Just an advice, don't place all of your reactors in one area. It's also better to efficiently distribute your crew, reactors, shields and weapons into "stations", then isolating each station to avoid the crew from going anywhere on the ship. This helps on making the crew focus on a specific task rather than doing everything. Ex: if you isolate your crew with a reactor and some shields, they will only operate on those shields. As a result, the shield will be constantly receiving energy thus making it active most of the time even when under enemy fire.
What I said about ditributing and isolating crew only works effectively for huge ships.