Work has been scaled back on the site of former nuclear power station following an outbreak of Covid.
Magnox said a "small number" of employees tested positive at the Trawsfynydd site in Gwynedd, where they are working to remove nuclear reactors and towers.
Those affected and others found through track and trace are self-isolating.
It said some key work will continue on site but other staff can work from home.
"The site will maintain safety, security and environmental compliance activities and progress routine maintenance activities to ensure our decommissioning programme can resume quickly," a statement from Magnox said.
"Magnox will keep the situation under review, and determine when a resumption of those activities paused should restart with employees returning to site. We will monitor the health and wellbeing of the workforce during this period."
Trawsfynydd was shut down in 1991 after operating for a quarter of a century.
The twin reactors will become the first in the UK to be fully decommissioned.
Over 20 years the remaining reactor buildings will be demolished, while a new low-level radioactive waste store is built on the site to hold the material.