Ageing nuclear reactors pose a “challenge” according to the UN atomic watchdog’s annual review.
The International Atomic Energy Agency (IAEA) report said significant progress was made last year in strengthening nuclear safety around the world, but it said a large number of ageing reactors were a safety concern.
The report was released shortly before a protest by Greenpeace activists this morning demanding the closure of one of France’s oldest nuclear plants, EDF's Tricastin plant, which was built more than 30 years ago – an age it shares with many other atomic energy facilities, according to the IAEA survey.
Overall, however, the UN agency report was upbeat about developments, saying "the world nuclear community has made noteworthy progress in strengthening nuclear safety in 2012".
The report came more than two years after reactor meltdowns at Japan's Fukushima nuclear plant, triggered by an earthquake and tsunami in March 2011, unnerved the nuclear industry and raised questions over whether atomic energy was safe.
Since the Fukushima disaster, a large majority of IAEA member states with nuclear power plants have undertaken so-called safety stress tests, the UN agency said.
"Many have introduced additional safety measures," the report, posted during the weekend on the IAEA's website (http://www.iaea.org/About/Policy/GC/GC57/Documents/), said.
Operational safety remained high, it said, referring to performance indicator data on 437 nuclear power plants globally.
Of those, 162 have been in operation for more than 30 years and 22 for more than 40 years. Long-term operation and ageing of nuclear power plants "is an on-going challenge for regulators, operators and utilities", the IAEA said.
There are growing expectations that existing plants should meet enhanced safety objectives, closer to those of recent designs, it said, and many countries had already taken action to address ageing reactors.
For example, authorities in the USA, Switzerland and Canada have "published guidelines and ageing lessons learned to ensure that safety and performance remain within acceptable limits throughout the life cycle" of nuclear power plants.
But environmental activists are not convinced that the measures are enough. About 30 Greenpeace activists climbed fences to break into EDF’s Tricastin plant at dawn, saying they wanted to expose security flaws and demand its closure.
The activists, dressed in red, said they reached the walls of two reactors at the Tricastin plant, one of France's oldest. EDF denied they had got into any "sensitive areas" and said production was not affected.
Interior Minister Manuel Valls called for an investigation into the intrusion which raised questions about the security of France's 19 nuclear plants and 58 reactors.
The protesters unfurled a yellow and black banner on a wall above a picture of President Francois Hollande, marked with the words: "Tricastin, nuclear accident – President of the catastrophe?"
"With this action, Greenpeace is asking Francois Hollande to close the Tricastin plant, which is among the five most dangerous in France," Yannick Rousselet, in charge of nuclear issues for Greenpeace France, said in a statement.
"If being physically able to touch the reactors is not being in a sensitive place, I don't know what is," Rousselet told Reuters. "People with bad intentions could have posed a threat to the reactor's safety."
Most of Monday's protesters were arrested by 6.30am GMT but about eight of them were still clinging to metal structures and ladders, EDF said.
"So far, the incident did not have any impact on the facility's safety," France's nuclear safety agency ASN said.
The agency did not include Tricastin in a list released in April of six nuclear plants with the lowest safety performance in 2012.
The action echoed tensions between the Socialist government and ecologists, who accuse Hollande of not doing enough to reduce France's reliance on nuclear power and increase the use of renewable sources of energy.
Hollande sacked his energy and environment minister for publicly criticising cuts to her budget earlier this month.
The President has pledged to cut the share of nuclear energy in the country's electricity mix to 50 per cent from 75 per cent by 2025. He has also said he wants to close the country's oldest plant at Fessenheim, near the German border, by 2017.
Greenpeace said to honour his promise, Hollande would have to close at least 10 reactors by 2017 and 20 by 2020. The campaign group said this ought to include Tricastin, which was built more than 30 years ago.