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To cap nuclear arsenals and make nuclear weapon reductions irreversible, it will be necessary to end the production of unsafeguarded highly enriched uranium and separated plutonium. To achieve this goal, in 1993, the UN General Assembly instructed the United Nations Conference on Disarmament in Geneva to begin negotiating a Fissile Material Cutoff Treaty (FMCT). This chapter explores the possible scope and verification challenges for an FMCT. These challenges include monitoring of operating and shutdown enrichment and reprocessing plants and ruling out potential clandestine production including at military nuclear facilities and in naval nuclear fuel programs in the nuclear weapon states. The latter two goals will pose some additional challenges beyond those of verifying the Non-proliferation Treaty in non-weapon states. Many non-weapon states also would like to see the nuclear weapon states place their pre-existing stocks of civilian and excess weapons material under international safeguards.

Fissile materials can sustain a nuclear chain reaction and are used in weapons and as reactor fuels. This chapter explains the production and use of the most common fissile materials, highly enriched uranium and plutonium. The highly enriched uranium typically used in weapons is enriched to 90 percent or higher in the isotope uranium-235. Gaseous diffusion was used to produce most of the highly enriched uranium in the world, but has been replaced by gas centrifuge technology. Plutonium is produced from uranium in reactors and separated from spent nuclear fuel in a reprocessing plant. Plutonium of almost any isotopic composition, including that produced in civilian power reactors, is weapons usable. In the bomb that destroyed Hiroshima, a gun-type assembly was used to create a supercritical mass of highly enriched uranium able to sustain an explosive chain reaction, while the Nagasaki weapon used a plutonium implosion compression assembly. In modern thermonuclear weapons, an implosion fission “primary” ignites a fusion-fission “secondary.” Such weapons generally typically contain about 3?4 kilograms of plutonium and 15?25 kilograms of highly enriched uranium.

As of 2013, globally there were about 1400 tons of highly enriched uranium and 500 tons of plutonium. Almost all of the highly enriched uranium and about half of the plutonium were originally produced for weapons and remain outside International Atomic Energy Agency safeguards. Since the 1970s, some non-weapon states have acquired the capability to separate plutonium and to enrich uranium as part of their civilian nuclear power programs. This chapter focuses on the amounts of fissile material in different categories of current or intended use, and includes material available for weapons, declared excess for weapon purposes, assigned for naval and civilian use, and material that has been disposed of. The United States and United Kingdom have declared their stocks of fissile materials. Stockpile estimates for the other weapon states carry significant uncertainties and combined they are equivalent to several thousand nuclear warheads. Increased transparency by all weapon states will be required for the negotiation and verification of deep reductions and the eventual elimination of their nuclear weapons.

Since the end of the Cold War, Russia and the United States have declared substantial quantities of their highly enriched uranium and plutonium excess to any military need and agreed to dispose of them. Much more fissile material could be declared excess by each of these states and by the other nuclear weapon states. In both Russia and the United States, excess highly enriched uranium recovered from weapons is blended down to low-enriched uranium, which is then used in light water power reactor fuel. So far, almost 700 tons of excess highly enriched uranium have been blended down. The disposal of excess weapons plutonium and civilian plutonium is much more costly and more hazardous. The first choice of disposal route has been via mixed-oxide (MOX, uranium-plutonium) reactor fuel. France has pioneered this approach in recycling its separated civilian plutonium. Japan, the United Kingdom and the United States all have been less successful, however, and the United States has begun to consider other disposal options. In selecting disposal strategies, key considerations should be the degree of irreversibility being sought, materials security, cost, and international verifiability.

The world has struggled for over six decades with the dangers posed by huge quantities of plutonium and highly enriched uranium, the chain reacting fissile materials that are the key ingredients of nuclear weapons and that were described by the eminent physicist Niels Bohr in 1944 as possibly posing a “perpetual menace” to humankind. Since the failure of the post-World War II efforts to ban nuclear weapons and control fissile materials, nine other states have followed the United States and produced fissile materials and nuclear weapons. This chapter provides an overview of the book and an introduction to the fissile material problem and the proposals to cap, reduce, and eventually eliminate fissile materials. It explains why such initiatives are critical to support deep reductions and eventual elimination of all nuclear weapons, to make such nuclear disarmament more difficult to reverse, to raise the barriers to nuclear weapon proliferation, and to prevent nuclear terrorism.