(UPDATE) According to Washington Post, workers were preparing a chemical solution to treat waste water when the accident happened. (H/T reader Atomfritz)
The cause of the accident is, according to the AFP article below, "oxygenated steam [which] had escaped after hydrogen peroxide reacted with water in a reservoir" during a maintenance operation.
(Now why are they pouring hydrogen peroxide in the reservoir? Killing the algae, perhaps?)
From AFP (9/5/2012):
Two slightly injured in accident at French nuclear plant
FESSENHEIM, France — A steam leak due to an accidental chemical reaction on Wednesday at France's oldest nuclear plant led to two people being slightly burnt and renewed calls to reduce the country's heavy reliance on atomic energy.
The accident occurred at the Fessenheim nuclear power plant in northeastern France within 1.5 kilometres (one mile) of the border with Germany and about 40 kilometres (25 miles) from Switzerland.
"It was not a fire," the local prefecture said, adding that oxygenated steam had escaped after hydrogen peroxide reacted with water in a reservoir.
About 50 firefighters were deployed, an official from the service said.
French power supplier EDF said "two people were slightly burnt through their gloves."
"It was a problem that cropped up during a maintenance operation," in an "auxiliary building in the nuclear complex but not in the building housing the reactor," the electricity giant said.
France is the world's most nuclear-dependent country, operates 58 reactors and has been a leading international proponent of atomic energy.
But in a deal with the Greens before this year's parliamentary and presidential elections, President Francois Hollande's Socialist party promised to reduce reliance on nuclear energy from more than 75 percent to 50 percent by shutting 24 reactors by 2025.
France's reliance on nuclear power has been increasingly called into question since the Fukushima disaster in Japan, which prompted Germany to announce plans to shut all of its reactors by the end of 2022.
Jean-Luc Cardoso, an official with the CGT union at the Fessenheim plant, said: "There was no fire, no death and two colleagues were slightly injured."
France's ecology ministry said there was no safety threat. Ecology Minister Delphine Batho termed it a "workplace accident" and promised that "a complete report on this incident will be made public."
On stream since 1977, Fessenheim has two water reactors. It is built along a huge canal and draws water for cooling from the Rhine river.
Due to its location, it is considered vulnerable to seismic activity and flooding and is provisionally scheduled to close in 2017.
After Wednesday's scare, former Green presidential candidate Noel Mamere said: "This incident proves that we must close Fessenheim as soon as possible," adding that it would be better to spend "billions of euros" on developing renewable energy.
Greenpeace and French environmental group Sortir du Nucleaire (Phase out the Nuclear Age) also mirrored the call.
Fessenheim is France's oldest nuclear power plant, which came on line on January 1, 1978 according to Wiki.
President Francois Hollande of France was expected to announce the closure of the plant once he was elected. (Has he done that yet?)