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 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 U235 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 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 minute 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 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.
Insecure nuclear materials
Nuclear materials are easier to monitor than materials suitable for chemical and biological weapons. This is because the key materials - Pu239 and HEU - require complex facilities to isolate. Even so, there are some difficulties.
The International Atomic Energy Agency (IAEA) has established a regime of safeguards on nuclear facilities in order to prevent diversion of fissile material for weapons purposes. Non-nuclear weapon States (NNWS) parties to the Non-Proliferation Treaty are required to sign safeguards agreements with the IAEA. In 1991 the discovery of Iraq's nuclear weapons program indicated shortcomings in the safeguards system. The IAEA thus developed a strengthened safeguards system and invited NNWS to join. However, not all NNWS parties to the NPT have joined. More significantly, the non-Parties to the NPT and the NWS are not required to place their facilities under IAEA safeguards. The possibility of States diverting nuclear materials for weapons purposes therefore continues to exist.
In addition, there are large stockpiles of fissile material, and the security of some of this material is under question. In August 1994 German police confiscated a suitcase used to smuggle plutonium from Moscow to Munich. On October 13, 1997 the New York Times reported on a number of examples of nuclear material smuggling from an insecure Russian system. The US has been assisting Russia in securing its fissile material under the Nunn-Lugar Program, but in recent years the government has been cutting funds for this.
Fissile Material Cut-Off Treaty
A treaty banning fissile material has been on the agenda of the Conference on Disarmament (CD) for many years. However, differences in what it should cover has prevented negotiations. Some countries - including the NWS -wanted it to cover just the production of fissile material, while others -including Pakistan - wanted it to also address current stockpiles. Some states also want to see concurrent progress by the NWS on nuclear disarmament. There is also the question of how to deal with the production of non-fissile nuclear materials, especially tritium.
In 1998, some progress appeared possible when the CD established an ad hoc committee to discuss a proposed fissile material cut-off treaty. However, US plans to develop ballistic missile defence have added another damper on the situation. China hinted that it may increase its nuclear arsenal in response thus requiring more fissile material. Due to the difficulties in the CD, it may be preferable for existing moratoria on fissile material production by the NWS to be codified in a treaty negotiated outside the CD, thus not requiring support from all CD members.