The loose piece of bolt, about 4 inches by half an inch, was discovered in one of 43 freshly manufactured canisters that featured a new design aimed at improving storage capability. The bolt was part of a redesigned system called a shim that aims to improve the balance and storage of the spent fuel assemblies.

The work ceased on March 5 after the discovery and later resumed, the company said, using 30 other canisters that do not include the new design.

Edison, which already filled four of the newly designed canisters with radioactive waste, is unable to check whether those casks have the same flaw. The utility said the unattached part was found while a fifth canister was being filled with spent fuel assemblies.

"Safety is our top priority," Tom Palmisano, the Edison vice president and chief nuclear officer, said in a statement. "Our first step was to confirm this fabrication change poses no safety risk to workers or the public."

Edison acknowledged the mishap at a meeting Thursday night of the community engagement panel, a utility-funded group monitoring the plant decommissioning. San Onofre was shut down after a small amount of radiation leaked in 2012 and was closed for good the following year.

"We also directed the manufacturer to conduct extensive evaluations to ensure we have a comprehensive understanding of this change," Palmisano said. "Additionally, we briefed the U.S. Nuclear Regulatory Commission and have kept the regulator apprised of our actions."

Several people at the Thursday night meeting were alarmed by the announcement that one of the canisters malfunctioned even before it was buried in the ground. On Friday, activists and lawyers suing over plans to bury the waste on site were calling on Edison to stop transferring the spent fuel into dry storage.

"We warned them that this was going to happen, and nobody listened to us," said Donna Gilmore, an Edison critic who runs a group called SanOnofreSafety.org. "Now they are trying to tell us: 'Everything is OK. Don't worry.' This is insane. Edison has proven they can't keep us safe."

The plant, which has been in the decommissioning process since 2013, generated some 3.6 million pounds of spent nuclear fuel that remains on the property just north of Oceanside.

Current plans call for Edison to transfer the fuel assemblies from cooling pools on the grounds of the abandoned power plant into steel-lined canisters that will be placed in a concrete encasement just over 100 feet from the shoreline.

Gilmore and others worry that the canisters could leak or become weakened by saltwater intrusion or other factors, threatening the 8 million people who live within 50 miles of the plant. Utility executives say the storage is safe.

The transfer process, approved by state and federal regulators, has been underway since January even though Edison agreed last summer to settle a lawsuit by exploring ways to move the nuclear waste away from the beach.

Earlier this month, utility officials and plaintiffs in the San Diego County Superior Court lawsuit agreed to set up an independent panel of nuclear experts to debate alternatives to storing the spent fuel at San Onofre.

Potentially, the waste could be moved to a more permanent storage facility under development in New Mexico or to the grounds of another nuclear plant in Arizona co-owned by Edison.

The canisters being used at San Onofre were designed and built by Holtec International of New Jersey. The company apparently altered the design midway through the fabrication process. Forty-three of the 73 canisters feature an amended shim-basket element, which is designed to support spent fuel assemblies for improved storage and transport.

Holtec issued a statement Friday saying that a loose bolt was found broken off at the bottom of a canister. The company is investigating what happened and checking if other canisters might be vulnerable to the same weakness that caused the bolt to break.

"Analyses demonstrate that the separation of the bolt does not impact the safety of the basket or the performance of the dry storage system in storage or transportation," Holtec project manager Fred Bidrawn wrote.

Edison said it will continue the fuel-transfer project using the balance of canisters that do not feature the altered shims.

"The remaining canisters with the new design are on hold until Holtec completes its root cause evaluation," Edison said.