It also called for a "rapid reaction force" to be operational by the end of 2014, with the capacity to intervene in a nuclear accident within 24 hours.
The recommendations, handed to Prime Minister Francois Fillon, were drafted by the Nuclear Safety Authority (ASN) as part of a post-Fukushima inspection of France's nuclear industry.
"Following additional safety evaluations of priority nuclear installations, the ASN believes that the installations that have been assessed have a sufficient level of safety to warrant it not to request any immediate shutdown," it said.
"At the same time, the ASN believes that continuing operations require existing safety margins to be strengthened as swiftly as possible."
It gave plant operators six months in which to itemise work to upgrade safety in response to "extreme situations" such as floods and earthquakes, and also to outline clearly procedures for handling any crisis.
The measures will require "tens of billions of euros in investment," the ASN's president, Andre-Claude Lacoste, told a press conference.
"The work and financing are on a massive scale, requiring the hiring and training of hundreds of people," Lacoste said.
He noted that a single emergency diesel generator, designed to be protected against floods, costs "tens of millions" of euros.
French nuclear power plants map
France is the most nuclear-dependent country in the world, deriving 75 percent of its electricity needs from 58 reactors, most of which were built in response to the oil shocks of the 1970s.
The programme led to the development of a massive state industry, with giants such as
the nuclear plant builder Areva and operator Electricite de France (EdF) as well as the Atomic Energy Commission (CEA), which carries out civilian and military research.
But a decades-long consensus among all the major parties in favour of nuclear power was shaken by Japan's Fukushima disaster and the issue is rising up the political agenda ahead of key elections this year.
In November, the opposition Socialist Party joined with the Greens to campaign for France to scale back its reliance on nuclear power to 50 percent by 2025 by shutting 24 reactors and boosting production from wind, solar and other renewable sources.
French President Nicolas Sarkozy, like his predecessors, is a champion of nuclear power. He condemned the Socialist-Greens proposal as "irresponsible" and potentially crippling.