The International Atomic Energy Agency (IAEA) has said Japan underestimated the risk of a tsunami hitting a nuclear power plant.
However, the response to the nuclear crisis that followed the 11 March quake and tsunami was "exemplary", it said, reports the BBC. The IAEA's draft report also said a "hardened" emergency response centre was needed to deal with accidents.
The Fukushima Daiichi nuclear plant, which was badly damaged by the tsunami, is still leaking radiation. The IAEA inspectors spent a week in Japan compiling their report on the Fukushima nuclear crisis, a summary of which has been handed to the Japanese government.
A full report will be submitted at an inter-governmental meeting in Vienna later this month, intended to improve nuclear safety around the world. The UN team was led by Britain's top nuclear safety official Mike Weightman and includes experts from France, Russia, China and the United States.
The inspectors pointed out a key failure, already admitted by Japan, to plan for the risk of waves crashing over the sea wall and knocking out the plant's back-up generators.
Even though a major faultline lies just offshore, the sea wall at Fukushima was less than 6m (20ft) high. The height of the tsunami wave was about 14m. "The tsunami hazard for several sites was underestimated," the UN team's three-page preliminary report said.
"Nuclear plant designers and operators should appropriately evaluate and provide protection against the risks of all natural hazards," it said. Amid concerns about safety, just 17 of the country's 54 reactors are operating.
Continued monitoring of the health and safety of the nuclear workers and the general public was necessary, it added.
The report also emphasized the importance of independent regulators in the nuclear industry. In Japan there has been criticism because the Nuclear Safety Agency is part of the industry ministry, which promotes nuclear power.
"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 co-operation," the IAEA report 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.