The explosion occurred on Thursday at 10am in the engine room of the Flamanville nuclear power plant on the Normandy coast.
Authorities said the explosion took place outside the plant's nuclear zone and posed no risk of contamination.
The cause of the explosion at the power station, which has been in operation since the 1980s, was not immediately known.
"It is a significant technical event but it is not a nuclear accident," senior local official Olivier Marmion told AFP.
Marmion said five people had reported feeling unwell but that there were no serious injuries, adding that rescue services were at the site.
One of the plant's two reactors was shut down after the explosion and the incident was declared over at 11am, the authorities said.
An emergency intervention plan, designed to protect local residents in case of a leak, was not triggered.
The plant "did not explode in a fireball" as was falsely reported by some British tabloids.
Flamanville was supposed to be France's flagship nuclear plant but a project to build a new EPR reactor on the site has been beset with problems in recent years leading to it becoming a costly calamity.
After being beset by hold-ups and snags - the project to build a new state of the art nuclear reactor, France is getting a reputation for all the wrong reasons and London has been left looking on nervously.
France's neighbours including Germany, Luxembourg and Switzerland have all expressed concerns in recent months about the dangers linked to some of France's older nuclear plants.
And in April last year a worrying report was released that concluded France was dangerously ill-prepared for a nuclear accident.
“France is not ready to face a serious nuclear accident,” warned Jean-Claude Delalonde, the president of the ANCCLI commission which reports on the state of the country's nuclear facilities.
“Even though a national response plan was made public in February 2014, nothing has been put in place,” he said.
Flamanville – A timeline of costly setbacks
- 2005 – The green light is given for construction to begin on a new third-generation EPR (European Pressurized Reactor) at the Flamanville nuclear power plant. The overall cost of construction is estimated at €3.3 billion.
- 2007 – Construction on the new reactor begins in December 2007.
- 2011 - A construction worker on the new EPR reactor dies after an 18-metre fall as the footbridge he was walking on was hit by a crane.
- 2012 – In the year that the reactor is due to begin operating, EDF announced that costs have in fact risen to €8.5 billion and the completion will be delayed until 2016.
- 2014 – In November EDF says construction was delayed again until 2017, due to delays in component delivery by Areva.
- April 2015 – Areva informs French nuclear safety regulator ASN that anomalies have been detected in the reactor vessel steel, meaning a delay of at least another year.
- October 2015 – Results of further tests on the vessel are set to be handed to authorities. The future of the whole project could hinge on those results.