Weapon Materials Basics

Enriching uranium is both technically difficult and expensive,as it requires separating isotopes that have very similar chemical and physical properties. The enrichment process is thus the main barrier to producing uranium suitable for use in nuclear weapons.

Low-enriched uranium (LEU) contains between 0.7 percent and 20 percent uranium-235, and highly enriched uranium (HEU) contains 20 percent or more uranium-235. LEU is not directly usable for weapons. HEU produced for weapons ("weapon-grade" uranium) is typically enriched to 90 percent uranium-235 or greater, but all HEU can be used to make nuclear weapons. The difficulty and expense of the enrichment process has an important consequence: HEU can be diluted with natural uranium to produce LEU, effectively eliminating the risk that it could be used to make a nuclear weapon if stolen by terrorists.

However, as sophisticated enrichment technology spreads around the world, more groups will be able to overcome the technical barriers to producing HEU for weapons. For this reason, Pakistan's illicit transfer of advanced enrichment technology to Iran, Libya and North Korea is of grave concern to the international community. Moreover, the commercial enrichment facilities used to make LEU fuel for power reactors can be reconfigured to produce HEU for weapons. Without strong regulations in place, these dual-use facilities pose major risks of nuclear terrorism. In addition, the continued use of HEU for both civilian research and naval propulsion reactors increases the risk of terrorist access to this material.

Plutonium. Plutonium occurs only in trace amounts in nature. It is produced as a matter of course in power reactors when the uranium-238 in reactor fuel absorbs neutrons. Countries producing plutonium for weapons have generally operated their reactors to maximize the production of plutonium-239—the isotope most useful for nuclear weapons—and to minimize the production of other plutonium isotopes such as plutonium-240.  Weapon-grade plutonium contains less than 7 percent plutonium-240. Under normal nuclear power plant operation, the plutonium in spent reactor fuel contains roughly 24 percent plutonium-240; such plutonium is often referred to as "reactor-grade." However, essentially all isotopic mixtures of plutonium—including reactor-grade plutonium—can be used for nuclear weapons.

In order to use plutonium in nuclear weapons or nuclear fuel, however, it must be separated from the rest of the spent fuel in a reprocessing facility. Plutonium separation is easier than uranium enrichment because it involves separating different elements rather than different isotopes of the same element, and it uses well known chemical separation techniques. However, since the spent fuel is highly radioactive, this process requires heavily shielded facilities with remote-handling equipment.

Relatively large amounts of plutonium-240, as would be contained in reactor-grade plutonium, can cause a weapon to detonate early and "fizzle," causing a smaller explosion than intended. However, even a weapon that fizzles would cause an explosion roughly equivalent to 1,000 tons (1 kiloton) of TNT. A weapon of this size could kill tens of thousands of people if detonated in a city, which clearly demonstrates that even reactor-grade plutonium would present a potent danger in the hands of terrorists.

MOX fuel. Another terrorist risk arises from the manufacture, transportation, and storage of mixed plutonium-uranium oxide (MOX) fuel, used in some research and power reactors. While MOX fuel itself is unlikely to be used to make nuclear weapons, the plutonium can be separated from the uranium by a straightforward chemical process. Moreover, MOX does not contain the highly radioactive components that make spent fuel difficult and dangerous to reprocess. As a result, MOX is as great a terrorist and proliferation concern as plutonium itself.

How much is needed to build a bomb?

The amount of HEU needed to make a nuclear weapon varies with the degree of enrichment and the sophistication of the weapon design. In general, the higher the enrichment level, the less HEU is needed to make a bomb. For a HEU-based nuclear weapon, there are two basic design options: a "gun-type" weapon where two pieces of HEU are brought together quickly and explode, and an "implosion weapon," where a sphere of HEU is rapidly compressed in a highly symmetrical manner. Gun-type weapons are far simpler in design and could likely be built by some terrorist groups. The second is more difficult technically but requires less HEU. Plutonium-based nuclear weapons only work as implosion weapons, with more sophisticated weapons using less plutonium.

The Union of Concerned Scientists' Preventing Nuclear Terrorism Project seeks to prevent terrorists from acquiring nuclear weapons and the fissile materials—plutonium or highly enriched uranium (HEU)—needed to make them. This fact sheet series covers specific problems relating to nuclear terrorism and the steps the United States and other countries should take to address them.

For more information, contact Dr. Edwin Lyman at 202-223-6133.