For the basics, to qualify for a Q clearance, besides there being a specific need for the individual to qualify for the clearance (that is, they don't just process people for the clearance, there has to be a specific position or need for the person they'll process for the clerance; they don't process several people for the clearance and then see which of the "now cleared" people they'll give the position to. (That's because, in part, as recently as 15 years ago, running a Q clearance investigation for someone who was otherwise qualified, cost approximately $3,500 per investigation).
Q clearance require a single-scope background investigation (that is, background checks involving checks of employment, education, organization affiliations, local agencies, where the subject has lived, worked, or gone to school, and interviews with persons who know the individual. The investigation may include a NACLC on the candidate’s spouse or cohabitant and any immediate family members who are U.S. citizens other than by birth or who are not U.S. citizens and generally goes back the past 10 years.
As a general rule, if you currently have a clearance with access to nuclear/atomic with another agency (such as Department of Defense Top Secret (TS) clearance + Critical Nuclear Weapon Design Information access), and are looking to work with the Department of Energy (which is where Q clearance is used) the other agency's access is not routinely adjudicated, but rather a Q clearance investigation is started from the beginning.
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