For those that are going after the fusion generator (as I did) you may want to consider some things.
1) If you scrap any current generator(s) you WILL take a huge hit on the scrapped material VS the building of it. It also depends on the size of the generator being scrapped.
2) If you build the fusion generator your settlement WILL suffer in the happiness category.
3) The fusion generator does take a lot more space than the standard generators.
I don't know if it is possible to have a melt down because it does show negative in the happiness category once built.