How It Works: Thermonuclear Weapons | Catalyst Spring 2007

Text SizeA A A

How it works logo

Thermonuclear Weapons

U.S. thermonuclear weapons derive their explosive energy from the combined power of nuclear fission and fusion. An initial fission reaction generates the high temperatures needed to trigger a secondary—and much more powerful—fusion reaction (hence the term “thermonuclear”).

Essentially, the destructive energy produced by such a weapon is the result of three separate but nearly simultaneous explosions. The first is the detonation of chemical explosives that surrounds a sphere (or “pit”) of plutonium metal. The force from this blast is directed inward, compressing the pit and bringing its atoms closer together. Neutrons (atomic particles with no electric charge) that have been introduced into this dense core collide with the plutonium nuclei, sometimes causing them to split, or fission (see the sidebar). Together, this chemical and fission explosion is known as the nuclear “primary.”

The combination of both fission and fusion
processes contribute to the large explosive yield from a thermonuclear weapon. Click here to see a larger version of the image. Illustration: Amanda Wait/DG Communications

The primary explosion acts like a giant match that ignites a fusion reaction in the “secondary” device—more commonly known as a hydrogen (or H-) bomb. This term derives from the process by which fusion combines two hydrogen atoms to form helium, creating an even larger and more deadly explosion than fission can produce alone. For example, the fission-based nuclear weapon (or “A-bomb”) dropped on Nagasaki, Japan, in 1945 had an explosive yield equivalent to about 20 kilotons of TNT; thermonuclear weapons in today’s U.S. missiles commonly have explosive yields of several hundred kilotons.

A Question of Reliability

Every year since 1997, the nation’s nuclear weapons laboratories have certified that all U.S. nuclear warheads are safe and reliable, and that renewed nuclear explosive testing is not currently needed to gauge reliability. However, the laboratories have recently voiced concern that warheads may not be reliable over the long term.

It must be noted that the definition of “unreliable” in this context is a weapon that falls short of its designed yield by more than 10 percent. In other words, an “unreliable” nuclear weapon can still produce a devastating explosion. A weapon with a 300-kiloton yield could be deemed unreliable if it exploded with a 270-kiloton yield—13 times more energy than that released by the Nagasaki bomb.

The Physics of Destruction

The interaction of virtually invisible particles can produce devastation on a massive scale.

Early in the twentieth century, scientists discovered that atoms of certain radioactive elements can split—or fission—if bombarded with fast-moving neutrons. The by-products of this fission are two lighter atoms, free neutrons, and energy in the form of heat and light.

Scientists then discovered that certain isotopes of radioactive elements (i.e., variations of the same element with different numbers of neutrons in the nucleus), such as plutonium-239 or uranium-235, can emit two neutrons when they fission. These secondary neutrons then collide with other nearby nuclei, causing them to fission and release two more neutrons. Each fission reaction doubles the amount of neutrons and energy released, causing a chain reaction.

After only a few microseconds, this chain reaction can produce an explosion equivalent to the detonation of many thousands of tons of TNT—yet even this force pales in comparison with the energy released in modern thermonuclear weapons. Similar fission processes (though controlled) generate the energy in nuclear reactors.

A nuclear fission chain reaction. Click here to see a larger version of the image. Illustration: Amanda Wait/DG Communications

A nuclear weapon’s reliability depends on the primary fission reaction, which in principle can be prevented from triggering the secondary fusion reaction by many variables. For example, if the chemical explosives surrounding the spherical plutonium pit do not detonate simultaneously and symmetrically, the plutonium will not implode properly and will not release all of its energy. However, the chemical explosives and all the other non-nuclear components that make up a nuclear weapon can be fully tested without inducing fission, and the United States conducts such tests on an ongoing basis.

The only component that cannot be tested without actually setting off a nuclear explosion is the plutonium pit itself, and the United States conducted its last nuclear explosive test in 1992. Every type of U.S. nuclear weapon currently deployed underwent explosive testing, but it is theoretically possible that the properties of the plutonium could change as it ages, resulting in a weaker primary.

Understanding Plutonium

Plutonium is a relatively young element; scientists produced it for the first time in 1941. Since the oldest plutonium pits currently deployed are only several decades old, it has been unclear how they might change over longer periods. However, there are certain known physical properties that could affect the viability of the plutonium pit.

As plutonium ages, it slowly emits high-energy alpha particles (helium nuclei consisting of two protons and two neutrons) that can damage the pit as they bore through nearby plutonium atoms. Over time, bubbles of helium gas can also build up, causing the metal to expand. Because the implosion of the pit induced by chemical explosives could be affected by these imperfections, it is conceivable that a weapon using “old” plutonium might not explode with the intended yield. On the other hand, plutonium repairs at least some of the alpha particle-related damage to itself through a process called self-annealing, in which the plutonium atoms fill in the gaps made by the alpha particles. Because these processes are occurring simultaneously, it is difficult to calculate their net effect on pit integrity.

The oldest warheads in the U.S. weapons stockpile were assembled almost 30 years ago. Until very recently, the minimum lifetime of plutonium pits was conservatively estimated to be 45 years, which would mean that the pits in every U.S. warhead might have to be replaced within the next two decades. This is the rationale behind the Bush administration’s proposed Reliable Replacement Warhead (RRW) program, which would redesign and replace all 10,000 U.S. warheads.

Over the past several years, however, the U.S. weapons laboratories have effectively eliminated this rationale by conducting “accelerated aging” experiments to re-evaluate the age at which reliability would realistically decline. By simulating the behavior of aged plutonium, scientists concluded that all existing U.S. plutonium pits have minimum lifetimes of 85 years, and most will remain reliable for at least 100 years. (The lifetimes could be much longer, but further experiments are needed.)

As these results make clear U.S. thermonuclear weapons will remain highly reliable for many decades, undercutting the primary reason for the Bush administration’s RRW plans. For other reasons why this plan represents a step backward in U.S. nuclear policy, visit the UCS website.

Robert Nelson is a senior scientist in the Global Security Program.

Also in This Issue of Catalyst