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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.

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 Gaseous diffusion was used to produce most of the highly enriched uranium in the world, but has been replaced by gas centrifuge technology.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

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.

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.

The 1953 U.S. Atoms for Peace initiative launched the dissemination of nuclear technologies to non-weapon states. It also led to the establishment in 1957 of the International Atomic Energy Agency and to the Nonproliferation Treaty of 1970. The spread of nuclear power programs also has led to the spread of sensitive enrichment and reprocessing technologies which has given some non-weapon states the means of producing fissile material and thereby a “latent” proliferation capability, where a state could quickly produce nuclear weapons should it decide to do so. Even a small nuclear power program can provide a nuclear weapon breakout potential. The proliferation dangers associated with today’s dominant nuclear fuel cycle come from the fact that the uranium enrichment plants that produce low-enriched uranium for fuel could be rapidly converted to produce highly enriched uranium for weapons and that some countries reprocess spent fuel to recover plutonium, a weapons material, to recycle as fuel.

Today there are about 500 tons of separated plutonium, about half of which was produced for weapons during the Cold War. The remainder is a result of civilian programs in the United States, the Soviet Union, the United Kingdom, France, China, India, and countries inspired by their examples which have sought to use plutonium as a reactor fuel, initially for liquid-sodium-cooled plutonium breeder reactors. This chapter surveys the history of breeder reactor and the costs and dangers of reprocessing and plutonium use and explains why they have largely been abandoned, with only six of the 31 countries with nuclear power reactors now separating plutonium. While, the UK has decided to stop reprocessing, Russia, India, and China continue to separate plutonium for their breeder development programs and, in France and Japan, plutonium separation has become so institutionalized that they are extracting plutonium for mixed-oxide (MOX, uranium-plutonium) fuel even though it is not economic, The chapter also assesses the challenges facing long-term storage and disposal in a final geological repository of spent fuel from nuclear power reactors and high-level radioactive waste from reprocessing operations.

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 elements making up the Actinoid Metals are those with atomic numbers from 89 through 103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr. The name is meant to parallel the lanthanoids. They are generally abbreviated as An. Their valence electron structures are 7s26d0−25f0−14. These elements resemble the lanthanoids somewhat, but they have a much wider variation in oxidation states. Nor do they resemble each other to the extent that the lanthanoids do, this being a result of the oxidation state variations. Ac resembles La greatly, but Th, Pa, and U resemble their vertical congeners (Hf, Ta, W) more than they resemble Ce, Pr, and Nd. From Np onwards, the resemblance to the lanthanoids increases such that by Am, the actinoid elements are behaving very similarly, showing a predominant oxidation state of III. All of this occurs because the 7s, 6d, and 5f levels are much closer in energy than the 6s, 5d, and 4f levels. Table 18.1 lists the actinoids with several of their pertinent characteristics. No stable isotopes of any of these elements exist, the last element in the Periodic Table with a stable isotope being Bi (Bi-209). However, some of the An elements have isotopes with very long half lives, which means that they are found in nature in relative abundance, most notably as Th-232 (1010.1 years), U-235 (108.8 years), and U-238 (109.7 years). Others are products of the decay of the above isotopes, so even though they are shorter lived, they persist in nature since they are continually being produced. The most important nuclides of this type are Ac-227 (21.8 years) and Pa-231 (104.5 years), both coming from U-235 decay. In U ores, very small amounts of Np-237 (106.3 years), Np-239 (2.4 days), and Pu-239(104.3 years) arise from the interaction of neutrons with U isotopes. Isotopes of the elements beyond U are produced artificially, Np and Pu by neutron capture by U, Am and Cm by multiple neutron capture by Pu, and elements beyond Cm by further neutron captures or bombardment of lower atomic number actinoids with ions of He, B, C, N, or O.