It's a nuclear reaction when you are messing around with atomic nuclei, as in forming helium from two hydrogens. But, the reactions are well known, and calculations can be done to know how much energy is required to make helium versus how much energy you get back out.
But, it isn't solely about the energy! If you take two hydrogens to make a helium, you now have two less hydrogens in your atmosphere! Thus, we need a source of hydrogen--the oceans? No, the earth needs its water thank you.
This is actually an interesting problem. This will be my next article.