A total of £7.8 million was paid at the close of the deal with the outstanding amount to be paid over the next three years dependent on performance targets being met.
The purchase follows Amec’s strategy to expand its power and process division.
Amec chief executive Samir Brikho said: “AllDeco will strengthen our capabilities and improve our standing as a leading player in nuclear decommissioning and reactor services in the UK, Europe and around the world.
"This acquisition will further strengthen our strong position in the nuclear sector, having been recently named as a member of the consortium appointed preferred bidder in the Sellafield competition.”
AllDeco has principal locations in Slovakia and the Czech Republic and employs 112 nuclear engineers, designers, chemists and operators.