Nuclear chiefs have admitted samples from a demolished Sellafield chimney stack were disposed of “in error” in the south of England.

An investigation has now been launched, though industry bosses have stressed that the mistake “posed no radiological risk to the public, the workforce, or the environment”.

Fifty cylindrical samples from the concrete base of a Reprocessing Plant Stack were taken to a lab more than 260 miles away.

The materials were dumped in the lab’s own designated landfill site after being wrongly categorised and using arrangements for uncontaminated materials.

However, according to the strict rules governing the industry the materials should have been returned to Sellafield for disposal following analysis.

David Moore, Copeland council’s nuclear portfolio-holder, described the error as “disappointing”.

He said: “But I know they try very hard to make sure that things like this don’t happen, and I am sure the Environment Agency will investigate.

“Fortunately there was no harm to the public, to the community or anyone who carried any of the work.

“This is about confidence in the industry, and we don’t want to see even the smallest of mistakes made.”

A Sellafield Ltd spokesman said: “During the successful demolition of the First Generation Reprocessing Plant Stack, we took some samples of its concrete support.

“These were sent to a laboratory for analysis using standard techniques in the civil engineering industry.

“This analysis informed the demolition process and helped us better understand the long-term performance of concrete of this age.

“These were sent from the Sellafield site, in error, using arrangements for materials that are not subject to radiological classification.

“We have conducted extensive research and analysis which confirms that these samples posed no radiological risk to the public, the workforce, or the environment.

“Following the discovery we issued a communication to the Sellafield workforce involved in releasing materials from site to reinforce our procedures and expectations. We also placed a temporary embargo on this activity.

“We informed our relevant partner agencies and our regulators. An investigation into the circumstances is ongoing.”

An Environment Agency spokeswoman confirmed that they were aware the incident and are now seeking further information from Sellafield.

“We will then consider what further action needs to be taken,” she added.

Reacting to news of the error, anti-nuclear campaigner Marianne Birkby of Radiation Free Lakeland, said this raised yet “more questions and concerns”.

She claimed the latest mistake is part of a wider issue, involving “large volume of nuclear materials now being dispersed to the wider environment” from the Sellafield site.

She said: “How much concrete has been dumped in ordinary landfill from this 1950s plutonium separation building? How radioactive is this concrete? “Where has it been dumped?

“Radiation Free Lakeland has vehemently opposed the ongoing reclassification or rather declassification of nuclear materials to be dumped in landfill, incinerated or in the case of radioactive metals ‘recycled’.

“We have argued that this so called ‘moving waste up the hierarchy’ would open novel routes for the ‘accidental’ dumping of materials.

“This dumping of radioactive material that is so called ‘free release’ is now happening with regularity.

“Sellafield have admitted once again that ’they got it wrong’. Recent assurances from Sellafield that no radioactive clothing will be sent to the newly-approved Energy Coast Laundry on the Lillyhall site 16 miles from Sellafield are not credible.

“We urge nuclear bosses, the councils and regulators to move the nuclear laundry back onto the Sellafield site.

“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.”