William Bookless, principal deputy administrator of the National Nuclear Security Administration (NNSA), assumed the role of acting administrator of NNSA after Lisa Gordon-Hagerty stepped down from the post.

NNSA said Friday Gordon-Hagerty, who received Senate confirmation in February 2018 to serve as head of NNSA, also resigned as undersecretary of energy for nuclear security.

Bookless brings more than 35 years of nuclear security experience to the role. He spent more than three decades at Lawrence Livermore National Laboratory in California as a senior physicist.

He previously served as a senior adviser to the NNSA administrator for policy and as assistant laboratory director for policy and planning at Brookhaven National Laboratory in New York.