Sellafield Ltd has made improvements to fire safety at a building on site which was in the final stages of construction.
ONR had issued a Sellafield Ltd with a fire enforcement notice in July 2014 following a fire safety inspection.
ONR concluded that the inadequate means of escape in part of the building were in breach of the Regulatory Reform (Fire Safety) Order 2005.
Sellafield Ltd has since made improvements to escape arrangements within the building.
This work was completed six weeks in advance of the 1 May deadline.