A UN nuclear safety team on a visit to the country has said that Japan underestimated the risk of a tsunami hitting a nuclear power plant.
However, the response to the nuclear disaster which followed the 11 March quake and tsunami was “exemplary”.
The International Atomic Energy Agency (IAEA) team also said a “hardened” emergency response centre was needed.
The UN team is studying how a number of meltdowns occurred at the Fukushima Daiichi plant on Japan’s Pacific coast.
The agency has compiled a report on the nuclear crisis which will be will be presented to an international meeting in Vienna on 20-24 June.
The team was led by Britain’s top nuclear safety official Mike Weightman and includes experts from France, Russia, China and the United States.
It pointed out the key failure, already admitted in Japan, to plan for the risk of waves crashing over the 5.7m (19 feet) wall and knocking out the plant’s back-up generators.
“The tsunami hazard for several sites was underestimated,” the UN team’s three-page summary report said.
“Nuclear plant designers and operators should appropriately evaluate and provide protection against the risks of all natural hazards,” it said.
Continued monitoring of the health and safety of the health of workers and the general public was necessary, it added.
The report also emphasised the importance of independent regulators in the nuclear industry.
“The planned road map for recovery of the stricken reactors is important and acknowledged. It will need modification as new circumstances are uncovered and may be assisted by international cooperation,” the summary said.
Goshi Hosono, an aide to Prime Minister Naoto Kan, accepted the report and said the government would need to review its nuclear regulatory framework.
The Fukushima nuclear plant, which was badly damaged by the tsunami, is still leaking radiation.
