Eighty percent of the world's nuclear power plants are more than 20 years old, which could impact safety, a draft U.N. report says a year after Japan's Fukushima disaster.
Many operators have begun programmes, or expressed their intention, to run reactors beyond their planned design lifetimes, said the International Atomic Energy Agency (IAEA) document which has not yet been made public.
"There are growing expectations that older nuclear reactors should meet enhanced safety objectives, closer to that of recent or future reactor designs," the Vienna-based U.N. agency's annual Nuclear Safety Review said.
"There is a concern about the ability of the ageing nuclear fleet to fulfil these expectations and to continue to economically and efficiently support member states' energy requirements."
The Fukushima tragedy 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 stricken 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 told Reuters 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 ageing nuclear plants, 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 percent 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.
The 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.
After the accident, Germany, Switzerland and Belgium decided to move away from nuclear power altogether to grow reliance on renewable energy instead.
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. (Editing by Robert Woodward and Alessandra Rizzo)
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