The order to Express Reinforcements for the Hinkley Point C project in Somerset is understood to be worth about £100m.
It was placed by the Bouygues and Laing O'Rourke, the main contractors.
Reinforced steel will be supplied from the firm's Neath and Newport manufacturing centres using rod supplied by Celsa Steel in Cardiff.
EDF Energy, which plans to build the power station, said: "In February this year, said that Hinkley Point C would be a significant opportunity for UK steel.
"The company has helped steel producers understand the requirements needed for nuclear construction so that a large proportion of its requirement could come from UK companies, subject to a competitive process."
The multi-billion pound project is on hold while the government reviews the details.
Prime Minister Theresa May is expected to make an announcement on Hinkley later in September.
A Welsh firm has been named as the preferred bidder for a 200,000-tonne order of steel for the UK's first new nuclear power plant in decades.