The U.S. National Nuclear Security Administration (NNSA) confirmed Wednesday that Stephen Younger will retire as director of the Sandia National Laboratories in New Mexico effective Dec. 31, after two years managing the nation’s nuclear-weapon engineering laboratory.

The local Albuquerque Journal reported the news first, on Tuesday.

“I am profoundly grateful to Dr. Younger for his exceptional leadership at the helm of Sandia National Laboratories,” NNSA Administrator Lisa Gordon-Hagerty said in a statement. “During his tenure as Labs Director, he has made significant contributions to NNSA’s national security missions through many scientific and engineering advancements to maintain our nuclear deterrent.”

The board of Sandia’s Honeywell [HON]-owned management and operations contractor, National Technology and Engineering Solutions of Sandia, “is working to identify a new Director,” Gordon-Hagerty said in the statement.

Dori Ellis, the deputy labs director at Sandia, will take over as interim director after Younger departs. Ellis became deputy director in June after David Douglass retired.

National Technology and Engineering Solutions has managed Sandia since April 2017, when it wrapped up a four-month transition period to take over from the Lockheed Martin [LMT]-led Sandia Corp. Younger has been at the helm that whole time.

The Sandia management contract, awarded in 2016, is worth around $25 billion over 10 years, with options. The company is in the second year the contract’s five-year base period.

Sandia designs the non-nuclear parts of nuclear weapons, and certifies each year that the U.S. nuclear arsenal can perform as designed. The lab also maintains the Z machine high-energy density physics facility. The laboratory had a roughly $3.5 billion budget in 2018. About 70% of that comes from the Department of Energy, with the Department of Defense providing the remaining 30%.

Younger is a former weapons designer from the Lawrence Livermore National Laboratory in California, where he worked from 1982 to 1989. Before running Sandia, he was president of the former site-management contractor for the Nevada National Security Site, National Security Technologies. He also did stints at the Los Alamos National Laboratory in New Mexico and the Defense Threat Reduction Agency.