OK I feel like I can offer you alot of different advice on this. First I am a Nuke electrician, second I am a recruiter, and most importantly I know all these programs pretty much first hand, so here it goes.
As far as STA goes, You are ineligible for it do to your college credits. The only routes you have Are NUPOC, then NRI and NRE. NRI is Naval reactors instructor, ie a teacher at NNPTC, NRE is Naval reactors Engineer. All three of these are basically closed off to you do to your GPA, a 3.2 is competitive at the most basic nature. With Your gpa you would have to score Perfect On the OAR portion of the astb( the officer asvab) Now as far as enlisted side goes, with a D in your class, regardless of which class your going to incur a waiver, before listening to everyone on here knocking recruiters saying how evil we are blah blah blah (yes I am angered and insulted by these comments, we bust our butt to help people out, obviously if you hated your time in the navy, then its no one else's fault but yours. Remember no one forced you to get off your mothers couch and do something with your life, anyways) Your going to have to blow away the asvab, and If you meet minimum score requirements you will have to take whats called the NAPT(navy advanced placement test) You will also incur an age waiver if you wait any longer and an additional waiver if its been two years since math class. The only other option you have is IF you enlist you can apply to go to OCS or ODS to get nuke that way but once again it is unlikely with your GPA. Your best bet if you wanna get in Nuclear power is enlist in Navy. You can be assured NUKE contract long before ever joining, Take the asvab and then take the NAPT and have the Nuke coordinator submit your waiver request to the Nuke waiver shop. If its approved great your on your way to becoming a nuke, if not well at least you tried, PM me you need any other specifics.