Two of the three shots in the Lawrence Livermore National Laboratory’s Nimble series of subcritical experiments should fire by early fiscal year 2024, with the third and final to follow not more than two years after, a lab spokesperson said this week.
“It is anticipated that two shots, Twin Peaks and Nob Hill, will be executed in late [fiscal year] FY[20]23 and early [fiscal year] FY[20]24, respectively,” the Livermore spokesperson wrote Wednesday in an email. The third shot, Mission Hills, was on the slate for fiscal year 2026, which begins Oct. 1, 2025.
The Nimble series was supposed to fire its first shot in spring 2022, but that didn’t happen because of “various delays,” the Livermore spokesperson wrote.
Those delays piled up with the Nevada National Security Site’s U1a complex, home of the National Nuclear Security Administration’s (NNSA) underground subcritical test laboratory, in cold standby for much of the year, according to the federal Defense Nuclear Facilities Safety Board.
U1a is the underground lab for conducting subcritical experiments, in which nuclear-weapon stewards blow up small pieces of fissile materials inside of thick steel spheres. High-speed x-ray cameras observe the explosive compression of the materials, which creates no nuclear yield, the U.S. says. This allows the NNSA to certify the effectiveness of existing nuclear weapons without nuclear-explosive tests.
At the moment, however, subcritical experiment packages are not even allowed to be transported down into U1a using the complex’s usual hoist. That’s in part due to a very active desert monsoon season in August and September. The prohibition on hoist use, put in place on Sept. 22, should be lifted on March 30, a Nevada site spokesperson told the Exchange Monitor this week.
After the summer rains, Nevada site personnel discovered that muck had accumulated at the bottom of the entrance shaft to the U1a complex. The muck at one point had risen to such a height in the U1a hoist sump that site personnel deemed it unsafe to transport subcritical experiment packages using the hoist.
With muck piled high, there might not be enough stopping distance to prevent damage to a subcritical experiment package riding the hoist down to the underground, according to a Defense Nuclear Facilities Safety Board report about the issue.
