The contract, worth £288m, is for the decommission of all 10 Magnox-operated sites on behalf of the Nuclear Decommissioning Authority (NDA).
The NDA, a government body, has been charged with decommissioning and cleaning up civil nuclear sites in the UK.
The contract is expected to be worth up to £30m each year, and will run initially for five years with options to extend up to a further five years.
The project work, jointly undertaken by FTSE 250 firm Balfour and Aim-listed Costain, will involve looking at the design, construction and maintenance of permanent buildings and structures, infrastructure maintenance and extension works around civil engineering and ground works at the nuclear sites.
Costain has worked for Magnox, the management and operations contractor for 10 nuclear sites and one hydroelectric plant in the UK, in the past. Last June it worked with AMEC and Jacobs Engineering Group to process radioactive waste across all 10 Magnox sites in the UK.
Costain chief executive Andrew Wyllie said yesterday that the maintenance of the assets will form a “major part” of the work to make sure the Magnox sites are wound down carefully.
Balfour Beatty rose 3.72 per cent in trading yesterday, while Costain rose 0.51 per cent.