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; a modern thermonuclear weapon weighing little more than a thousand kilograms can produce an explosion comparable to the detonation of more than a billion kilograms of conventional high explosive.

In the Fallout world, nuclear weapons weren't developed to be as destructive as the ones in our world, allowing humanity to survive and eke out an existence in the ashes of the old world. However, what they lacked in raw power they made up in numbers, as China, USA, the European Commonwealth member states and other countries around the world (likely the Soviet Union too) possessed massive nuclear stockpiles and when the Great War entered its last phase, sent them skyward, on bombers and intercontinental ballistic missiles. Entire continents were swallowed in flames and fell under the boiling oceans, while the EMP released from the blasts destroyed electronics worldwide.

Fallout
In Fallout, the Glow is testament to the horror of nuclear war, a radioactive hellhole destroyed by a direct nuclear hit. In the same game, the Vault Dweller also discovers an unused nuke sitting in the Master's vault, to be used as last resort against an undefeatable enemy.

Fallout 2
A nuclear bomb also rests on the Enclave Oil Rig, and is, once again, used to obliterate the main enemy of the game.

Fallout 3
Nuclear weapons feature prominently in Fallout 3, in the form of Megaton's nuke, the Fat Man nuclear catapult and its unique variant, the Experimental MIRV, exploding nuclear weapons, a warehouse full of nuclear bombs, Vertibirds with nuclear minibombs and a massive robot throwing nuclear bombs at everything that moves.

Fallout Tactics
A nuclear ICBM warhead appears first (called Plutonius) in Kansas City, worshipped by a ghoul cult. It is later used to gain entrance to Cheyenne Mountain installation, the Vault 0.

Van Buren
The B.O.M.B.-001 space station, the endgame location, was an orbital ballistic missile launch platform, that Victor Presper wanted to use to reshape the world as he envisioned it.

Inconsistencies
The yield of the weapons in the games is never explicitly stated. While it is stated that entire continents were scorched by nuclear weapons, their effects in the game are not even remotely similiar to that description.

Additionally, the way the weapons are portrayed in the games is inconsistent - while in the "classic" Fallout games nuclear weapons are feared, respected and exceedingly rare, not to mention that arguably the most intelligent being in the Fallout world, the Master, is unwilling to unleash the power of atom again, in Fallout 3 they are commonplace and devoid of their traits from previous games - you can detonate a city with a nuclear bomb in the first few hours of the game, blow up cars in nuclear explosions and have a personal nuke launcher.