Variable yield — or dial-a-yield — is an option available on most modern nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

s. It allows the operator to specify a weapon's yield

Nuclear weapon yield

The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...

, or explosive power, allowing a single design to be used in different situations. For example, the Mod-10 B61 bomb

B61 nuclear bomb

The B61 nuclear bomb is the primary thermonuclear weapon in the U.S. Enduring Stockpile following the end of the Cold War. It is an intermediate yield strategic and tactical nuclear weapon featuring a two-stage radiation implosion design....

 had selectable explosive yields of 0.3, 5, 10 or 80 kt, depending on how the ground crew set a dial inside the casing when it was loaded onto an aircraft.

Variable yield technology has existed since at least the early 1960s. Examples of variable yield weapons include the B61 nuclear bomb

B61 nuclear bomb

The B61 nuclear bomb is the primary thermonuclear weapon in the U.S. Enduring Stockpile following the end of the Cold War. It is an intermediate yield strategic and tactical nuclear weapon featuring a two-stage radiation implosion design....

 family, B83

B83 nuclear bomb

The B83 nuclear weapon is a variable yield gravity bomb developed by the United States in the late 1970s, entering service in 1983. With a maximum yield of 1.2 megatons, it is currently the most powerful atomic weapon in the US arsenal...

, W80, W85

W85

The W85 was a thermonuclear warhead developed by the United States of America to arm the Pershing II missile. It had a variable yield— often referred to as "dial-a-yield" — which could be set between 5 and 80 kilotons.-Overview:...

 and WE177A warheads.

Most modern nuclear weapons are Teller–Ulam design type thermonuclear weapons, with a fission

Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 primary stage and a fusion

Nuclear fusion

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 (or fission) secondary stage which is collapsed by the energy from the primary. These offer at least three methods to vary yield:

• Varying primary yield by boosting
  Boosted fission weapon
  A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
  
   with fusion
  Nuclear fusion
  Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
  
  , using small amounts of deuterium
  Deuterium
  Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
  
   / tritium
  Tritium
  Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
  
   gas inside the primary fission bomb to increase its yield. Typically, the gas is injected a few seconds before detonation and the amount used can be preset.
• Varying primary yield by varying the timing or use of external neutron initiators (ENIs). These are small particle accelerators which cause a brief fusion reaction by accelerating deuterium into a tritium target (or potentially vice versa), producing a short energetic pulse of neutrons. Precise timing of the ENI pulse as the nuclear primary's pit is collapsing can significantly affect yield, and the rate of neutron injection can also be controlled.
• Shutting down the thermonuclear secondary, either by firing the primary at low enough yield that it does not compress the secondary sufficiently to ignite, or by blocking energy transport inside the warhead briefly as the primary is firing using shutters or a similar mechanism. If the primary's energy starts to disperse through the radiation case before being focused on the secondary then the secondary will likely never detonate.

All current British

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

nuclear warheads incorporate variable yield technology as standard.