About 27 tonnes of fresh fuel is required each year by a 1000 MWe nuclear reactor. In contrast, a coal power station requires more than two and a half million tonnes of coal to produce as much electricity. (1)Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then pressed to form small fuel pellets, which are then heated to make a hard ceramic material. The pellets are then inserted into thin tubes to form fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are several meters long. 

Uranium is a naturally-occurring element in the Earth's crust. Traces of it occur almost everywhere, although mining takes place in locations where it is naturally concentrated. To make nuclear fuel from the uranium ore requires first for the uranium to be extracted from the rock in which it is found, then enriched in the uranium-235 isotope, before being made into pellets that are loaded into assemblies of nuclear fuel rods.

Scientists say plutonium may be the worst of all the fission byproducts that could enter the environment as a result of the Fukushima nuclear disaster. That's why MOX fuel rods that are piled up in spent fuel pools near the Unit 3 reactor, which consist of a mix of plutonium and uranium isotopes, have become the number one concern of workers at the plant.

Control rods are usually used in control rod assemblies (typically 20 rods for a commercial PWR assembly) and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to increase or decrease the neutron flux, which describes the number of neutrons that split further uranium atoms. This in turn affects the thermal power, the amount of steam produced and hence the electricity generated.

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning. Because these elements have different capture cross sections for neutrons of varying energies, the composition of the control rods must be designed for the reactor's neutron spectrum. Boiling water reactors (BWR), pressurized water reactors (PWR) and heavy water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons.

The ‘uncanny’ might serve us better. One of the most chilling traces of the Anthropocene is the global dispersal of radioactive isotopes since mass thermonuclear weapons testing began in the middle of the 20th century, which means that everyone born after 1963 has radioactive matter in their teeth. The half-life of depleted uranium (U-238) is around 4.5 billion years, roughly the same as the age of the Earth, while that of the plutonium in Chernobyl’s nuclear reactor is 240,000 years. Such timescales resist the imagination, but exist as a haunting presence in our daily lives.