Eighty percent of nuclear power plants are more than 20 years old, raising safety concerns, the UN atomic agency warned in a draft report seen by AFP on Tuesday, a year after Japan's Fukushima disaster.

This "could impact safety and their ability to meet member states' energy requirements in an economical and efficient manner," the International Atomic Energy Agency's draft annual Nuclear Safety Review said.

Countries opting for what it called "long term operation (LTO) must thoroughly analyse the safety aspects related to the ageing of irreplaceable' key components," said the report, due to finalised and published in mid-2012.

The IAEA, which promotes the peaceful use of nuclear technology, said that five percent of the world's 435 nuclear facilities have been in operation for more than 40 years and 32 percent for more than 30 years.

It said that there were "growing expectations that older nuclear reactors should meet enhanced safety objectives, closer to that of recent or future reactor designs."

"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," it said.

It also said that 70 percent of the world's 254 research reactors -- for producing medical isotopes and other uses -- have been in operation for more than 30 years, many of them "exceeding their original design life."

This has raised "serious concerns" amongst research reactor operators, regulators and the public, it said.

IAEA chief Yukiya Amano said last week that measures put into place since the Fukushima disaster in March 2011, caused by a massive earthquake and tsunami but also by "human and managerial failings," had made nuclear power safer.

Amano said that "good progress" has been made implementing the IAEA's nuclear safety action plan, involving stress tests on nuclear power plants, peer reviews and the strengthening of defences against natural disasters.