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.

Constellation (CEG) trades sharply lower despite reporting better than expected Q3 adjusted earnings, helped by higher power demand, and raising its full-year profit forecast.

Amazon (AMZN) had sought to hook the data center directly into the plant's reactor so it would not need to go through the traditional electricity grid, but nearby utilities objected to the plan, saying Amazon was siphoning power away from the grid and not helping pay for the infrastructure that keeps electricity flowing to regular consumers.

FERC ruled the Amazon (AMZN) plan should not move forward as proposed, but it will still allow Amazon to use 300 MW worth of power from the reactor, without an additional 180 MW as contained in the original proposal.

Talen (TLN) said it believes the amendment is just and reasonable and in the best interest of consumers, "but our direct-connect configuration is just one of several commercial solutions to the demand of large loads, and we are exploring other solutions as we move forward."

"The data center economy will require an all-of-the-above approach to satisfy the increased demand, including co-location such as Talen's arrangement with [Amazon], hybrids that co-locate primary power behind the meter while using grid power for back-up, and front-of-the-meter connections to utility transmission," the company said.

Talen (TLN) -3.1% but off earlier lows, while other nuclear-related names also are declining, including Vistra (VST) -3.2%, Public Service Enterprise Group (PEG) -4.8%, Cameco (CCJ) -2.3%, Nuscale Power (SMR) -4.1%, Oklo (OKLO) -5.6%, Nano Nuclear Energy (NNE) -7.5%, Centrus Energy (LEU) -18%.

FERC's rejection appeared to be more about technical issues than broader policy, UBS analysts said, believing "investors should focus on the strong fundamental backdrop... and see through the noise created by the FERC ruling."