I should let an officer answer this, but I'm not sure who's available on these boards.
If you're already being recruited for OCS, you can enter the nuclear field that way. I'm not sure how competative you would be with your GPA, but it can't hurt to try.
There are three different nuclear options, and they are apples, oranges, and pears.
The enlisted Nuclear Field program trains technicians to operate, maintain, and repair the electrical, electronic, and mechanical aspects of a submarine or aircraft carrier power plant.
Since you already have a degree, your only practical choice for commissioning once enlisted is OCS. Your performance in Nuclear Power school should offset your low college GPA and prove that you have technical aptitude. Nuclear officers are expected to not only lead, but to understand in complete detail(of both operation and theory) the power plants that they oversee.
The submarine nuclear officer program trains junior officers for submarine duty. All submarine officers are nuclear trained. After spending a year at Nuclear Power School and a nuclear prototype(similar to the last 2/3 of the enlisted pipeline) and officer submarine school, you would work in engineering on a submarine for one sea tour, and be in charge of other departments on subsequent tours. You will have needed to take at least two semesters of calculus and two semesters of calculus-based physics to be eligible.
(If you don't qualify, you could go to OCS as an unrestricted line officer and try to do a lateral transfer once you have create a good military record)
The SWO(Surface Warfare Officer) nuclear program will send you to a ship(destroyer, cruiser, etc.) after commissioning. After a successful time at sea, you will undergo the same training as the submarine officers, and be assigned to the engineering department of an aircraft carrier.
You really need to talk to a nuclear officer recruiter. Tell him what you want, and he should be able to help you get there. We always need nuclear trained officers. Ask to speak with current nuclear officers who have been to sea and back. If you consider the enlisted route, speak to enlisted sailors. Better yet, try to get your recruiter to put you in touch with a nuclear officer who started as an enlisted man. There are also many former or retired enlisted nuclear trained sailors on these boards.