Paris: A French nuclear watchdog on Tuesday called for the country's plants to beef up safety to cope with natural disasters under a programme likely to cost tens of billions of euros (dollars), but said none faced any immediate shutdown.

It also called for a "rapid reaction force" to be operational by the end of 2014 that could intervene in a nuclear accident in less than 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 operators until June 30 to spell out measures to strengthen safety in response to floods and earthquakes, providing for instance backup systems for power, coolant and plant operations.

The measures should aim at "preventing a serious accident or limiting its spread" and "limiting massive releases (of radioactivity) in an accident scenario," the ASN said.

In addition, operators should itemise their procedures for handling a crisis.

The measures will require "tens of billions of euros (dollars) 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. Another major expense would be building "bunkers" to serve as emergency backup for plant controllers.

"I don't see how this cannot have an impact on
(electricity) prices," he warned.

Fillon's office said the government would ensure that the requests would be carried out "in their entirety (and) on time."

An anti-nuclear group, the Nuclear Observatory, dismissed the ASN report as a "predictable trick."

"The ASN is absolutely not independent and plays the role allotted to it by the government -- to have the public believe in the feasibility of nuclear safety," it said.

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.