The IAEA also stressed the importance of "regulatory independence and clarity of roles". Japan’s nuclear watchdog Nuclear and Industrial Safety Agency is part of the ministry of trade and industry, which promotes atomic power.
The IAEA sent an 18-member team of its own experts and specialists from 12 countries, including the United States, China, Russia and South Korea, on a fact-finding mission to Japan.
It has given a preliminary report on the disaster at the Fukushima Daiichi nuclear plant to the Japanese government and will present a full report at a ministerial meeting in Vienna on 20-24 June.
The operator, Tokyo Electric Power Company (Tepco), has said it hopes to bring the plant to a stable state of "cold shutdown", with low pressure and temperatures, some time between October and January.
Map: Nuclear plants in Japan
The IAEA report on lessons learnt from the disaster said that nuclear plants should be designed to withstand "extreme external events, particularly those with common mode implications such as extreme floods".
The IAEA report summary also said that at nuclear plants "simple, effective, robust equipment should be available to restore essential safety functions in a timely way for severe accident conditions".
But the report praises Japan’s government, plant operators and agencies for being "extremely open in sharing information" and praised the country's initial response to the disaster.
"The response on the site by dedicated, determined and expert staff, under extremely arduous conditions has been exemplary and resulted in the best approach to securing safety given the exceptional circumstances," it said.
It also calls the longer-term response “ impressive and extremely well organised", advocating "a suitable and timely follow-up programme on public and worker exposures and health monitoring would be beneficial".