement cannot occur during significant thermal stress on the assembly in the core of the reactor. Each type of reactor requires its own type and size of nuclear fuel, which is why fuel assemblies are manufactured exactly to measure for each specific reactor.

The last place falling into the front end of the fuel cycle is the nuclear power plant. New fuel assemblies are transported in protective containers from the production plant to the fresh fuel storage in the power plant and during the fuel exchange, they are put into the reactor core by a loading machine. The average time of the fuel in the reactor is four years, during which the fission reaction takes place in the fuel and energy is released. By removing the fuel from the reactor, it becomes spent fuel and by storing the fuel assembly in the storage pool, the nuclear fuel enters the second, the back end of the fuel cycle.

The back end of the nuclear fuel cycle: reprocessing and storage of spent fuel

The back end of the nuclear fuel cycle can have two distinct scenarios. In the first, the spent nuclear fuel is temporarily stored in an interim storage and, after the radioactivity has decreased and the heat production has decreased, it is directly deposited in a deep geological repository — we call it an open fuel cycle. In the second scenario, the fuel is reprocessed sometime after it is taken out of the reactor, uranium and plutonium is obtained, which are then used to make new fuel. Reprocessing is thus linked to enrichment and fuel production and in this case, we speak of a closed fuel cycle.

After removal from the reactor, nuclear fuel is most often stored in water pools in close proximity to the reactor. It is highly radioactive and still generates a lot of heat at this stage, so the pool water ensures its continuous cooling and also acts as a shield against ionizing radiation. After 3—4 years, when the radioactivity of the fuel is reduced by about half, the fuel can be transferred to another interim storage, where it can remain for decades or it can be taken for reprocessing.

Interim storage facilities for spent nuclear fuel can be wet or dry. In the wet interim storage, the individual fuel assemblies are constantly cooled by circulating water, similar to the reactor pool, even though the cooling requirements are no longer so high. The assemblies can be easily visually inspected in the water for a long time, on the other hand, it is necessary to ensure the circulation and treatment of the cooling water.

Dry interim storages require significantly easier maintenance. The cooling of the spent fuel in them is ensured by the natural circulation of the surrounding air. In dry storage, groups of fuel assemblies are hermetically sealed in special steel storage containers and these are spread out either in the open air or in light, well-ventilated halls. Alternatively, dry fuel assemblies can be placed in thin-walled closed containers and then stored in special concrete cellars or in thick-walled concrete containers, which are again cooled by circulating air.

A closed fuel cycle is characterized by the reprocessing of spent nuclear fuel. This complex but useful process is beneficial for two main reasons — obtaining up to 25% more energy from the spent fuel and reducing the amount of high-level waste by up to 80%. Reprocessing the spent fuel, uranium and plutonium isotopes are obtained, which can be reused with some limitations in the production of new fuel. The limitation lies in the difficult handling of the fuel due to the presence of isotopes that are strong sources of gamma radiation or the need to quickly use plutonium to produce new fuel due to the short half-lives of some of its isotopes into strong gamma emitters.

The main method used in reprocessing plants to extract uranium and plutonium is the PUREX method, in which chopped fuel rods are dissolved in nitric acid and the useful isotopes are then separated from the unwanted fission products by various chemical reactions. These are eventually processed through the vitrification process into highly active glass, which is then disposed of as waste.

For highly radioactive waste, which is expected to emit dangerous radiation for thousands of years, it is necessary to build a deep repository in a stable geological formation. It should be a site with no groundwater and no danger of earthquakes, tsunamis or inundation by seawater or lava and should provide natural barriers to prevent accidental release of radionuclides into the environment. Work on final repositories is underway in many states, but none have yet begun to store spent nuclear fuel. The reason may be the small amount of spent fuel so far, but also the development of new transmutation technology, which would help to significantly reduce the amount of waste stored in the future.