The Nuclear Fuel Cycle

The nuclear fuel cycle is the series of industrial processes which involve the production of uranium 235 for use in nuclear energy power reactors. Uranium 238 (uranium) is a relatively common element that is found throughout the world, and is mined in a number of countries. But before uranium can be used as fuel for a nuclear reactor, it must first go through a number of processes known as “enrichment.”

The various activities associated with the production of electricity from nuclear reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste (this is called an open fuel cycle). If the fuel is reprocessed after use, this is called a closed fuel cycle (note: even reprocessing produces a small amount of nuclear waste which cannot be re-used and must be disposed of).

Yellowcake

The first step in the nuclear fuel cycle is the mining of uranium ore, which takes place around the world.

Uranium ore is usually around .1% uranium, which is further concentrated in the milling process. The milling process produces a uranium oxide concentrate that is often referred to as yellowcake. Yellowcake is concentrated into uranium oxide of about 80%, but is not yet suitable for use in a nuclear reactor; the uranium oxide concentrate requires additional processing before being used as nuclear fuel.

Uranium Hexafluoride

For uranium yellowcake to be suitable for use in a nuclear reactor, the uranium isotope U-235 must be concentrated within the uranium. To do this, the yellowcake is first converted to uranium hexafluoride, a gas, at a conversion facility. This uranium hexafluoride gas is then taken to an enrichment plant where the U-235 is further concentrated, in order for the uranium to be able to undergo nuclear fission and produce energy. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide.

Enriched Uranium Fuel

Next, the enriched uranium oxide must be fabricated into nuclear reactor fuel. Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide (UO2) which is sintered (baked) at a high temperature (over 1400°C). The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor.

Controlled Nuclear Reaction

Inside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the process, release energy. This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator which produces electricity. The fissioning of U-235 is used as a source of heat in a nuclear power station in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant, the difference being that uranium does not combust (a chemical reaction) but fissions (a nuclear reaction).

Spent Fuel

With time, the concentration of fission fragments and heavy elements in the fuel will increase to the point where it is no longer practical to continue to use the fuel. So after 12-24 months the spent fuel is removed from the reactor. The amount of energy that is produced from a fuel bundle varies with the type of reactor and the policy of the reactor operator.

When removed from a reactor, the fuel will be emitting both radiation, principally from the fission fragments, and heat. Used fuel is unloaded into a storage pond immediately adjacent to the reactor to allow the radiation levels to decrease. The pond water shields the radiation and absorbs the heat. Used fuel is held in such pools for several months to several years. It may be transferred to ventilated dry storage on site.

Reprocessing & Disposal

Depending on policies in particular countries, some used fuel may be transferred to central storage facilities. Ultimately, used fuel must either be reprocessed or prepared for permanent disposal.

Used fuel contains almost 1% U-235 that has not fissioned, around 1% plutonium, and 4% fission products (highly radioactive elements, with other transuranic elements formed in the reactor). In a reprocessing facility the used fuel is separated into its three components: uranium, plutonium and waste. Reprocessing enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste (compared with treating all used fuel as waste).

The uranium from reprocessing, which typically contains a slightly higher concentration of U-235 than occurs in nature, can be reused as fuel after conversion and enrichment.

The plutonium can be directly made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides are combined. In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal uranium oxide fuel.

At the present time, there are no disposal facilities (as opposed to storage facilities) in operation in which spent fuel, not destined for reprocessing, and the waste from reprocessing, can be placed. A number of countries are carrying out studies to determine the optimum approach to the disposal of used fuel and wastes from reprocessing. The general consensus favors its placement into deep geological repositories, initially recoverable before being permanently sealed.

Taken from World Nuclear Association, ”The nuclear fuel cycle,” August 2010, accessed at http://www.world-nuclear.org/info/inf03.html.