Nuclear Materials

Nuclear materials are the key ingredients in nuclear weapons production. They include fissile, fussionable and source materials. Fissile materials are those which are composed of atoms that can be split by neutrons in a self-sustaining chain-reaction to release energy, and include plutonium-239 and uranium-235. Fussionable materials are those in which the atoms can be fused in order to release energy, and include deuterium and tritium. Source materials are those which are used to boost nuclear weapons by providing a source of additional atomic particles for fission. They include tritium, polonium, beryllium, lithium-6 and helium-3.

Plutonium
Plutonium is not found naturally in significant quantities. It is produced in a nuclear reactor through the absorption of neutrons by Uranium 238. The Plutonium emerges from a nuclear reactor as part of the mix in spent nuclear fuel, along with unused uranium and other highly radioactive fission products. To get plutonium into a usable form, a second key facility, a reprocessing plant, is needed to chemically separate out the plutonium from the other materials in spent fuel.

Once plutonium is separated, it can be processed and fashioned into the fission core of a nuclear weapon, called a "pit". Nuclear weapons typically require three to five kilograms of plutonium. Plutonium can also be converted into an oxide and mixed with uranium dioxide to form mixed-oxide (MOX) fuel for nuclear reactors. Britain, France, Russia, India, Japan, Israel and China operate reprocessing plants to obtain plutonium (the last two only for military purposes). U.S. plutonium production reactors were shut down in 1988.

A number of isotopes of plutonium are produced in a reactor, the most common being Pu-239 which is easily fissionable, and Pu-240 which is not. The relative proportion of Pu-239 determines the weapons grade of the plutonium. Reactor grade Pu, i.e. Pu with 18% or more Pu-240, can still be used to make a “crude” nuclear bomb.

Plutonium is an alpha particle emitter and so does not penetrate the skin. However, when ingested into the body, plutonium is incredibly toxic as alpha particles cause a very high rate cell damage. It is possible, for example, to contract lung cancer from one millionth of a gram.

Uranium
Uranium occurs naturally in underground deposits consisting of a mixture of 0.7% uranium-235, which is easily fissionable, and about 99.3% uranium-238, which is not fissionable. Nuclear weapons require “enrichment” to increase the proportion of U-235 to 90% or more. This is called Highly Enriched Uranium (HEU). Nuclear reactors require enrichment to about 3 - 5 % of U-235. This is called Low Enriched Uranium (LEU).

HEU can be combined with plutonium to form the "pit", or core of a nuclear weapon, or it can be used alone as the nuclear explosive. The bomb dropped on Hiroshima used only HEU. About 15-20 kgs of HEU are sufficient to make a bomb without plutonium.

Tritium
Tritium is a relatively rare form of hydrogen isotope with an atomic mass of three (one proton and two neutrons). It is used commercially, but only in small quantities, for medical diagnostics and sign illumination. Tritium's primary function is to boost the yield of both fission and thermonuclear weapons. It is produced in fission reactors and high-energy accelerators by bombarding lithium or lithium compounds with high energy neutrons.
Tritium decays rapidly with a half-life of 12.5 years, and thus must be replenished over time. For example, the U.S. has produced 225 kilograms since 1955. This has now decayed to an inventory of 75 kilograms.

Deuterium
Deuterium is a stable, naturally-occurring isotope of hydrogen with an atomic mass of two (one proton and one neutron). There is approximately 1 part of deuterium to 5000 parts of normal hydrogen found in nature. Deuterium is sometimes called heavy hydrogen. In thermonuclear bombs deuterium is fused with tritium to release energy.