A Starter Glossary of Nuclear Terms

From nuclear parity to plutonium pits, the Corporation’s Sharon Weiner and Samara Shaz define some frequently used words and acronyms to increase your nuclear understanding

By Sharon K. Weiner and Samara ShazSeptember 6, 2023

Carnegie Reporter Fall 2023 Nuclear Threats

Control panels are seen in an underground launch control center for intercontinental ballistic missiles (ICBMs) outside of Great Falls, Montana, on July 1, 2018. (Credit: Lido Vizzutti for The Washington Post via Getty Images)

Nuclear security is a major global concern. It affects the safety and stability of individuals, communities, and nations. Delving into this specialized field can be daunting, as it can involve grappling with unfamiliar terminology, concepts, and field-specific language. For those eager to expand their knowledge, the following terms may serve as a good place to start.

ABM Antiballistic missiles, sometimes referred to as missile defense, are launched from earth with the goal of destroying incoming ballistic missiles.

Annual Threat Assessment of the U.S. Intelligence Community is a report on threats to the United States published annually by the U.S. director of national intelligence.

ICBMs Intercontinental ballistic missiles are land-based missiles that carry nuclear warheads. Along with SLBMs (submarine- launched ballistic missiles) and bombers, they make up the nuclear triad.

Modernization refers to the program that, over the next 30 years, will replace or add improvements to most parts of the U.S. nuclear arsenal.

Mutually Assured Destruction, or MAD, is a condition that exists when two adversaries each have a secure second strike — that is, a nuclear arsenal capable of surviving an attack by the other and then responding with devastating force.

The New START Treaty is the last remaining U.S.-Russian arms control agreement. It limits each side to 1,550 warheads that are deployed and ready for launch on the nuclear triad. The treaty does not constrain the number of warheads in the hedge, meaning warheads that are not deployed but which are kept in storage for potential future use.

The Nuclear Non-Proliferation Treaty (NPT) established global nuclear norms and responsibilities for nonproliferation, disarmament, and the peaceful use of nuclear energy.

Nuclear parity is the state of two countries having a similar number of nuclear weapons, where neither side has an advantage in terms of capability.

A nuclear weapon is composed of a delivery system (a missile, submarine, or bomber) and a warhead (a device that explodes because of a nuclear reaction).

Peaceful nuclear use is when a nuclear reaction is used for nonweapons purposes, such as generating electricity in a power plant or for making medical isotopes.

A plutonium pit is the sphere-like shell of plutonium that sets off the nuclear reaction in a warhead.

The Treaty on the Prohibition of Nuclear Weapons (TPNW), adopted by the United Nations in 2017, prohibits nuclear weapons.

Triad (nuclear triad) Comprised of ICBMs (intercontinental ballistic missiles), SLBMs (submarine-launched ballistic missiles), and bombers.

Uranium-235 (U-235) is an isotope of uranium that undergoes a chain reaction in nuclear weapons. (An isotope is a form of an element with the same number of protons but a different number of neutrons.) Highly enriched uranium (HEU), or weapons-grade uranium, is material that is at least 20 percent U-235. Low-enriched uranium (LEU) has a concentration of U-235 lower than 20 percent and is used as fuel for producing nuclear energy in power plants.

Sharon K. Weiner is a senior resident fellow in the International Peace and Security program at Carnegie Corporation of New York and an associate professor at American University’s School of International Service. Her teaching, research, and policy engagement are at the intersection of organizational politics and U.S. national security. Her work also focuses on civil-military relations and nuclear weapons programs and nonproliferation. Samara Shaz is a program assistant for the International Peace and Security program. Previously, she was a research assistant at MIT’s Security Studies Program, investigating the salience of nuclear weapons in American politics.