The Los Alamos National Laboratory and the Nevada National Security Site are resetting the underground U1a facility at the former Nevada Test Site for the second in three planned Nightshade subcritical experiments.

Nightshade B is scheduled to fire in “spring 2021,” a spokesperson for the Los Alamos National Laboratory said Tuesday. The shot will follow the successful Nightshade A experiment fired on Nov. 3. That experiment, the first subcritical test in more than a year, went off “without issue,” the lab spokesperson said. Nightshade C, the series finale, will notionally follow by September 2021.

Subcritical experiments, carried out inside a thick metal confinement vessel, explosively compress pieces of plutonium to the point of a near criticality. The tests are intended to produce no nuclear yield and help weapons designers determine if crucial nuclear material is aging as expected. Together with computer simulations, the regimen allows the Department of Energy’s National Nuclear Security Administration to certify the destructive potency of existing nuclear weapons without resorting to underground nuclear detonations.

Nightshade got a late start. Once slated to begin in December 2019, the series’ first shot slid deep into the COVID-19 pandemic after difficulties in February 2019 with the final experiment in the previous subcritical series, Ediza. The Lawrence Livermore National Laboratory in California led that series.

The last Ediza shot breached a diagnostic port on the experiment’s confinement vessel and leaked contamination into the U1a zero room in Nevada. After that, personnel had to decontaminate the area and reinforce confinement vessels for future tests.

At one point, there were four Nightshade tests on the slate. The fourth, Nightshade-L, would have been run by Livermore, but the lab scrapped the shot after deciding Los Alamos’ Nightshade platform could not provide the desired data.