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The blaze at the Tricastin plant in Drôme in the Rhône valley sent a thick cloud of black smoke into the sky. A mistral wind sent it south over a nearby motorway on one of the busiest travel days of the year as the French left for their summer holidays.
EDF, which runs the power station, said the incident took place in an electric transformer situated in the non-nuclear part of the plant and had not resulted in any radiation leak or any other contamination.
A statement issued by the French energy giant raised further concerns as it omitted to mention the explosion – only a fire – and did not give the cause of the blaze.
"This event happened in the non-nuclear part of the installation and had no radiological consequence on the environment and the population. The fire brigade was immediately called and the fire was rapidly brought under control. Nobody was hurt," it said.
EDF added that the plant's number one reactor was not in operation at the time of the fire, having been "closed for its annual maintenance". Police confirmed there was no environmental contamination.
On Thursday France's nuclear safety authority, the Autorité de Sûreté Nucléaire (ASN), demanded 32 safety measures at the Tricastin number one reactor, a 900MW water pressurised reactor built in 1974 and put into operation in 1980.
In 2007, an ASN report had concluded that "the site must make improvements in management and training" and criticised the plant's procedure for dealing with fires as "taking too long". The following year uranium leaked from the number two reactor at Tricastin during a cleaning operation and contaminated local rivers.
Swimming and fishing were banned as a precaution. Although one of the oldest reactors in use in France, the ASN agreed last year that Tricastin's working life could be extended for another decade.
The safety measures revealed last week include greater protection against fire, flooding and earthquakes, including improvements to the methods of cooling of the nuclear fuel rods in order to lessen the risk of an explosion of hydrogen at the heart of the reactor. The ASN said the faults in the reactor were "known and under surveillance" and ordered that the new safety measures should be completed before December 2014.
A week ago the French president, Nicolas Sarkozy, bucked the anti-nuclear trend sweeping Europe since the Fukushima disaster by pledging €1bn of investment in atomic power as well as "substantial resources" to strengthen research into nuclear safety.
France has 58 nuclear reactors, 34 of which are reactors of the type at Tricastin and have an average age of 28 years. About 74% of the country's electricity comes from nuclear sources and it is the world's largest net exporter of nuclear energy.
Sarkozy said France was known to be "considerably ahead" of other countries in terms of atomic power technology and safety. "Our power stations are more expensive because they are safer," he said last week.
After the Fukushima nuclear accidents in March, caused by a combination of earthquake and tsunami, the French prime minister, François Fillon, asked the ASN to carry out an "open and transparent" audit of the country's nuclear installations, examining the risks of flood and earthquake damage, loss of power and cooling, as well as the emergency accident procedure, to examine if any improvements could be made. Its conclusions are expected in September.
Corinne Morel-Darleux, a local member of the Parti de Gauche, said: "It was not a nuclear accident but it's an incident that was seen and raises questions about the security of this plant."
Kim Willsher for the Guardian