An estimated 1,200 tonnes of concrete is needed to construct the heavy duty floor slab.
Dounreay awarded Graham Construction a £22m contract in 2018 to construct the intermediate level waste facility, where drums of waste will be stored. It is scheduled to be finished by the end of 2021.
The project is part of the decommissioning of the site that is being delivered by Dounreay Site Restoration Limited (DSRL), a company owned by Cavendish Dounreay Partnership, on behalf of the Nuclear Decommissioning Authority.
Graham won the contract having previously built the £14m low level waste store on the site.
Dounreay project manager Dave Busby said: “It’s great to see the progress of the floor slab, which is now 25% complete slightly ahead of schedule.”