A lack of preparedness for a disaster and failures in the response to it exacerbated the effects of the nuclear accident at Japan's Fukushima plant in March, a new report says.
The government-commissioned study said plant operators and regulators had failed to adequately anticipate a huge tsunami and its likely impact.
The interim findings were issued by an independent panel set up in May.
More than 20,000 people were killed when an earthquake and tsunami struck.
Tens of thousands had to be evacuated as radiation leaked into the atmosphere, sea and food chain.
The six-reactor Fukushima Daiichi nuclear plant was badly damaged by the earthquake and tsunami, with blasts occurring at four reactors after the cooling systems went offline.
Last week, the authorities declared the plant had been stabilised, but said it would take decades to dismantle it completely.
Lack of precautions
The panel said its aim was not to apportion blame for the disaster, but to learn why the accident happened in the way it did, AFP news agency reported.
Tokyo Electric Power (Tepco), the company which operated the plant, did not "take precautionary measures in anticipation that a severe accident could be caused by (a) tsunami such as the one (that) hit... Neither did the regulatory authorities," the report said.
It also accused Tepco of failing to "incorporate measures against tsunamis exceeding the design basis. This indicates the limit of voluntary safety measures".
Further, the government's nuclear regulatory body "did not require Tepco to take specific measures, such as additional construction, after they received simulation results from Tepco in 2008 and early in 2011 regarding the impact of tsunamis on their facilities".
Tepco's own report on the disaster, by contrast, said there was no way it could have been prepared for a 9.0 magnitude quake and huge waves that triggered meltdowns and explosions at the plant.
The panel's report said the situation was also made worse by;
- delays in relaying information to the public
- managers' lack of knowledge of procedures to deal with emergencies
- poor communications - between the workers and the government, among the workers themselves, and between government bodies.
"Collection of accurate and most up-to-date information is a pre-requisite for timely and appropriate decision-making. This issue, together with the need for providing information to the nation, is of a major concern," it said.
It said Tepco staff at the plant were not trained to handle emergencies like the power shutdown that struck when the tsunami destroyed back-up generators, AP news agency reported.
Staff also misunderstood problems that arose with the cooling systems for reactors 1 and 3.
The 506-page report was based on interviews with more than 450 people, including government officials and plant workers.
The 12-member panel is headed by Yotaro Hatamura - an engineering professor at Tokyo University who specialises in the study of failures - and includes seismologists, former diplomats and judges.
It was set up in May by then prime minister Naoto Kan, and is expected to issue its final report in mid-2012.
In an earlier report, submitted to the UN nuclear watchdog, the Japanese government said it had been unprepared for a nuclear accident on the scale of the one at the Fukushima plant.