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)

Creation and World War II
German scientists at the Kaiser Wilhelm Institute in the 1930s and 1940s were the first humans to discover how to split the atom. (AUDIO: The Alchemists) During World War II, American scientists, assisted by British scientists as well as scientists from other nations, developed the first nuclear bomb. With the Allies still at war with Japan in 1945, the scientists hoped that a demonstration of the bomb on an uninhabited island would be enough to convince the Japanese to surrender. However, the Allies were worried that clearing the Japanese from the Pacific Islands could extend the war for as long as another five years. The first nuclear bomb was dropped on Hiroshima, where it detonated with the power of 13,000 tonnes of TNT, killing 100,000 Japanese people and wiping out the entire city. Four days later, and even more powerful plutonium bomb was dropped on Nagasaki. (PROSE: Endgame) A third bomb was intended for Kyoto but the aircraft carrying the bomb, the Sky Jack, fell into a black hole. (COMIC: Sky Jacks) The bombing of Hiroshima and Nagasaki resulted in the surrender of the Japanese and the end of the war. (PROSE: Endgame)

Cold War
The scientists who developed the bomb had hoped that it would never be put to practical use. In 1949, convinced that it was wrong for one country to have a monopoly on nuclear weapons, some of the scientists passed on the secrets to the Soviet Union. They developed their own nuclear weapons, bringing about the nuclear standoff between the United States and the Soviet Union which largely defined the Cold War. (PROSE: Endgame) The major powers aimed nuclear missiles at one another, ensuring a stalemate through mutually assured destruction. (TV: Cold War)

In 1963, according to Barbara Wright, the US and USSR had enough bombs between them to destroy the entire population of Earth thirty times over. Ian Chesterton found her attitude surprising given her support for the Conservative Party, but she claimed her view on the bomb was conservative. (PROSE: Nothing at the End of the Lane) 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)

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)

Post-Cold War
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 the early 21st century, 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 Doctor 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)

In the wake of the Southport Incident, the world governments stopped stockpiling nuclear weapons. (PROSE: Just War)

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 alternate timeline, it was thought by James Stevens 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 alternate 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)

Parallel universes
In the Unbound Universe, Mike Yates made a sacrificial trip back to the era of the Silurians, detonating forty nuclear weapons to render them extinct, saving the human race.

When the psychic parasites who powered the Keller Machine could no longer control the Ke Le Divisions, the Chinese government prepared to destroy them via a nuclear bomb detonation in Mongolia, disguising the detonation as a test. The Doctor brought the last parasite to the detonation, killing them all and restoring the memories of the Ke Le Divisions while the bomb's shockwave broke the inhibitor on his TARDIS, allowing him to resume his travels across time and space. (AUDIO: Sympathy for the Devil)