* D'Agostino to leave National Nuclear Security Administration
* Said wants to retire from federal service
* Decision unrelated to break-in at Y-12 site in July-official
By Roberta Rampton
WASHINGTON, Dec 21 (Reuters) - The head of the agency responsible for the U.S. nuclear weapons stockpile said on Friday that he will leave his post in January, six months after three elderly peace activists broke into the government's maximum-security facility for weapons-grade uranium.
The departure of Thomas D'Agostino, the administrator at the National Nuclear Security Administration (NNSA), has nothing to do with the security breach, an NNSA official said.
D'Agostino, who worked for the government for more than 36 years and had been head of the NNSA for more than five years, had been asked to stay in the job but declined, the official said.
"I am a strong believer that organizations are healthier when leadership changes on a periodic basis," D'Agostino said in a statement, noting he would step down on Jan. 18, just before the second term of the Obama administration begins.
In July an 82-year-old nun and two other aging peace activists made their way past multiple layers of security and vandalized a building that was supposed to be one of the most secure complexes in the United States.
Neile Miller, currently serving as the NNSA's principal deputy administrator, will serve as the acting chief of the NNSA, said Energy Secretary Steven Chu. The Energy Department oversees the agency.
Chu lauded D'Agostino's service and said he led the agency "through a period of unprecedently international attention and complex transition" as it worked to reduce the number of deployed nuclear warheads and clean up contaminated sites.
The department's review of the break-in at the Y-12 site in Oak Ridge, Tennessee, is still under way.
So far, the department's Inspector General has found that contractors at the plant ignored a broken security camera for months and routinely ignored motion sensors.
After the incident, the government also found that guards at the facility were given a copy of a test and its answers before they were to take it.
The government fired the contractor, owned by international security firm G4S, which was the focus of political scrutiny last summer for failing to provide enough guards for the London Olympics.