Are we talking about TOS? I only recall fusion
being brought up in relation to the engines at one point in time... when Kirk was talking about overloading the Constellation's impulse engines...
Kirk: Am I correct in assuming that a fusion explosion of 97 megatons will result if a starship impulse engine is overloaded?
Spock: No sir... 97.835 megatons.
So, considering that fusion explosions are not only created by nuclear fusion reactors, I'd say that is inconclusive.
Also, the energy of a fusion reaction isn't as straight forward as E=mc^2
... what you are actually talking about is providing enough energy to overcome the repulsive electromagnetic forces to gain the even greater potential energy of the strong nuclear forces. There is a resulting loss of mass, but it is not equal to the original mass of the fuel (that type of reaction, 100% conversion, would be from a one-to-one matter/anti-matter process).
I have other threads on math and physics and would be very happy to discuss this subject in more detail in them. But for our purposes here, I tend to ignore post-TOS (though welcome others to inject whatever they want into their own derivative works) and stay about as vague on this subject as the writers guide was...
"The Enterprise has a secondary propulsion system. These are the impulse engines (same principle as rocket power), located at the rear of the "saucer section." Vessel speed, when using the impulse engine, is less than the speed of light. In case of total failure of all engine power sources, the vessel's gravitational and life support systems can be switched to battery power, with a full-load capacity of about one week."
But as an interesting sidebar, how much hydrogen do you think one would need to create an approximately 100 megaton explosion? A hint... the largest fusion based bomb ever exploded was the Tsar
hydrogen bomb which yielded about 50 megatons as I recall.
If we were to theorize about what an impulse engine might be... it could be similar to an ion engine only using particles accelerated to relativistic velocities to create the impulse thrust while expelling very little actual matter in the process. The thrust would be based on the relativistic momentum of the particles rather than a more standard momentum (based on the real mass of the particles).
But in the end we don't really know... and while it is fun to play with ideas, the plans I'm working on will avoid the limitations of todays ideas on this subject. And everyone else is free to put in as much of this as they want as I'll be leaving those areas undefined and undetailed.
But you are right... there are lots of places for fuel storage if you wanted to use them that way. I would suggest this diagram when hunting for those spaces.
The grey-to-black areas on the individual decks represent space too short for standard personnel deck use... but would be just fine for equipment and storage. I plan on leaving a lot of that type of space open for others to interpret it as they want.