This chapter begins with an introduction to the characteristics of NEM, the means by which these materials are produced, and current stocks and flows of NEM in the military and civilian sectors. (This treatment is supplemented with more detail in Appendix A) The chapter then addresses the challenges of transparency and monitoring for NEM, first in conceptual terms and then in terms of the specific bilateral and multilateral measures that have been undertaken up until now in connection with cooperative efforts to account for, secure, and protect both military and civilian materials1


All nuclear weapons rely on the energy released by an explosively growing fission chain reaction—a process in which heavy nuclei split into lighter ones following absorption of free neutrons and, in splitting, release more neutrons that in turn induce more fissions, and so on. Only a few nuclides2 of the hundreds that exist are capable of sustaining the explosive nuclear chain reaction needed for a nuclear weapon. Such nuclear-explosive nuclides include U-235, U-233, and all the isotopes of plutonium, among others. A nuclear-explosive material is one in which the proportions of nuclear-explosive nuclides and nonexplosive nuclides of the same elements are such as to permit an explosive chain reaction if the material is present in suitable quantity, density, chemical form and purity, and configuration.

In the simplest nuclear weapons, the fission chain reaction is the only source of the nuclear energy that is released. In more advanced nuclear weapons, such as “boosted” fission weapons and thermonuclear weapons, some of the energy is generated by fusion reactions that are ignited by energy from the fission explosion.


The arguments in this chapter build on those in National Academy of Sciences, Committee on International Security and Arms Control, Management and Disposition of Excess Weapon Plutonium, 2 vols. (Washington, DC: National Academy Press, 1994 and 1995); Steve Fetter, Verifying Nuclear Disarmament, Occasional Paper 29, Henry L. Stimson Center, Washington, DC, 1996; and Independent Bilateral Scientific Commission on Plutonium Disposition, Final Report, Washington, DC: President's Committee of Advisors on Science and Technology, The White House, and Russian Academy of Sciences, June 1997.


“Nuclide” is the general term for a species of atom as characterized by both its atomic number (equal to the number of protons in the nucleus, which determines the element to which a nuclide belongs) and its mass number (equal to the number of protons and neutrons combined, which determines which isotope of the element it is). See Appendix A.

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