UPDATE 1-Head of U.S. agency overseeing nuclear weapons fleet resigns -Energy Department

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WASHINGTON, Nov 6 (Reuters) - The head of the agency that oversees the U.S. fleet of nuclear weapons has resigned, the U.S. Energy Department said in a news release on Friday, without providing a reason.

Lisa Gordon-Hagerty resigned as administrator of the National Nuclear Security Administration, a semi-autonomous branch of the Department of Energy.

William Bookless is now serving as acting administrator of NNSA, the department said. He had been serving as NNSA principal deputy administrator for the last year-and-a-half.

Gordon-Hagerty became the first woman to serve as administrator when she took up her post in early 2018.

NNSA oversees the development and maintenance of nuclear weapons, but not the delivery systems, which are managed by the Department of Defense.

DefenseNews was first to report Gordon-Hagerty’s resignation. The news outlet said she had had clashes over her budget with Energy Secretary Dan Brouillette. (Reporting by Timothy Gardner in Washington Editing by Leslie Adler and Matthew Lewis)