Safety concerns have been raised over the handling of asbestos during the removal of nuclear waste on Anglesey.
The last nuclear reactor was switched off at Wylfa on the island in December 2015.
The Office for Nuclear Regulation (ONR) has served an enforcement notice on Magnox Ltd to improve its management of asbestos at the site.
There is no risk of exposure to the public. Magnox Ltd said it had already started improvements.
The company, charged with removing nuclear fuel from the decommissioned site, said there was no suggestion that any staff had been exposed to asbestos fibres.
It has until 28 July to make changes.
An inspection of the site, carried out by the ONR, found that while the company had committed to making improvements, action was needed to make sure legal standards were met for managing asbestos-containing materials.
Chief nuclear insp Dr Richard Savage said it had "no impact on nuclear safety".
"We do require improvements to ensure that any arrangements to manage the risk arising from the presence of asbestos are adequate and appropriate," he said.
A statement from Magnox said the Wylfa site contained "significant quantities of asbestos in a wide range of forms" and had an asbestos management plan, as required by law.
It reads: "Since this issue was first identified we have mobilised significant additional resources to ensure that it is dealt with as a priority.
"We are also reviewing the asbestos management plans across our business to ensure that they are all of the appropriate standard."