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The production of highly enriched uranium and/or plutonium is the key challenge for a state seeking to acquire nuclear weapons. This chapter summarizes these efforts in the 10 states that have built nuclear weapons, starting with the World War II United States’ Manhattan Project, which pioneered key technologies and set the template for the nuclear programs to follow. As part of the Cold War arms race, the Soviet Union (Russia) produced both more highly enriched uranium and plutonium than the United States. The United Kingdom, France and China had much smaller scale military fissile material production programs. All five of these states have ended the production of fissile materials for weapons. Israel produces its plutonium in a dedicated reactor and reprocessing plant provided by France. India and North Korea initially focused on plutonium, and Pakistan on highly enriched uranium but all now produce both fissile materials. South Africa based its weapons on indigenously produced highly enriched uranium and has ended production and dismantled all its nuclear weapons.

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 Gaseous diffusion was used to produce most of the highly enriched uranium in the world, but has been replaced by gas centrifuge technology.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.

Highly enriched uranium (HEU) was used in the Hiroshima bomb, a gun-type device the design of which is well within the capabilities of terrorist groups. This chapter explores the Use of HEU as fuel in research reactors and in naval propulsion reactors and efforts to end such use. Beginning in the 1950s, HEU began being used to fuel research reactors that the United States and Soviet Union exported to about 40 countries. Proliferation concerns led the United States and Soviet Union to launch programs to these reactors to low-enriched uranium (LEU) fuel. Russia, however, has been slow to convert or shut down its own HEU-fueled research reactors which now constitute about half of the global total of about 100. The United States, United Kingdom, Russia and India also use HEU fuel in about 200 naval propulsion reactors, with the United States responsible for over half of the world’s nuclear-powered submarines and ships. Discussion has just begun of a norm that would require future naval reactors to be LEU fuelled, like those already used by France and China.

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.

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.