Two workers were injured after a steam leak during maintenance at Fessenheim nuclear power plant in Alsace, eastern France, according to power company EDF.
The accident, sparked by an involuntary chemical reaction, triggered a brief fire alert on Wednesday but was quickly brought under control, officials said.
French power utility EDF said there was a steam leak at the plant, but denied reports from the firefighters that a blaze had broken out.
A spokeswoman said: "No fire broke out. During a maintenance operation steam escaped which set off the fire alarm."
She added that two EDF workers "were slightly burned through their gloves".
Fifty firefighters were deployed to the plant after the alert.
Fessenheim, France's oldest nuclear power plant located on the border with Germany, has two reactors that went into service in the 1970s.
It is provisionally scheduled to close in 2017.