Costain has been awarded a decommissioning framework contract by Magnox South at Bradwell nuclear power station on the Essex coast.
The deal is worth £75m over 10 years with possible extensions.
The contract is to design plant and equipment, and to procure, construct, operate and refurbish plant prior to transfer to further sites.
The work forms part of the station's accelerated decommissioning programme, and centres on the delivery of Fuel Element Debris (FED) dissolution.
Magnox South sites at the end of their operational life are undergoing decommissioning, and Costain's work will remediate and dispose of FED produced during the generating life of the power stations.
The client will shortly be awarding contracts for similar programmes at its other sites, Hinkley Point, Sizewell, and Oldbury.
Andrew Wyllie, Costain chief executive, said: "It is testimony to Costain's growing reputation within the nuclear industry as one of the few major contractors in the UK with the capability to carry out such complex and critical work."