A micro-fusion generator was an outdated, outlawed and cheap generator. It produced a very harmful toxic waste. It was loud and "antisocial". The waste from the generator required a specific containment unit in order to be disposed of properly, but the waste could simply be dumped anywhere.
There was a micro-fusion generator aboard the SS Humphrey Bogart. When the ship crash landed on Laylora, it was used as an emergency power source and the waste was dumped outside the ship. Petra Shulough considered it a "drop in the ocean"; this enraged the Tenth Doctor. The Witiku kept attacking the ship until they eventually destroyed the generator. (PROSE: The Price of Paradise)