The largest supplier of contract workers to Grand Gulf Nuclear Station’s recent upgrade is being acquired by a process technology firm in a $3 billion cash-and-stock deal.

Shaw Group Inc., based in Baton Rouge, and The Woodlands, Texas-based Chicago Bridge & Iron Co., will form one of the world’s most complete engineering and construction companies focused on the energy industry, according to a statement this week announcing the deal.

A combined company would employ about 50,000 people and have a backlog of more than $28 billion in jobs worldwide. Shaw’s operations will operate under the brand name CB&I Shaw after the deal closes in early 2013, subject to shareholder approval.

Under the deal, Shaw shareholders will receive $41 in cash and CB&I stock worth about $5 for each of their Shaw shares. Three separate legal inquiries are under way by separate securities law firms that center on whether the deal is adequate for Shaw shareholders.

Shaw, an engineering and energy services conglomerate, is one of three Fortune 500 companies in Louisiana and is a key player in the nuclear power industry. The company sent about 2,500 technical and engineering specialists to the Claiborne County plant earlier this year to refuel the boiling-water reactor and upgrade its core to generate 13 percent more power. Overall, more than 5,000 contract employees from 76 companies and 15 subcontractors worked on the $874 million project, completed in June.

CB&I’s landscape of operations spreads into hydrocarbon, water and nuclear industries, including storage tanks and nuclear containment vessels. Its U.S. base is in Texas; its worldwide headquarters is in The Hague.

Jim Bernhard Jr., Shaw’s chairman, president and CEO, is leaving the company after the deal closes.

“While Shaw has been growing in our business and has many opportunities ahead of us, we believe this transaction is in the best interest of … our employees and our customers,” Bernhard said.