Unfortunately, your commercial security clearance will mean little to the DOE. DOE L and Q clearances are a little more thorough than the commercial nukes and you will need to get a job with the DOE that requires a clearance in order for them to spend the time and money on you.
The good news is that there are a lot of DOE RCT jobs that don't require a clearance. You will most likely need to start with one of these and work your way into the "cleared" sector.
Good luck with your career change. It sounds like you are on the right track!!
John