If you're working 80 hours a week now and you qualified for nuke, you should be fine. The program requires a work ethic, integrity (honesty), and a certain baseline level of knowledge.
Most of the guys I saw kicked out or re rated were alcohol busts, integrity violations, and medical conditions, in that order. I know 3 people who were honestly kicked out for lack of knowledge, and they got decent jobs in the navy afterwards. Well, one of them is still awaiting re rating, the other two did though.
This is coming from a qualified student at prototype, awaiting graduation.