Fusion bombs were a devastating weapon. They were tested in the 23rd century. If detonated in sequence, their effectiveness increased logarithmically. The effects were:
- 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
They were banned before any larger combination could be tried.
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. It did not reach its destination, thanks to the Seventh Doctor. (PROSE: Cold Fusion)