S&P 500 nuclear stock Constellation Energy fell in afternoon trade on Wednesday after a report emerged that it's close to finalizing a deal for Calpine, a natural gas and geothermal power generator. Constellation Energy is in discussions with Calpine's private-equity owners about a deal that could value the company at about $30 billion including debt,

Shares of Constellation Energy Corp. CEG slid 4.61% to $243.84 Wednesday, on what proved to be an all-around great trading session for the stock market, with the S&P 500 Index SPX rising 0.16% to 5,918.

Constellation Energy shares fell roughly 5% after reports it is in advanced talks to acquire closely held Calpine in a cash-and-stock deal. Shares in Constellation, the largest U.S. owner of nuclear-power plants,