The letter means that your file has kicked out of the Office of Personnel management (OPM) as complete, and is with the adjudicators at the agency you applied with. The fact that you had to sign the paperwork means that you are very likely to get your clearance fairly soon. There is a huge backlog at the NRC, and I am sure the DOE is backlogged as well. Rest easy, it is almost over, your clearance should come in soon.