Older nuclear plants pose a safety challenge, a draft UN report says following the anniversary of the Fukushima disaster.
Eighty per cent of the world's nuclear power plants are more than 20 years old, said the report by the International Atomic Energy Agency (IAEA).
Many operators have begun programmes, or expressed their intention, to run reactors beyond their planned design lifetimes, and the IAEA raised concern over whether the plants could meet safety objectives.
"There are growing expectations that older nuclear reactors should meet enhanced safety objectives, closer to that of recent or future reactor designs," said the U.N. agency's annual Nuclear Safety Review, which has not yet been made public.
"There is a concern about the ability of the ageing nuclear fleet to fulfil these expectations."
The Fukushima crisis in Japan was triggered on March 11, 2011, when an earthquake unleashed a tsunami that left 19,000 people dead or missing.
It also smashed into the coastal power plant causing a series of catastrophic failures at the facility.
Images of the stricen plant shook public confidence in nuclear power and forced the nuclear industry to launch a campaign to defend its safety record.
IAEA Director General Yukiya Amano said last week that nuclear power is now safer than it was a year ago.
The report said the "operational level of NPP (nuclear power plant) safety around the world remains high".
It cited steady improvements in terms of unplanned reactor shutdowns in recent years.
But the 56-page IAEA document also highlighted an ageing nuclear fleet, with eighty percent of the 435 facilities more than two decades old at the end of last year.
This "could impact safety and their ability to meet member states' energy requirements in an economical and efficient manner", said the report, which has been submitted to IAEA member states but not yet finalised.
Operators and regulators opting for so-called long term operation "must thoroughly analyse the safety aspects related to the ageing of 'irreplaceable' key components", it added.
About 70 per cent of the world's 254 research reactors have been in operation for more than 30 years "with many of them exceeding their original design life," it said.
The document was debated by the IAEA's 35-nation governing board last week, almost exactly a year after the world's worst nuclear accident in 25 years.
Last year's tsunami overwhelmed Fukushima on Japan's northeast coast, knocking out critical power supplies that resulted in a nuclear meltdown and the release of radiation.
The reactors were stabilised by December, but high radiation levels hamper a cleanup that is expected to take decades.
The crisis sparked a rethink about nuclear power and countries such as Germany, Italy and Switzerland have decided to phase out their reactors.
But other states, for example fast-growing China and India, continue to look to nuclear energy to meet their growing energy needs, the IAEA report said, adding that some "are even accelerating their nuclear energy programmes".
France is building its first "advanced" reactor and Russia is seeking to double its nuclear energy output by 2020, it said.
"All countries that are using nuclear power are much more serious about nuclear safety," Amano said last week.
But environmental group Greenpeace said no "real lessons" appeared to have been learnt from Fukushima.