LAING O'Rourke is tipped to win the second part of a £1.3 billion decommissioning project at the Trawsfynydd power station in north Wales.
The firm is set to see off competition from Norwest Holst, Balfour Beatty and Nuttall to secure the £20 million deal for the British Nuclear Group.
Work will involve building an intermediate level waste storage facility on the site to store new waste from the process.
Preparations for the care and maintenance began in 1993 when the station stopped operating. Since then much work has been done to begin the decommissioning process, clearing site hazards and processing legacy wastes, leaving only three buildings for the care and maintenance phase.
Trawsfynydd is set to become a greenfield site, initially cleared by 2088 according to Nuclear Decommissioning Authority guidelines, and will be returned to the local community at the end of the century.
The station is on the edge of the Snowdonia National Park so work has to be done in such a way as to protect the landscape.
Maintenance and care work is expected to last 80 years after decommissioning.
The contract marks the beginning stages of Britain's massive nuclear clean-up, estimated to be worth £56 billion.
Last month the British Nuclear Group continued the large scale clean-up work by tendering for contractors to work on sites across the UK.
The contract for the Trawsfynydd site is due to be awarded in the next few weeks.