VIENNA - Significant progress was made last year in strengthening nuclear safety around the world, the UN atomic watchdog said in its annual review, despite the "challenge" posed by a large number of ageing reactors.

The International Atomic Energy Agency (IAEA) published the report shortly before a protest on Monday by Greenpeace activists targeting one of France's oldest nuclear power plants and demanding its closure.

The EDF's Tricastin plant was built more than 30 years ago - an age which it shares with many other atomic energy facilities, the IAEA survey showed.

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 web site 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 ongoing 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 United States, 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.