Thermonuclear weapon design

A common misunderstanding about thermonuclear weapons (those that employ tritium-deuterium fusion as well as the fission of uranium or plutonium) is that most of the extra energy produced comes from fusion. In fact, the great majority comes from additional fission encouraged by neutrons produced by the fusion reaction. Each atom that undergoes fission generates 180 million electron volts (MeV) of energy, equivalent to 74 terajoules per kilogram. Tritium-deuterium fusion produces only 17.6 MeV per incident, though the materials that undergo fusion are far less massive than those that undergo fission.

The general functioning of a modern thermonuclear bomb (Teller-Ulam configuration) is something like the following:

  1. A neutron generator bombards the plutonium pit of the primary (fission device).
  2. Exploding-bridgewire or slapper detonators initiate the high explosive shell around the pit.
  3. The pit is compressed to a supercritical density.
  4. The pit undergoes nuclear fission, aided by the neutron reflecting properties of a shell made of beryllium, or a material with similar neutron-reflection properties.
  5. The fission process in the primary is ‘boosted’ by the fusion of tritium-deuterium gas contained in a hollow chamber within the plutonium.
  6. The x-rays produced by the primary are directed toward the secondary through an interphase material.
  7. Within the secondary, heat and compression from the primary induce the production of tritium from lithium deuteride.
  8. Tritium and deuterium fuse, producing energy and high-energy neutrons.
  9. Those neutrons help induce fusion within a uranium-235 pit within the secondary (called the spark plug). Layers of uranium-235 may alternate with layers of lithium deuteride, and the whole secondary may be encased in a sphere of uranium-235 or 238. This tamper holds the secondary together during fission and fusion. Uranium-235 or 238 will also undergo fission in the presence of neutrons from fusion.

Throughout this process, the whole device is held together by a uranium-238 (depleted uranium) case. This is to ensure that the reactions proceed as far as possible before the whole physics package is blasted apart.

One important security feature can be built into the detonators that set off the explosive shell around the primary. By giving each detonator a fuse with a precisely set random delay, it is possible to ensure that only those who know the timing of each detonator can cause the bomb to explode as designed. If the detonators do not fire in a very precisely coordinated way, the result is likely to be the liquefaction of the plutonium core, followed by it being forced out of the casing as a fountain of liquid metal. Nasty as that would be, it is better than the unauthorized detonation of the weapon.

The detonators are also an important safety feature since their ability to cause very stable explosives to detonate means that the high explosive shell can be made of something that doesn’t detonate easily when exposed to shock or heat. That is an especially valuable feature in a world where bombs are sometimes held inside crashing planes, and where fires on submarines can prove impossible to control.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

21 thoughts on “Thermonuclear weapon design”

  1. Lessons From the Accidental Nuke Flyby
    Popular Mechanics | Adam Pitluk | April 07, 2008

    During the analysis of the incident by the Defense Science Board (DSB), released this month, the ugly truth came out: America’s nukes are so neglected that they are stored alongside conventional missiles, with nothing but an 8.5 x 11-in. sheet of paper to differentiate the two. The last day in August, Air Force personnel loaded the nuclear warheads on a routine repositioning of weapons stocks, believing them to be cruise missiles.

  2. “Each atom that undergoes fission generates 180 million electron volts (MeV) of energy”

    It was my understanding that a volt is a unit of energy intensity, not of energy proper. Isn’t saying something produced “this many volts of energy” exactly like saying, “this thing produces this many pounds per square inch of energy”?

  3. The electronvolt (symbol eV) is a unit of energy. It is the amount of energy equivalent to that gained by a single unbound electron when it is accelerated through an electrostatic potential difference of one volt, in vacuo. In other words, it is equal to one volt (1 volt = 1 joule per coulomb) multiplied by the (unsigned) charge of a single electron.

  4. Keeping Presidents in the Nuclear Dark
    (Episode #2: The SIOP Option that Wasn’t)
    Bruce G. Blair, Ph.D, CDI President,
    Feb. 16, 2004

    One of the most rarefied experiences of a newly installed president is his receiving of the “nuclear football” conferring the right to order the use of nuclear weapons in defense of the American national interest. Few, if any, presidents have had a firm grip on the “football” however, as all U.S. presidents receive a misleading briefing on their nuclear weapons rights and responsibilities, and options.

    From the time of this highly classified orientation briefing given immediately upon his assumption of the presidency through the end of his tenure, a president is made to believe that he is the nuclear quarterback in control of the nuclear football and would call the shots in the event of a nuclear show-down or enemy missile attack. In the latter case, the short flight time of missiles launched from half way around the planet – 30 minutes from Russia to the American heartland – or from submarines lurking off the U.S. coasts – 10 to 15 minutes to Washington, D.C. — puts the president in the hot seat. He must evaluate early warning information, weigh his response options, and render a decision within minutes and seconds.

    Given the awesome responsibility and authority of the commander in chief in a situation of apparent incoming nuclear missiles, one can only hope for a deliberate, rational act of leadership and prudence that impels a president to refrain from ordering retaliation in the event of a false alarm triggered by faulty sensors or human error.

    What is misleading about the briefing is that the president’s supporting command system is not actually geared to withhold retaliation in the event of enemy missile attack, real or apparent. It is so greased for the rapid release of U.S. missiles forces by the thousands upon the receipt of attack indications from early warning satellites and ground radar that the president’s options are not all created equal. The bias in favor of launch on electronic warning is so powerful that it would take enormously more presidential will to withhold an attack than to authorize it. The option to “ride out” the onslaught and then take stock of the proper course of action exists only on paper. That is what presidents never learn during their tenures. Their real control is illusory. What’s more, the truth has been kept from the presidents intentionally.

  5. It’s amazing how man has endeavored to improve, and refine the design, and construction of a thermo-nuclear weapon, while leaving the important challenges unchecked.

    Power isn’t measured in megatons; rather, it’s measured in the wealth that a Society builds, while simultaneously caring for the least fortunate people. If you want to “crush” your opponent, you don’t do it with military might – you do it by adding value to both your own Country, and the World at large.

    We need thermo-nuclear weapons in our Arsenal, but only to deter the crazy wackos from launching a first-strike offensive against us. Whoever pushes, as Elmer Fudd refers to, “the wed button” first, is both selfish, and maniacal, and will secure for himself, “that special place in Hell”.

  6. US Forgets How To Make Trident Missiles

    Hugh Pickens writes “The US and the UK are trying to refurbish the aging W76 warheads that tip Trident missiles to prolong their life and ensure they are safe and reliable but plans have been put on hold because US scientists have forgotten how to manufacture a mysterious but very hazardous component of the warhead codenamed Fogbank. ‘NNSA had lost knowledge of how to manufacture the material because it had kept few records of the process when the material was made in the 1980s, and almost all staff with expertise on production had retired or left the agency,’ says the report by a US congressional committee. Fogbank is thought by some weapons experts to be a foam used between the fission and fusion stages of the thermonuclear bomb on the Trident Missile and US officials say that manufacturing Fogbank requires a solvent cleaning agent which is ‘extremely flammable’ and ‘explosive,’ and that the process involves dealing with ‘toxic materials’ hazardous to workers. ‘This is like James Bond destroying his instructions as soon as he has read them,’ says John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament, adding that ‘perhaps the plans for making Fogbank were so secret that no copies were kept.’ Thomas D’Agostino, administrator or the US National Nuclear Security Administ

  7. Pingback: In Mortal Hands
  8. The Mark 15 is widely described as a transitional design between fission and thermonuclear weapons. The Mark 15 was a staged weapon (see Teller-Ulam design), using radiation implosion from a fission nuclear primary (Cobra) to implode a secondary stage. Unlike most more modern thermonuclear bombs, the Mark 15 used a secondary which was primarily HEU (highly enriched uranium), which generated most of its energy from nuclear fission reactions once the primary imploded it. There was a thermonuclear core which underwent fusion reactions, but most of the energy came from the HEU fissioning. The HEU fission was enhanced by fusion stage neutrons, but would have generated a very significant fission yield by itself.

    Some later bombs used depleted uranium fusion stage tampers, and neutrons from the fusion would fission some of the tamper, but the primary energy release (50% or more) was from the fusion reaction.

  9. 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 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.

  10. Perhaps you could help. After a country, Iran for example, has mastered the technology to make an A bomb of circa 1946 vintage and the political will was present, is it feasible that they would then be able to go on develop thermo-nuclear armanants for available medium to long distance missile delivery systems?

  11. Almost all Soviet SLBMs, including Russia’s entire operational SLBM arsenal today, have used liquid propellant. This has formed the foundation of the Russian experience with naval ballistic missiles. By contrast, the U.S. Navy never fielded a single liquid-fuel SLBM even though it delayed the Polaris program considerably. The U.S. Navy was uncomfortable with cramming the highly toxic and corrosive liquid fuels then available into the tight spaces of a nuclear submarine. As an alternative, the United States did pioneering work in solid-fuel SLBMs in the late 1950s, even as the Soviets were fielding crude ballistic missile submarines armed with liquid-fuel SLBMs.

    In addition to the benefits in terms of safety, solid fuel has been found to be ideal for storing for long periods and at the same time being ready to launch quickly. Solid-fuel missiles also generally burn and accelerate faster. The most modern land-based intercontinental ballistic missiles (ICBMs) in both the United States and Russia are all solid fuel.

    Yet the Soviets had different design and manufacturing considerations than their competitors in the West, such as their greater problems with quality assurance. Though the Soviets were responsible for a number of important innovations, their missile programs were often a bit more force and a bit less finesse. It was only in 1983, after more than a decade of work, that the Soviets were able to field a solid-fuel SLBM: the SS-N-20 “Sturgeon.” It was the largest SLBM in history and required the largest submarine in history to carry it.”

  12. “On March 9, 1951,” Bethe notes, “… Teller and Ulam had published a [classified] paper which contained one-half of the new concept.”

    But “within a month,” Bethe goes on, “the very important second half of the new concept occurred to Teller, and was given preliminary checks by [Frederick] de Hoffmann. This immediately became the main focus of attention of the thermonuclear design program.” The second half of the new concept was probably a further nesting of cylinders within cylinders: an outside casing of U238 to scatter X rays from the primary into the plastic; a layer next of plastic; a layer next of U238 tamper; a layer next of thermonuclear materials; and at the axis of the cylinder a stick of plutonium. Now the imploding plastic would work not only on the thermonuclear materials. It would also start a second fission chair reaction in the stick of Pu by squeezing it to critical mass. That would add a further huge flux of heat and pressure to the thermonuclear materials and push the fusion reactions over the top. The U238 later, in turn, would benefit from the dense flux of neutrons released in thermonuclear burning and would fission above the 1 MeV U238 fission threshold. Neutrons from that fission would then contribute to preparing the thermonuclear materials for further burning.” Such a design is usually described as fission-fusion-fission. Not without reason did Robert Oppenheimer call the two part Teller-Ulam invention “technically… sweet.”

    Rhodes, Richard. The Making of the Atomic Bomb. p.776 (paperback)

  13. Some time in the spring, a new warhead for the Polaris arrived, the first of many that were to be shipped to the submarine fleet. I went through the warhead manual and found a number of things that disturbed me. This particular warhead was designed for use against cities. It was very compact, a weapon with a small bang and a small cross-section, but its ablative shield was an alloy of uranium, and it produced very heavy alpha fallout downwind. I thought about the world laid waste by these warheads. I wondered if you could be a good soldier and have an imagination.

    There was also this. When installed on the rocket, the main warhead connection was safety-wired in place and hidden. But in our bay and out in the storage depots these warheads were stored with an unlocked weapons connector. Unzip the weather cover, and it was right there. The safety mechanisms were in the fusing assembly, not in the warhead. Bang the right pins with the right voltage, and the warhead would blow. I wrote a technical change memo, suggesting a locked cover for the warhead connector while it was in storage.

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