What is the source for your information?
Which information? I already said I couldn't track down my source for the last thing. The others are just basic physics.
The first instance where fusion is associated with the impulse engines is "The Doomsday Machine".
Although the second pilot's reference to the impulse points decaying to lead implies that they were meant to be made of uranium, i.e. that the impulse engines were assumed to be fission-powered.