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,

The stock's fall snapped a three-day winning streak.

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,