Eighty percent of the world's nuclear power plants are more than 20 years old, raising safety concerns, a draft UN report has said 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 UN agency's annual Nuclear Safety Review said. "There is a concern about the ability of the ageing nuclear fleet to fulfil these expectations."
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.
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 safety around the world remains high".
It cited steady improvements in terms of unplanned reactor shutdowns in recent years.
However, the 56-page IAEA document also highlighted an ageing nuclear fleet, with 80% of the 435 facilities more than two decades old at the end of last year.
About 70% 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.