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)

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.”

For instance, thermonuclear weapons require tritium (produced in nuclear reactors), as well as advanced interphase materials.

For much more information, see: Rhodes, Richard. Dark Sun: The Making Of The Hydrogen Bomb.

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.

It discusses various ways in which the accidental and unauthorized detonation of thermonuclear weapons can be avoided.

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.

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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