FIREPIT CHATS - Encouraging Conversation about National Security Issues Important to YOU - Understanding the Nuclear Realm: Physics, Reactors, Weapons, and the Force
The Steady State | by Margaret Henoch and Dr. Tony Fainberg, August 6, 2025
Understanding the Nuclear Realm: Physics,
Reactors, Weapons, and the Forces Within
From the anniversary of the Hiroshima bombing, to Iranian nuclear reactors to nuclear submarines, it may be useful to have a basic understanding of The Nuclear Realm for discussion and consideration.
In a series of three articles, we will attempt to demystify the core concepts and components, and distinct characteristics of both, alongside critical safety considerations and associated dangers.
While the terms "nuclear reactor" and "nuclear weapon" often evoke strong images and sometimes fear, they represent fundamentally different applications of the same powerful forces. While both nuclear reactors and nuclear weapons harness the immense power of the atom, their design, purpose, and the implications of their use are vastly different. Reactors aim to control nuclear reactions for beneficial energy production, with safety systems meticulously designed to prevent uncontrolled release. Weapons, conversely, are engineered for uncontrolled, virtually instantaneous destruction. Understanding these distinctions is crucial for informed discussions and policy decisions about nuclear technology, non-proliferation, and global security.
The Elements Involved
Our explanations will center, broadly speaking, on Uranium-235 and Plutonium-239, both of which are used in fission and fusion. The number following the name of the element is the particular form of the element, or the isotope, involved. An isotope of an element has a nucleus with the number of protons that define the element (92 for uranium, 94 for plutonium) and a number of neutrons which, when added to the number of protons, define the isotope. For example, uranium-235 has 92 protons and 143 neutrons; plutonium-239 has 94 protons and 145 neutrons.
Summarizing some other, related points, natural uranium is composed of the fissionable isotope 235 (0.7%) and the most common isotope, 238 (99.3%). There is one other natural isotope, 234, but this is present only at the level of 0.005%. Plutonium 239 is the nuclear weapon and reactor fuel. The even isotopes, 240, 242, and 244, are too radioactive to work well as nuclear explosives. They would pre-detonate a device, causing a large reduction in explosive yield. Pu239 (to a pretty high level of isotopic purity) is what is needed for a nuclear explosive (and in a reactor that uses Pu as fuel).
The Atom: Fission and Fusion
At the heart of all nuclear technologies lie two fundamental atomic processes: fission and fusion.
Fission: This is the process where the nucleus of a heavy atom, such as uranium-235 or plutonium-239, is split into two or more smaller nuclei, typically by being hit with a neutron. This split releases a tremendous amount of energy, and additional neutrons. These newly released neutrons can then go on to strike other heavy atoms, triggering a chain reaction. In a reactor, this chain reaction is controlled, and electric power can be generated. An uncontrolled chain reaction is the principle behind a fission bomb.
Fusion: This is the process where two light atomic nuclei, such as isotopes of hydrogen (deuterium and tritium), combine under extreme heat and pressure to form a heavier nucleus. This process also releases an enormous amount of energy, even greater than fission, as mass is converted into energy according to Einstein's famous equation E=mc2. Fusion is the energy source of the sun and stars, and it is the basis for the most powerful nuclear weapons, but controlled fusion for power generation remains a significant scientific and engineering challenge.
Enrichment
Uranium 235 is the only naturally occurring material that can sustain a nuclear chain reaction, powering both reactors and the bombs; but most naturally occurring U235 does not have the concentration of U235 required in either a reactor or a bomb. That uranium, however, can be enriched, and enriched U235 will work in most nuclear reactors and weapons. The process of enrichment involves separating the heavier, not useful, molecules U238, from the U235 molecules. That separation is done by spinning the atoms at high speeds, something like separating cream, using a centrifuge. Any nation that wants a self-contained nuclear energy program must be able to enrich their own uranium, such that they do not have to rely on another country to provide the (low) enriched uranium needed for reactor fuel. Which means that any country with a self-contained nuclear energy capability must have centrifuges and the means to enrich their own uranium.
The problem with that is as follows: 3-5% 235 enrichment is needed for a standard light water reactor, but that concentration is totally useless for making a nuclear weapon. Which is great!! The downside, HOWEVER, is that if a country can enrich to that lower-than-bomb-concentration-level, that nation is using pretty well-understood engineering techniques, and is half-way to enriching to greater than 90%, the very concentration needed for a weapon. So, can a nation be self-sufficient in nuclear fuel for economic (electricity production) purposes, without also having the capability of producing nuclear weapons? Unfortunately, not.
Today, the favored technique for uranium enrichment is the centrifuge, which can produce nuclear reactor fuel. But in a “cascade” arrangement, in which some centrifuges feed others in series, one can also make nuclear weapon fuel… with the exact same type of centrifuge.
The solution to that problem has been the near-universal acceptance of the international nuclear safeguards regime under the International Atomic Energy Agency (IAEA), in which international inspectors make on-site visits to verify that all is kosher in a nation’s enrichment plant. If there is NO international verification, a nation could develop nuclear weapon fuel without constraint, and obtain the highly enriched uranium which is the most difficult step in constructing a nuclear weapon.
Under the JPCOA, Joint Comprehensive Plan of Action, a 2015 agreement between Iran and several world powers (China, France, Germany, Russia, the United Kingdom and the United States) an IAEA safeguards regime was attached to Iran, which the Iranians tried to evade and render impotent. For now, Iran has left the safeguards regime entirely. Understanding or controlling what is going on in Iran’s centrifuge halls and other parts of their nuclear system is fundamental to assure that they cannot make a nuclear weapon. This is why controlling the centrifuge issue is central to the Iran problem.
There is another route to build a bomb using Plutonium 239 (Pu239) which does not occur naturally, and has to be manufactured. Pu239 is created using a heavy water reactor fueled with natural uranium (U238). During the reaction, the U238 absorbs neutrons initiating a chain reaction which converts U238 into Pu239. That Pu239 contains uranium isotopes, which must be separated from the Pu using chemical processes. This separation process is crucial to produce Pu for use in nuclear weapons and other applications.
The Manhattan Project used both paths - U235 and Pu239. The Atomic Bomb, used at Hiroshima, was assumed to be virtually certain to work and was not tested. The plutonium fueled bomb was, however, tested, successfully, at the Alamogordo test site, about 200 miles south of Los Alamos. That test was for the bomb headed to Nagasaki. And, both the bomb for Alamogordo and the one headed to Nagasaki were plutonium weapons.
Other Important Terms in the Nuclear World
Dirty Bomb (Radiological Dispersal Device - RDD): A dirty bomb is not a nuclear weapon. It is a conventional explosive device designed to scatter radioactive material over a wide area. Its primary purpose is not to cause mass casualties through the explosion itself, but to create widespread panic, contamination, and disruption, making areas uninhabitable and requiring costly cleanup. While dangerous, the immediate casualties from radiation sickness are typically limited compared to a nuclear weapon, though long-term health effects from exposure are a concern. The radioactive material usually comes from industrial, medical, or research sources.
Radioactivity: The spontaneous emission of radiation from the nucleus of an unstable atom as it decays to a more stable state. This radiation can be in the form of alpha particles, beta particles, gamma rays, or neutrons. Radioactivity is produced by nuclear reactions and found in nuclear waste. Radioactivity is also a natural phenomenon e.g., radon gas and bananas. Bananas are very mildly radioactive, due to natural decay of potassium-40, which is radioactive and represents about one in ten thousand potassium nuclei on earth.
Radiation Illness (Acute Radiation Syndrome - ARS): A severe illness caused by exposure to a large dose of penetrating radiation over a short period. Symptoms can include nausea, vomiting, diarrhea, fatigue, hair loss, skin burns, and damage to bone marrow (leading to impaired immune function and bleeding). The severity and onset of symptoms depend on the dose received, and very high doses are often fatal. Long-term effects of radiation exposure can include increased risk of cancer, genetic mutations, and other chronic health problems.
Next on “Firepit Chats” Nuclear Reactors - What you need to know!
(Much of this Primer is based on or quoted from information in the Manhattan Project Interactive History, which was produced by the Department of Energy and is available on the internet. ) For a Deeper Dive please see “The Nuclear Realm” https://thesteadystate.org/media-and-posts/
Margaret Henoch was born in Los Alamos and developed her understanding of the nuclear world, nuclear reactors and nuclear weapons by listening to her father at the dinner table over about a decade and a half. After college she worked for SRI, International assessing the technical capabilities of airborne advanced radar and communication systems on fighter and bomber aircraft, and after five years of that, joined the Clandestine Service of the CIA, where she served for 25 years. She is a member of The Steady State.
Dr. Tony Fainberg has spent a career in basic and applied research in physics, as applied to national security issues, such as counter terrorism and nuclear nonproliferation. He received his PhD from University of California Berkeley in 1960. He has served on congressional staff, in several agencies of the Executive Branch and with a government-sponsored research institution. Mr. Fainberg is a member of The Steady State.
Founded in 2016, The Steady State is a nonpartisan, nonprofit 501(c)(4) organization of more than 300 former senior national security professionals. Our membership includes former officials from the CIA, FBI, Department of State, Department of Defense and Department of Homeland Security. Drawing on deep expertise across national security disciplines including intelligence, diplomacy, military affairs and law, we advocate for constitutional democracy, the rule of law and the preservation of America’s national security institutions.