Constellation Energy (NASDAQ:CEG) -10.1%, the biggest loser on the S&P 500, in early trading Monday after the Federal Energy Regulatory Commission on Friday rejected the amended PJM interconnection agreement for an Amazon (AMZN) data center connected directly to rival Talen Energy's (NASDAQ:TLN) Susquehanna nuclear power plant in Pennsylvania.

Microsoft's nuclear power partner Constellation Energy saw its stock drop on Monday as strong earnings couldn't overcome a ruling from the FERC late Friday that was pressuring nuclear stocks across the board.

Talen said it believed FERC had erred and was evaluating its options. The company can still restructure its expanded deal with Amazon, according to Oppenheimer analysts.