Nuclear weapon

A nuclear weapon was a powerful and destructive device based on the principles of nuclear fission. The Eighth Doctor once commented that a nuclear device tapped into the power of the planet itself. (AUDIO: The Last)

Nuclear weapons on Earth
In the 1970s, nuclear missiles were used against Eldrad when she took over the Nunton Experimental Complex, but they had no effect on her. (TV: The Hand of Fear)

During the Cold War, the major powers aimed nuclear missiles at one another, ensuring a stalemate through mutually assured destruction. In 1983, Skaldak attempted to launch a Soviet submarine's nuclear missile so that the two sides would wipe out all of humanity. (TV: Cold War)

In 2008, agents of Cell 114 attempted to explode nuclear missiles stored in an old mine near Cardiff, but were thwarted by members of Torchwood Three. (TV: Sleeper)

In 2009, UNIT attempted to fire nuclear weapons at the Sontaran flagship to prevent the Sontaran invasion of Earth, but this was secretly thwarted by a clone of Martha Jones who averted the countdown. As noted by the Tenth Doctor, those missiles wouldn't have even dented the Sontaran ship, making him curious as to why the Sontarans would take actions to stop the launch, later finding out that it was to preserve the clone feed that they had spread across Earth. After the Martha clone died, the Doctor had the real Martha continue cancelling the countdown, not wanting to start an interplanetary war. (TV: The Poison Sky)

In 2013, Kate Stewart threatened to detonate a nuclear weapon to destroy London and the Black Archive to prevent the Zygons from getting everything in the Archive. Kate and her Zygon duplicate fought over the countdown of the bomb, but the War, Tenth and Eleventh Doctors forced them both to cancel the countdown by erasing their memories of who was who so they would be forced to make peace. (TV: The Day of the Doctor)

At an unknown point in history, in the wake of an unspecified event known as the Southport Incident, the world governments stopped stockpiling nuclear weapons. (PROSE: Just War) However, Zoe Heriot recognised the effects of an atomic blast on the Island of Death, likely indicating that nuclear weapons were still around in her time. (TV: The Dominators)

Nuclear weapons on other planets
On Skaro, nuclear weapons were used early on in the Thousand Year War. In the later years of the war, the combatants were reduced to primitive weaponry. (TV: Genesis of the Daleks)

On Dulkis, the Dulcians tested nuclear weapons on the Island of Death, and were so horrified by their destructive capabilities that they outlawed war. (TV: The Dominators)

Nuclear weapons were also used in the war between the Argolins and the Foamasi. (TV: The Leisure Hive)

The Marshal of Atrios attempted to end the war between Atrios and Zeos by firing a nuclear missile from his Command Capsule at Zeos, but the Fourth Doctor redirected his attack onto the Third planet. (TV: The Armageddon Factor)

On Alrakis, Tonska rebels used a nuclear bomb to destroy Zaniah's laboratories, inadvertently allowing a Cerunnos to escape to Earth. (PROSE: ...And Eternity in an Hour)

On Mars, a nuclear device was used by Adelaide Brooke to destroy Bowie Base One on 21 November 2059 after an infection known as the Flood broke loose. (TV: The Waters of Mars)

On Sunday, a Swamp Monster attempted to blow itself up with a nuclear weapon, thus spreading itself across the Galaxy. (PROSE: Wetworld)

Alternate timelines
In an alternative timeline, it was thought that U.S. President John F. Kennedy would have started a nuclear war on 22 January 1964 if he had not been assassinated on 22 November 1963. (PROSE: Who Killed Kennedy)

In another alternative timeline created by the Elder Gods in the hope of destroying Earth, World War III broke out on 9 November 1989 between the United States and its allies including the United Kingdom on the one hand and the Soviet Union on the other hand. Given that nuclear weapons were used by both sides, hundreds of millions of people were killed in the conflict. This timeline was ultimately negated by the Seventh Doctor. (AUDIO: Protect and Survive)