Part of a nuclear reprocessing plant chimney stack has been mistakenly disposed of using arrangements for uncontaminated materials.
Concrete samples from Sellafield in Cumbria were sent to a laboratory in the south of England for analysis.
They should have been returned to the plant but were sent to landfill.
Sellafield Ltd is investigating but said the mistake "posed no radiological risk to the public, the workforce, or the environment".
The samples were dumped in the laboratory's designated landfill site after being wrongly categorised, the Local Democracy Reporting Service said.
Sellafield said the material was disposed of "in error, using arrangements for materials that are not subject to radiological classification".
A temporary embargo on sending materials from Sellafield has been put in place and relevant staff have been reminded of "procedures and expectations", a spokesperson said.
David Moore, the nuclear portfolio-holder for Copeland Council, which encompasses Sellafield, said the error was "disappointing".
"This is about confidence in the industry and we don't want to see even the smallest of mistakes made," he said.
Marianne Birkby, from anti-nuclear campaign group Radiation Free Lakeland, said the mistake raised yet "more questions and concerns".
"We urge the regulators not to approve Sellafield's most recent wish list of deregulation which would see ever more dangerous materials leaving the site under new and more relaxed rules," she said.
The Environment Agency said it was seeking further information from Sellafield and would then consider what further action was needed.