The UN atomic agency said Friday that Japan's Onagawa nuclear power plant, despite having been closest to the epicentre of the March 2011 earthquake and tsunami, was in surprisingly good shape.

The structural elements of the plant were "remarkably undamaged given the magnitude of ground motion experienced and the duration and size of this great earthquake," the International Atomic Energy Agency said in an initial report.

Onagawa "experienced very high levels of ground shaking -- among the strongest of any plant affected by the earthquake -- and some flooding from the tsunami that followed, but was able to shut down safely," the Vienna-based IAEA said.

The Fukushima plant, 120 kilometres (75 miles) to the south, was not so lucky, suffering multiple reactor meltdowns and releasing radioactivity into the environment in the world's worst nuclear accident in 25 years.

The IAEA report followed a two-week trip by 19 international experts that will be followed by further visits at Onagawa and reviews of other Japanese nuclear plants.

Findings will be added to an IAEA database being compiled by its International Seismic Safety Centre (ISSC) that was part of the watchdog's "action plan" on nuclear safety drawn up in the wake of the Fukushima disaster.

"The data we are collecting will make an important contribution to improving safety," said mission head Sujit Samaddar.

"Information in the database will allow IAEA member states to measure the performance of their nuclear power plants in the face of external hazards."