Nuclear fuel

Nuclear fuel refers to any substance used by nuclear power stations or other devices to generate energy. Most of these substances are fissile materials, which means they can be split to release energy.

While many people think of fuel as something that burns, nuclear fuel works through different physical processes. The choice of fuel depends on the type of reactor being used. Different materials offer different advantages regarding heat, safety, and how long they last.

In many fission reactors, the fuel is based on uranium in the form of a metal oxide. Scientists prefer using uranium dioxide (UO2) rather than pure uranium metal. This is because the oxide has a much higher melting point than the metal. It also cannot burn because it is already in an oxidized state. Uranium dioxide is a black semiconducting solid. To make it, uranium hexafluoride can be reacted with ammonia to create ammonium diuranate. This solid is then heated, or calcined, to form uranium oxide. Finally, the UO2 is mixed with an organic binder and pressed into small pellets. These pellets are fired at very high temperatures to sinter them into a dense solid with few pores.

As the fuel operates inside a reactor core, its composition actually changes. During normal operation, the fuel is exposed to a neutron flux. A small percentage of the U-238 in the fuel absorbs excess neutrons and undergoes transmutation. It becomes U-239, which rapidly decays into Np-239, which then decays into Pu-239. Because Pu-239 has a higher neutron cross section than U-238, the nature of the chain reaction shifts over time. At the start of use, the reaction is mostly U-235 fission. By the end of an 18 to 24 month exposure period, the ratio shifts to about 70% U-235 and 30% Pu-239.

There are several distinct types of fuel used for different purposes. Mixed oxide fuel, or MOX, is a blend of plutonium and natural or depleted uranium. MOX behaves similarly to the enriched uranium used in most reactors. It is used as an alternative to low enriched uranium (LEU) fuel.

Metal fuels are another category and have a long history of use. They have been used since the Clementine reactor in 1946. Metal fuels can have a very high fissile atom density and carry heat very well. Some metal fuels are pure uranium, while others are alloys like uranium-aluminum or uranium-zirconium.

Other specialized types include TRIGA fuel, which uses uranium zirconium hydride (UZrH) to stay safe during temperature changes.

Researchers also study non-oxide ceramic fuels and liquid fuels. Uranium nitride is a ceramic that NASA produces because it has better thermal conductivity than UO2. Uranium carbide is another ceramic that is attractive for Generation IV reactors because it has a high melting point and no oxygen.

Liquid fuels are quite different because they contain dissolved nuclear fuel in a liquid state. These can be mixtures of actinide salts, such as thorium or uranium fluoride, kept above their melting points. One major advantage of liquid fuel is that it can be drained into a safe dump-tank. This was demonstrated during the Molten-Salt Reactor Experiment from 1965 to 1969.

History shows how the development of fuel has been shaped by global policy and safety research. In 1978, the U.S. Department of Energy launched a program to convert TRIGA reactors to low-enriched uranium. Today, there are 35 TRIGA reactors in the United States and 35 in other countries. The production of MOX fuel has also seen international movement. It is made in France at the Marcoule Nuclear Site, and also in Russia, India, and Japan. In the 1960s, the LAMPRE experiments at Los Alamos National Laboratory tested molten plutonium encapsulated in tantalum. These experiments helped scientists understand how different metal alloys behave under intense conditions.

Understanding these materials is vital for the future of energy and waste management. For example, about one percent of used fuel is plutonium, and two-thirds of that is fissile. Using MOX fuel is one way to dispose of surplus plutonium through transmutation.

Liquid-salt reactors can also increase efficiency by incinerating much of their own waste. As scientists develop new technologies like fast breeder reactors or diamond batteries, the way we use nuclear fuel continues to evolve. These advancements aim to make energy production more stable, efficient, and safe for the long term.