Valued at anywhere from £800m to £1.4bn, it is Mace’s first major deal on the nuclear waste clean-up programme.

The massive Silos Direct-encapsulation Plant is a bespoke, mechanical handling plant and building that will be used for the Magnox clear up programme.

The project was retendered after key contractors Vinci and Nuvia pulled around 450 staff off the job two years ago when the contract was halted after significant cost overruns.

The large and technically complex treatment plant will handle radioactive liquid waste, which will be mixed with cementitious material to allow it to be stored in containers within existing Sellafield stores.

It is the only project of its kind in the world and is expected to mobilise several hundred people as well as engaging the local supply chain as much as possible.

Sellafield Ltd will now enter a period of preferred bidder clarification and confirmation of commitment with the a.m.a joint venture while the contract is finalised and all legal processes are completed.

Scott Reeder, Sellafield’s chief projects officer, said: “The market recognised the challenges presented by this important and complex project and two strong joint venture organisations emerged to tender for the contract.”

The team will take the project through detailed design, construction, manufacture, procurement, installation, and inactive commissioning.