An employee at the Lethabo Power Station in the Free State died and another was seriously injured in a "major incident" on Wednesday afternoon, Eskom said.
The power utility said in a statement that police officials were on the scene at Unit 5 and emergency response teams had been activated.
"The Department of Labour has been notified and Eskom executives are travelling to the site... As a result of the rupture of a mainsteam pipe, the unit is not operational. Operations continue on the other units."
The employee's next of kin were being informed.
"Our condolences go out to the family and friends and we pray for the recovery of our injured colleague."
Energy expert Chris Yelland tweeted that there had been a boiler explosion.