Variable yield
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Variable yield, or Dial-a-yield, an option available on most modern nuclear weapons, allows the operator to specify a weapon's yield, or explosive power, allowing a single design to be used in different situations.
In some weapons, the yield is adjusted on the ground, with the addition or removal of tampers that control the rate of fission or fusion. For example, the Tsar Bomba's yield was halved when its uranium fusion tamper was replaced with a lead one.
Dial-a-yield can also be achieved with fusion neutron boosting. This can be accomplished by injecting a few milliliters of tritium gas into the vacuum of a hollow core pit inside of a fission-type nuclear weapon. When the dial is turned it may open a valve that will inject a little bit of tritium into the core of the device. Then the atomic core is plugged, and the high-explosive trigger is assembled.
One weapon that may use this approach is the W88 warhead currently used on American SLBMs. The W-88 with fresh tritium inside its pit may explode with a yield of 475 kilotons, but with no tritium inside the core it might explode with the force of just 20 kilotons. See the nuclear weapons archive to obtain more detailed information on fusion neutron boosting.
If the tritium has been in the pit for several years, half or more of it may have undergone radioactive decay. This might reduce the yield of a device such as a W-88 nuclear warhead to, for example, 40 kilotons.
Tritium has to be bred in nuclear fission reactors and its half-life is only 12.32 years. Most modern nuclear weaponeers now realize that any dependency on fusion neutron boosting is a nuclear weapon design flaw because of the extreme undersupply of tritium gas since about 1990.[citation needed]
