The JASON group of scientists has endorsed a plan to immediately begin building new infrastructure to produce fresh nuclear weapon cores, according to a letter written by the group’s program manager and published Monday by the Federation of American Scientists.

“[W]e urge that pit manufacturing be re-established as expeditiously as possible in parallel with the focused program to understand [plutonium] aging, to mitigate against potential risks posed by [plutonium] aging on the stockpile,” reads the letter signed to Tod Caldwell at National Nuclear Security Administration (NNSA) headquarters’ Defense Programs Office in Washington.

Gordon Long, director of the JASON program office at contractor MITRE Corp., McLean, Va., signed the letter, which is dated Nov. 23. The missive was an introduction to a classified report that the NNSA, the part of the Department of Energy responsible for nuclear weapons, chartered JASON to write in 2019.

Long added in the letter that “present assessments of [pit] aging do not indicate any impending issues for the stockpile.” Pits are the fissile cores that power the primary stages of nuclear weapons. In a 2007 report, the JASONs said that “most weapons system types in the stockpile have credible minimum lifetimes in excess of 100 years.”

Congress appropriated more than $700 million in fiscal year 2020 for the NNSA work on a two-state pit-production complex that the agency hopes will produce 80 pits annually by 2030.

The NNSA first plans to upgrade the PF-4 Plutonium Facility at the Los Alamos National Laboratory in New Mexico, which is supposed to produce 10 pits annually starting in 2024 and ramp up to 30 a year by 2036. Meanwhile, the agency plans in 2020 to finalize a design for converting the partially built Mixed Oxide Fuel Facility at the Savannah River Site in South Carolina into the Savannah River Plutonium Processing Facility: a planned factory the agency has tapped to cast 50 pits a year beginning in 2030.

Initial batches of pits at both sites will be for W87-1 warheads intended for the Ground Based Strategic Deterrent: a planned fleet of 400 silo-based, intercontinental ballistic missiles the Air Force wants to deploy starting in 2030 to replace aging Minuteman III missiles.

In April, the Pentagon announced it would no longer fund JASON’s independent investigations of defense science and technology, as the agency had done since the 1950s. After the Pentagon dropped the group, the NNSA continued funding three JASON studies, including the pit aging study. Congress has ordered the NNSA to use JASON for future studies on the civilian agency’s part of the ongoing round of nuclear modernization.