Chart of nuclides showing thermal neutron fission cross section values

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons (i.e. a thermal system) or fast neutrons. Fissile materials can be used to fuel thermal reactors, with a neutron moderator; fast-neutron reactors, with no moderators; and nuclear explosives.

Fissile vs fissionable[link]

According to the fissile rule, heavy isotopes with 90 ≤ Z ≤ 100 and 2 × Z – N = 43 ± 2, with few exceptions, are fissile (where N = number of neutrons and Z = number of protons).^[1]

"Fissile" is distinct from "fissionable." A nuclide capable of undergoing fission after capturing a neutron is referred to as "fissionable." A fissionable nuclide that can be induced to fission with low energy thermal neutrons is referred to as "fissile." Although the terms were formerly synonymous, fissionable materials include also those (such as uranium-238) that can be fissioned only with high-energy neutrons. As a result, fissile materials (such as uranium-235) are a subset of fissionable materials.

Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible. Consequently, uranium-238 is a fissionable material but not a fissile material.^[2]

An alternative definition defines fissile nuclides as those nuclides that can be made to undergo nuclear fission (i.e., are fissionable) and also produce neutrons from such fission that can sustain a nuclear chain reaction in the correct setting. Under this definition, nuclides that are only fissionable are those nuclides that can be made to undergo nuclear fission but produce insufficient neutrons, in either energy or number, to sustain a nuclear chain reaction.^[3] As such, while all fissile isotopes are fissionable, not all fissionable isotopes are fissile. In the arms control context, particularly in proposals for a Fissile Material Cutoff Treaty, the term "fissile" is often used to describe materials that can be used in the fission primary of a nuclear weapon.^[4] These are materials that sustain an explosive fast fission chain reaction.

Under all definitions above, uranium-238 (U-238) is fissionable, but because it cannot sustain a neutron chain reaction, it is not fissile. Neutrons produced by fission of U-238 inevitably inelastically scatter to an energy below 1 MeV (i.e., a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of U-238, so fission of U-238 does not sustain a nuclear chain reaction.

Fast fission of U-238 in the secondary stage of a nuclear weapon contributes greatly to yield and to fallout. The fast fission of U-238 also makes a significant contribution to the power output of some fast-neutron reactors.

Fissile nuclides[link]

In general, most actinide isotopes with an odd neutron number are fissile. Most nuclear fuels have an odd atomic mass number (A = the total number of protons and neutrons), and an even atomic number (Z = the number of protons). This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.

More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission, and they also have relatively much longer half-lives for alpha or beta decay. Examples of these elements are uranium-238 and thorium-232. On the other hand, isotopes with an odd number of neutrons and an odd number of protons (odd Z, odd N) are short-lived because they readily decay by beta-particle emission to an isotope with an even number of neutrons and an even number of protons (even Z, even N) becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton-proton and neutron-neutron pairing. The short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.

Nuclear fuel[link]

To be a useful fuel for nuclear fission chain reactions, the material must:

• Be in the region of the binding energy curve where a fission chain reaction is possible (i.e. above radium)
• Have a high probability of fission on neutron capture
• Release two or more neutrons on average per neutron capture (which means a higher average number of them on each fission, to compensate for nonfissions, and absorptions in the moderator)
• Have a reasonably long half life
• Be available in suitable quantities

Fissile nuclides in nuclear fuels include:

• Uranium-235 which occurs in natural uranium and enriched uranium
• Plutonium-239 bred from uranium-238 by neutron capture
• Plutonium-241 bred from plutonium-240 by neutron capture. The Pu-240 comes from Pu-239 by the same process.
• Uranium-233 bred from thorium-232 by neutron capture

Fissile nuclides do not have a 100% chance of undergoing fission on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the neutron capture cross sections for fission (σ_F), the cross section for neutron capture with emission of a gamma ray (σ_γ), and the percentage of non-fissions are in the table at right.

See also[link]

• Fertile material
• Fission product
• Special nuclear material
• Nuclear fusion
• Fissility (disambiguation)
• Fissile rule

References[link]

Robert L. Gallucci (born February 11, 1946) is an Italian American academic and diplomat, who currently works as President of the John D. and Catherine T. MacArthur Foundation. He previously served as Dean of the Edmund A. Walsh School of Foreign Service at Georgetown University from 1996 to June 2009. Before his appointment in 1996 he was employed for 21 years by various governmental and international agencies, including the Department of State and the United Nations.

Early life and education[link]

Gallucci was born in Brooklyn, New York. He attended the State University of New York at Stony Brook for his undergraduate studies, later earning his master's degree and doctorate in politics from Brandeis University. After his post-graduate studies, he taught at Swarthmore College, Paul H. Nitze School of Advanced International Studies at Johns Hopkins University and Georgetown University. He has received fellowships from the Council on Foreign Relations, the International Institute for Strategic Studies, Harvard University, and the Brookings Institution.

Career[link]

Gallucci left the world of academia in 1974 and went on to hold various positions relating to international affairs. He first found employment at the Arms Control and Disarmament Agency. Four years later, he became a division chief in the Department of State's Bureau of Intelligence and Research. Between 1979 to 1981, he was a member of the Secretary's policy planning staff. He then served as an office director in both the Bureau of Near Eastern and South Asian Affairs and in the Bureau of Political-Military Affairs for a year each.

Ten years after beginning his foreign affairs career, he left Washington, D.C., to serve as the Deputy Director General of the Multinational Force and Observers, the Sinai peacekeeping force headquartered in Rome. He returned in 1988 to join the faculty of the National War College, where he taught for three years. In April 1991 he moved to New York to take up an appointment as the Deputy Executive Chairman of the United Nations Special Commission (UNSCOM) overseeing the disarmament of Iraq. He returned again to Washington in 1992 to join the Office of the Deputy Secretary as the Senior Coordinator responsible for nonproliferation and nuclear safety initiatives in the former Soviet Union. In July of the same year his appointment as the Assistant Secretary of State for Political-Military Affairs was confirmed. During the North Korean nuclear crisis of 1994, Gallucci was the chief U.S. negotiator. He also has served as an Ambassador-at-Large with the Department of State since August 1994.

Gallucci returned to Georgetown University as Dean of the Edmund A. Walsh School of Foreign Service on May 1, 1996. In March 1998, the Department of State appointed him as Special Envoy to deal with the threat posed by the proliferation of ballistic missiles and weapons of mass destruction, a position which he held until January 2001. As a dean at Georgetown University, Gallucci recommended conservative Douglas J. Feith to a 2-year faculty position which Feith occupied in the fall of 2006, a move which generated protests from some liberal faculty and students.

External links[link]

• MacArthur Foundation profile
• "New Leader for MacArthur Foundation", New York Times, March 10, 2009.
• "Gallucci Looks Back" (Transcript), PBS Online NewsHour, May 8, 1996.
• Robert Gallucci, "Reflections on Establishing and Implementing the Post-Gulf War Inspections of Iraq's Weapons of Mass Destruction Programs". Address delivered at the Institute for Science and International Security, June 14 – June 15, 2001.
• Harry Kreisler, "U.S. Foreign Policy and Multilateral Negotiations: Conversation with Robert Gallucci," "Conversations with History," Institute of International Studies, UC Berkeley, 2002.
• Going Critical: The First North Korean Nuclear Crisis with Joel S. Wit (Author), Daniel Poneman