Fusion bombs were a devastating weapon. They were tested in the 23rd century. If detonated in sequence, their effectiveness increased logarithmically: one bomb could level a city and kill over 50 million people; five bombs could destroy a planet; and eight bombs could shatter every solid planet and ignite the gas giants in an entire solar system. They were banned before any larger combination could be tried.
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)
When the Sontarans invaded the Eleven-Day Empire on behalf of the Great Houses, they planted a fusion bomb when they stole Faction Paradox's biodata samples. Cousin Justine, instructed by Godfather Morlock, stopped the bomb by dropping her shadow-weapon and using her shadow to pick it up, allowing it to detonate harmlessly. (AUDIO: The Eleven Day Empire) When Justine was given Grandfather Paradox's shadow, she could access all its shadow-weapons, which included a fusion bomb. (AUDIO: The Shadow Play)