State-owned British Nuclear Fuels has agreed the sale of its power station construction arm to Japan's Toshiba.
BNFL chief executive Mr Mike Parker said the deal to sell US-based Westinghouse to Toshiba for £3.1 billion was an "excellent result" for Britain.
Toshiba faced stiff competition for Westinghouse, whose value has soared recently amid signs that governments around the world are preparing a return to nuclear power. In the summer, analysts thought Westinghouse could be worth around £1.13 billion.
Toshiba saw off competition from 13 other parties, including General Electric and Mitsubishi.
BNFL and the Government agreed to sell the business following a review of the company in 2003.
Mr Parker said Westinghouse was a "terrific business" but was secondary to BNFL's focus on nuclear decommissioning.
Nearly 50 per cent of nuclear power plants in operation worldwide are based on technology made by Westinghouse. It has 8,000 employees including 1,000 in the United Kingdom.
Prospect - a union representing 6,000 engineers, scientists and managers at Sellafield and 21 other sites - has attacked the disposal plan for robbing the United Kingdom of its chief reservoir of new-build expertise.