A cooling mechanism that’s part of a HEPA air filtering system was accidentally activated at Los Alamos National Laboratory’s plutonium facility in March, releasing 4,700 gallons of water that took crews weeks to clean up.

The spraying equipment initially flooded the system’s housing with water 1½ feet deep, resulting in about 300 gallons overflowing into the basement during the March 13 incident, according to a report by the Defense Nuclear Facilities Safety Board.

Crews finished cleaning up the water April 3, and the reason for the mishap has been identified, a lab spokeswoman wrote in an email.

“There was a short circuit in the activation system that was caused by adjacent upgrade work,” spokeswoman Laura Ann Mullane wrote.

No contaminants were found in the basement water, although personnel’s access to the wing of the facility was restricted while the water was being removed, the report said.

The safety board’s two subsequent reports after the water release described cleanup efforts continuing. In its latest report March 31, it said crews had begun siphoning water into a basement sump.

The board’s reports generally have a three-week lag after an incident.

The spray-down mechanism is designed to kick on if air going into a ventilation space exceeds a certain temperature — signaling a fire — to protect the HEPA filters, Mullane explained.

The lab’s operations center received a signal from the high-temperature detector when the spraying system activated, the March 17 report said. The Los Alamos Fire Department responded and found no evidence of a fire.

An anti-nuclear activist said although some safety incidents and problems can be expected at a large, complex facility where hazardous operations are conducted, this deluge is significantly worse than normal for the plutonium facility known as PF-4.

“This is not a run-of-the-mill incident but rather is connected to larger safety issues at PF-4 — namely the admitted obsolescence of the current fire alarm system,” wrote Greg Mello, executive director for the Los Alamos Study Group and a former safety inspector and hazardous materials incident commander for the state of New Mexico.

The National Nuclear Security Administration, which oversees the country’s nuclear arsenal, is spending a quarter-billion dollars to replace outdated fire systems, an undertaking that will take years to complete, Mello added.

Mello has argued the facility, built in the late 1970s, can’t feasibly be retrofitted to produce 30 nuclear bomb triggers, known as pits, per year by the end of 2026 — a goal set by the National Nuclear Security Administration, an Energy Department branch, to help modernize the country’s nuclear weapons stockpile.

The most pits the lab has made was 11 for Navy missiles more than a decade ago.

In this incident, the water being sprayed into the vent space was stopped by closing an outdoor valve, the report said.

Remediation included sampling, draining and processing the released water through the lab’s radioactive liquid waste treatment plant. It also involved replacing and testing new HEPA filters.

A similar incident occurred in 2019 with a water cooling mechanism protecting a ventilation system in a different facility. In 2021, the nuclear security agency’s field office approved disconnecting this cooling system based on computer modeling that showed diluting air was enough to keep the filters’ temperatures down during a fire.

Triad National Security, the lab’s primary contractor, might pursue a similar action at the plutonium facility, the report said.

Funding to upgrade the plutonium facility as it gears up to produce pits has climbed in the past several years.

The Energy Department’s proposed budget for the coming fiscal year calls for increasing the lab’s funding for plutonium modernization and operations to $1.76 billion, up from this year’s $1.55 billion and the $1 billion approved in 2022.