Fusion bomb

Fusion bombs were a devastating weapon. They were tested in the 23rd century. If detonated in sequence, their effectiveness increased logarithmically. The effects were: They were banned before any larger combination could be tried.
 * One bomb - levelled a city and could kill upwards of 50 million people
 * Five bombs - total destruction of a planet
 * Eight bombs - solar system-wide, shattering every solid planet and igniting the gas giants

While the bombs were believed unstoppable, the Doctor was able to disarm them using a technique described as being "beyond the science of the Humanian Era", namely reversing the polarity of the neutron flow.

Adjudicator Medford, believing the Time Lords were a threat to Earth, filled the Machine with more than twenty (which could potentially annihilate a galaxy) and had the Fifth Doctor and Patience send it back to ancient Gallifrey; the Fifth Doctor believed that he had already disarmed the bombs, but was unaware that the Seventh had done that before him and he had thus accidentally re-armed them. The Machine did not reach its destination, thanks to the Seventh Doctor tricking the Ferutu into holding it in place in the Vortex so that it would collide with its past self on its original trip, the bombs destroying the Machine of the present and sending the past Machine crashing while leaving the timeline intact. (PROSE: Cold Fusion)

The Daleks loyal to the Dalek Prime planted a fusion bomb on the Doctor's TARDIS when he was of no use to them during a civil war on Skaro. The Eighth Doctor defused it. (PROSE: War of the Daleks)

A fusion bomb was one of the shadow-weapons that Cousin Justine could access with Grandfather Paradox's shadow. (AUDIO: The Shadow Play)