Weapons-grade nuclear material

Weapons-grade nuclear material is any special kind of nuclear material that is pure enough to be used in making a nuclear weapon. The most common examples are plutonium and certain types of uranium that have just the right qualities for use in nuclear weapons.

Only certain types of elements, called fissile isotopes, can be used in nuclear weapons. For this purpose, the amount of uranium-235 and plutonium-239 must be high enough. Natural uranium is made richer through a process called isotope separation, and plutonium is created in a nuclear reactor.

Scientists have also tested uranium-233, which comes from the thorium fuel cycle. There have been ideas about using neptunium-237 and some types of americium, but it is not known if these have ever been used in real weapons. These materials are found among the minor actinides in spent nuclear fuel.

Critical mass

Main article: Critical mass

For a material to be used in a nuclear weapon, it must have a critical mass. This means it needs to be just the right size to keep a nuclear reaction going. Different types of materials, called isotopes, need different amounts to reach this critical mass. Some materials, like uranium-238, never can reach a critical mass. But others, such as uranium-233 and uranium-235, can.

The critical mass can change based on how pure the material is and its shape. A round shape, like a ball, usually needs the least amount of material. There is a table that shows the critical masses for some materials, but much of this information is kept private for safety.

Countries that have produced weapons-grade nuclear material

At least ten countries have made special materials that can be used to create powerful energy weapons.

• Five countries are known to have these materials under an agreement called the Nuclear Non-Proliferation Treaty: the United States, Russia, the United Kingdom, France, and China.
• Three other countries have also made these materials but are not part of this agreement: India, Pakistan, and North Korea.
• Israel is believed to have these materials but has not said so openly.
• South Africa once had these materials but later stopped and joined the agreement in 1991.

Weapons-grade uranium

Natural uranium can be changed into weapons-grade material through a process called isotopic enrichment. Normally, natural uranium has only about 0.7% of a special type called U-235, with most of it being U-238. These are separated because they have different weights.

When uranium is enriched to about 90% U-235, it is considered weapons-grade. Another type, U-233, comes from thorium-232 and does not need enrichment. However, making U-233 can also create small amounts of U-232, which has dangerous radioactive materials. This makes it harder to use U-233 in weapons.

Weapons-grade plutonium

See also: Reactor-grade plutonium

Pu-239 is made in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays into Pu-239. It can then be separated from the uranium in a nuclear reprocessing plant.

Weapons-grade plutonium is mostly Pu-239, usually about 93% Pu-239. Pu-240 is made when Pu-239 absorbs another neutron and does not split apart. Pu-240 and Pu-239 are not separated during reprocessing. Pu-240 splits apart easily on its own, which can cause a nuclear weapon to go off too early. This makes plutonium unsuitable for some types of nuclear weapons. To lower the amount of Pu-240 in the plutonium, special reactors used for making weapons-grade plutonium, like the B Reactor, expose the uranium to neutrons for a shorter time than normal nuclear power reactors.

This is a key difference between reactors used for making weapons and those used for making electricity. Some older reactors, like the British Magnox and French UNGG reactors, could make both electricity and materials for weapons. They operated at lower power and changed their fuel often to make weapons-grade plutonium. This is not possible with the light water reactors commonly used today to make electricity. In these reactors, the reactor must be stopped and taken apart to reach the used fuel.