SANTA FE, N.M. — A national laboratory’s workers producing a shell for a triggering device for nuclear weapons violated safety rules in August by storing too much material at one location in a facility for plutonium, a highly radioactive material, a federal oversight panel reported.

The Defense Nuclear Facilities Safety Board memorandum called the Aug. 18 incident at the Los Alamos National Laboratory a “criticality safety event” and said workers there discovered the placement error made by a casting crew three days later when they moved the grapefruit-sized shell again.

The workers at that point failed to follow proper procedures for reporting the Aug. 21 action, the safety board said in a one-page memorandum dated Sept. 1. The report doesn’t specify whether the shell itself contained plutonium.

The Albuquerque Journal and Santa Fe New Mexican reported Saturday on the safety board’s memorandum.

Michael Golay, a professor of nuclear science and engineering at the Massachusetts Institute of Technology who has served on the U.S. Nuclear Regulatory Commission’s research review committee, told the New Mexican there are strict controls on the use of nuclear materials because putting plutonium or other materials too close together could cause a nuclear reaction.

Golay said he could not comment on the specific conditions at Los Alamos.

Los Alamos released a statement saying “there was no criticality accident” and that the lab “employs extremely aggressive limits on the quantities nuclear material in any given area, to assure there cannot be an unexpected criticality. The Laboratory takes criticality safety very seriously and is conducting a full fact finding.”

In physics, the term criticality refers to the point at which a nuclear reaction is self-sustaining.

Efforts to obtain comment from the National Nuclear Security Administration, the Department of Energy agency that oversees Los Alamos, were not immediately successful Saturday.

Los Alamos — the birthplace of the atomic bomb and still a premier nuclear research facility — is resuming production of the plutonium triggering devices, which are called cores and which haven’t been made since 2011. The Energy Department wants to ramp up production.

While Los Alamos officials have said the plutonium facility is operating safely and that improvements have been made in recent years, the oversight board earlier this year found that many of the safety systems in place at Los Alamos date to the 1970s and needed to be upgraded.

The oversight board’s memorandum on the Aug. 18 incident said it notably followed a recently completed readiness review “and is one of the few operations where the crew that underwent readiness has not experienced personnel turnover.”

Also, the memo said, “this was the first shell cast in the facility in about four years and the second time that a restarted operation encountered conduct of operations issues related to the criticality safety of material movements shortly after resuming nuclear work.”

The memorandum said Los Alamos managers briefed National Nuclear Safety Administration officials on the Aug. 18 incident and steps taken in response. Those included pausing casting operations, requiring use of a paper checklist when moving material, implementing new requirements for approvals and monitoring by senior officials, and studying “longer-term improvements to the material movement process.”