An aerial image of reactors 3 and 4 at Fukushima Daiichi nuclear power station as decommissioning work continues. Photograph: The Asahi Shimbun/Getty
The catastrophic triple meltdown at the Fukushima Daiichi nuclear plant in March 2011 was "a warning to the world" about the hazards of nuclear power and contained lessons for the British government as it plans a new generation of nuclear power stations, the man with overall responsibility for the operation in Japan has told the Guardian.
Speaking at his Tokyo corporate headquarters , Naomi Hirose, president of the Tokyo Electric Power Company (Tepco), which runs the stricken Fukushima plant, said Britain's nuclear managers "should be prepared for the worst" in order to avoid repeating Japan's traumatic experience. "We tried to persuade people that nuclear power is 100% safe. That was easy for both sides. Our side explains how safe nuclear power is. The other side is the people who listen and for them it is easy to hear OK, it's safe, sure, why not?
"But we have to explain, no matter how small a possibility, what if this [safety] barrier is broken? We have to prepare a plan if something happens … It is easy to say this is almost perfect so we don't have to worry about it. But we have to keep thinking: what if …"
Tepco's Fukushima Daiichi facility on the coast about 124 miles (200km) north-east of Tokyo, comprising six nuclear reactors, was hit by a giant tsunami with waves peaking at 17 metres high caused by the Great East Japan earthquake on 11 March 2011. In what quickly became one of the world's worst nuclear disasters, operators lost control of the plant when the power supply, including emergency back-up, failed amid massive flooding. As cooling systems malfunctioned, reactors 1, 2 and 3 suffered meltdowns.
Reactor 4 was closed for routine maintenance at the time. But one of several hydrogen explosions blew the walls and roof off the reactor building. This week a delicate and lengthy operation to remove fuel rods from that reactor began.
Radiation leakage following the explosions forced the evacuation of tens of thousands of people from the surrounding area. An exclusion zone roughly 11 miles by 19 miles remains in force around the plant two and half years later. The entire facility is now being decommissioned, but Tepco's clean-up, which has been strongly criticised by environmentalists, is expected to take up to 40 years.
Hirose said that although the situation facing Fukushima Daiichi on 11 March was exceptional, measures could have been adopted in advance that might have mitigated the impact of the disaster. Tepco was at fault for failing to take these steps, he said.
"After I became president [in 2012], we formed a nuclear safety review committee. We focused mainly on what we could do, what we could learn. We had a lot of data by then. Three other reports, one from the Diet [Japan's parliament], one from government. We had a lot of information. Tepco's own report, too. We concluded that we should have avoided that catastrophic accident, and we could have. We could see what we should have done."
Preventative measures included fitting waterproof seals on all the doors in the reactor building, or placing an electricity-generating turbine on the facility's roof, where the water might not have reached it. In addition, wrong assumptions were made, he said.
"I don't know if I could have seen or thought this before the accident … Probably I assumed that people had discussed counter-measures to avoid a huge tsunami by something very special like a complete shutdown."
It transpired that the huge cost and technical complexity of a multiple shutdown, in what was considered the unlikely event of an abnormally large tsunami, had led managers to discount such a scenario as implausible and inefficient, he said.
"What happened at Fukushima was, yes, a warning to the world," he said. The resulting lesson was clear: "Try to examine all the possibilities, no matter how small they are, and don't think any single counter-measure is foolproof. Think about all different kinds of small counter-measures, not just one big solution. There's not one single answer.
"We made a lot of excuses to ourselves … Looking back, seals on the doors, one little thing, could have saved everything."
Tepco was willing to share its experience with British and other nuclear plant operators if they wished, Hirose said. "We can share all the information, all the data we obtained, that we learned from this accident, and then hope that people will use the data and information to prevent the same thing happening."
Hirose confirmed that his company has paid a large price for the disaster. It planned to "streamline" the business and shed hundreds of jobs through voluntary retirement to keep itself in business. "We have a huge debt for the compensation for damages and losses and for decommissioning … We have to be sustainable as a going concern."
Concerned that Tepco may be unable to cope and responding to criticism that the company has bungled parts of the clean-up operation, Japan's government has agreed to spend 47bn yen (£292m) on dealing with hundreds of static tanks to store radiated water at the plant.
It is also considering paying part of the cost to decommission Fukushima's damaged nuclear reactors. Tepco will reportedly seek 500bn yen (£3.1bn) in bank loans by the end of the year to help keep itself afloat.
Asked about the severe domestic and international criticism that followed the discovery in July of leaks from some of the tanks storing contaminated water, Hirose said the problem stemmed from a "simple mistake" in managing the tanks. Since the discovery, the monitoring system had been changed and new welded tanks installed, instead of the old bolted together versions.
Hirose said he could not state categorically that all leakage of contaminated groundwater into the sea had ceased, but the outflow was much reduced. "Probably there is some leakage. It is very difficult to say where it comes from and how much it is, but the harbour [radioactivity] level does not go down, so that means there is some leakage … We are trying to stop it."
Hirose said he felt deeply sorry for the estimated 150,000 local residents who have been forced to leave their homes due to potentially harmful radiation levels, and may in some cases never be able to return.
"I have visited Fukushima many times, met the evacuees, the fishing union, the farmers, many people whose businesses have been damaged very much. I feel very sorry for them. We have to compensate them fully for the damage we caused by our accident."
Tepco was investing in the area and creating jobs by building a new thermal power plant. But he acknowledged reconstruction and rebuilding would take many years.
But his report found 38 areas for improvement including risks associated with flooding and the state of preparedness for emergencies. In 2012, documents released under freedom of information rules showed that all eight coastal nuclear locations in the UK, including Hinkley Point, were at risk of flooding and coastal erosion, which would worsen with climate change. EDF Energy said it was confident its UK sites were adequately protected against storms and floods. "Without these arrangements in place the regulator would have the authority to close us down," said an EDF spokeswoman.
Hirose said that although there are currently no nuclear powerplants operating in Japan, nuclear power had a future in the country. Popular former prime minister Junichiro Koizumi called last week for Japan to abandon nuclear power altogether, saying it was demonstrably dangerous.
The best course for Japan and other developed countries was energy diversification, Hirose said, combining nuclear power with other forms of generation, including oil, gas and renewables.