Fizzle (nuclear explosion)

A fizzle occurs when the detonation of a device for creating a nuclear explosion (such as a nuclear weapon) grossly fails to meet its expected yield. The cause(s) for the failure can be linked to improper design, poor construction, or lack of expertise. All countries that have had a nuclear weapons testing program have experienced some fizzles. A fizzle can spread radioactive material throughout the surrounding area, involve a partial fission reaction of the fissile material, or both. For practical purposes, a fizzle can still have considerable explosive yield when compared to conventional weapons.
In multistage fission-fusion weapons, full yield of the fission primary that fails to initiate fusion ignition in the fusion secondary (or produces only a small degree of fusion) is also considered a "fizzle", as the weapon failed to reach its design yield despite the fission primary working correctly. Such fizzles can have very high yields, as in the case of Castle Koon, where the secondary stage of a device with a 1 megaton design fizzled, but its primary still generated a yield of 100 kilotons, and even the fizzled secondary still contributed another 10 kilotons, for a total yield of 110 kT.
Fusion boosting.
If a deuterium-tritium mixture is placed at the center of the device to be compressed and heated by the fission explosion, a fission yield of 250 tons is sufficient to cause D-T fusion releasing high-energy fusion neutrons which will then fission much of the remaining fission fuel. This is known as a boosted fission weapon.
If a fission device designed for boosting is tested without the boost gas, a yield in the sub-kiloton range may indicate a successful test that the device's implosion and primary fission stages are working as designed, though this does not test the boosting process itself.
Terrorist concerns.
One month after the September 11, 2001 attacks, a CIA informant known as "Dragonfire" reported that al-Qaeda had smuggled a low-yield nuclear weapon into New York City. Although the report was found to be false, concerns were expressed that even a "fizzle bomb" capable of yielding a fraction of the known 10 kiloton weapons could cause "horrific" consequences. A detonation in New York City would mean thousands of civilian casualties.