The contract, worth about $5m, involves improvements to the steel and concrete shelter covering the damaged number 4 reactor at Chernobyl, to prevent highly radioactive dust and water escaping.
AEA Technology has won nuclear contracts worth about pounds 20m, and a further pounds 11m contract for clean-up work at Chernobyl is due to be announced.
The company, which was floated off from the Atomic Energy Authority two years ago, has a 25 per cent stake in the Chernobyl consortium. Its partners are SGN of France and JGC of Japan. The contract is one of the first major orders to be placed since the Ukraine government and the G7 countries agreed on a clean-up programme for the stricken plant.
It is being funded through the Chernobyl Shelter Fund, which is administered by the European Bank for Reconstruction and Development on behalf of the G7.
The work also involves drawing up an emergency plan in the event of an accident occurring inside or near to the shelter.
The order is one of a burgeoning number of international nuclear clean- up contracts won by AEA Technology since its pounds 228m privatisation in September 1996.
The shares were sold at 280p and are now worth more than 800p.