"The tsunami has hit the side of the turbine buildings." It was not the TV, but a voice inside the bunker. Sato was jolted away from the television. What did they say? Only then did he realise the same wave had hit the Fukushima coast, too.

"The tsunami alarm said the height would be about three metres," said Atsufumi Yoshizawa, the director of Reactors 5 and 6 at Fukushima Dai-ichi, who was outranked only by plant manager Masao Yoshida. A lithe, quietly spoken man with neatly parted salt-and-pepper hair, Yoshizawa was a TEPCO veteran of 30 years.

The earthquake had already knocked out power to the nuclear plant, triggering the automatic shutdown of Fukushima Dai-ichi's only functioning reactors - 1, 2 and 3. But the plant's emergency defences appeared to be working. Thirteen diesel generators - each the size of a train locomotive - had kicked in, running the emergency pumps to cool the reactors. Most of the generators were in the basement of the turbine building; others were on the ground floors of Reactors 4 and 6.

But suddenly there was panic inside the bunker.

"I got some information from outside," says Yoshizawa. "Heavy oil tanks were being washed away by the waves. And then the pumps in the plant stopped and the power failed. That was when I realised a huge tsunami had come."

The diesel back-up generators had been submerged by the seawater that had flooded the basements of the turbine buildings and other generator sites. Electrical circuits had been shorted and generator fuel tanks washed away. There was no power source cooling the reactors. All that was left to stop Fukushima Dai-ichi's nuclear fuel from overheating and disgorging radiation were banks of what are called "coping" batteries. They had enough charge to last just eight hours.

With gauges showing that the water level inside Reactor 1 was above the fuel, Yoshida thought the situation was under control. But now the panel was showing there was a problem. A big problem.

Pressure inside Reactor 1's containment vessel was rising - a sign that the core cooling system was not working. The pressure was being caused by the reactor's residual "decay" heat. Even after a reactor stops operating, the nuclear fuel inside will continue to radiate enormous amounts of heat. If there is no cooling to remove this decay heat from the reactor, the temperature will continue to rise, to the point where it could melt the nuclear fuel. And because the coolant inside Reactor 1 was not being circulated, it was evaporating.

"We may have to vent the reactor," said Yoshida.

Yoshida's bosses at TEPCO weren't so sure about venting. This emergency procedure had never been done before in Japan, and it was certain to sow further doubts in the public consciousness about the safety of nuclear power, an industry that had earned TEPCO tens of billions of dollars. It could also leave the company wide open to substantial damages for radioactive contamination.

But Yoshida was choosing the lesser of two evils. If they didn't vent, pressure inside the reactor's containment vessel would continue to build and it could explode. That would spew infinitely more radiation into the atmosphere and across Japan. Once the reactor was decompressed, they could start injecting water to cool it.

Heads turned towards the TV again. Something was flashing across the screens. An evacuation order for everyone living within three kilometres of the nuclear plant had been issued by the government in Tokyo. Sato couldn't believe it. If his wife and daughter were alive, they now had to run.

In Japan, this terrible day would become known simply as san ten ichi ichi, or 3/11.

Takashi Sato had heard they'd started injecting fresh water from on-site supplies into Reactor 1 to try to cool it, but that the water level kept falling. He was also told Prime Minister Naoto Kan was coming. His helicopter would arrive within minutes.

Sato heard plant manager Yoshida complaining he did not want the prime minister in his way, holding up his work. The reactor water level was falling despite the injection; there was no power.

In another time, in another place, Yoshida and Kan would probably have liked each other. The plant manager didn't put up with bullshit. Known for his bluntness and independent streak, 56-year-old Yoshida would prove during this crisis that he was no TEPCO toady, or prime-ministerial push-over. At this point in the crisis, Yoshida believed the only value Kan could bring to the emergency response would be to stay in Tokyo.

Standing nearby, listening to his boss, Sato agreed. What could Prime Minister Kan do here? He would slow everything down. In fact, work would have to stop so they could listen to him. "We couldn't afford that," he thought.

Seconds later, the prime minister arrived, striding past sleeping workers under blankets on the floor. Kan was wearing pale-blue overalls. He had a dosimeter (radiation reader) around his neck and he looked like he was about to explode. Kan couldn't understand why the venting of Reactor 1 had not happened yet. He had told TEPCO to do it at 1.30am. "Do it. Let's do it," he had said.

But what TEPCO had not told him was that they couldn't do it. They simply had never imagined they would have to vent a reactor without electricity, and they did not know how to do it manually. Workers were right that minute in the Reactor 1 control room trying to figure it out. They were flicking through blueprints, while watching the radiation levels rise.

"They were working in a situation with extraordinarily high radiation," recalls Yoshizawa. "I cannot possibly say how difficult it was or how scary. It was something they risked their lives for."

The workers tried to open the vent valves remotely, but with no power they wouldn't budge.

Kan again demanded to know why venting had not started. Yoshida explained the pressure-release valves were hard to open without power. "We will do the vent as soon as possible," he assured the PM. "Even if we have to set up a suicide squad."

At 8.03am on March 12, Yoshida ordered the vent valve on Reactor 1 to be opened in an hour. At 8.04am, Prime Minister Kan left, knowing his orders could condemn men to their deaths.

The prime minister believed there were bigger things at stake than the lives of a few men - the very future of Japan hung in the balance. Later that day, Kan would even contemplate the worst-case scenario: a doomsday release of radiation caused by multiple meltdowns, a chain reaction requiring the evacuation of the world's most populous metropolis, Tokyo, and its 35 million people.

To Yoshida, it was also a war of survival. It was a fight to the death against an unstoppable enemy.

Across the Fukushima Dai-ichi compound, inside the Reactor 1 control room, the radiation level was so high that three teams of two workers were rotating in and out every 17 minutes. They were wearing full protection gear with oxygen tanks and had taken tablets to fill their thyroid glands with iodine, to stop them absorbing radioactive iodine from the contaminated atmosphere. While their suits would protect them from alpha and beta radiation, they wouldn't protect them from the more penetrative gamma rays.

One worker received a radiation dose of 106 millisieverts - more than twice the maximum allowable annual exposure for Japanese nuclear workers - and would later be sent to hospital for a check-up. They were working in the dark, in intense heat. After a while, they found the wheel for manually opening the vent, which has to be turned for hundreds of revolutions. Eventually, the aperture for the valve was set at 25 per cent. Then the suicide squad left.

At 2.30pm on March 12, applause erupted inside the Fukushima Dai-ichi bunker. Vapour was seen rising into the sky from the Reactor 1 stack, and pressure in the containment vessel began to fall. The venting had started, and it was working. But now there was a new problem. They were running out of fresh water to cool the reactors.

Right next to the Fukushima Dai-ichi site was the largest source of water on the planet: the Pacific Ocean. But TEPCO preferred not to cool the reactor with seawater, because the salt would damage it. Salt can clog cooling pipes and erode the zirconium oxide layer of the fuel rods. The reactor, which cost hundreds of millions of dollars to build and earned TEPCO many millions back, wouldn't be able to be used again, and the company would be forced to decommission it.

Later, during an attempt to cool Reactor 2, a TEPCO official from head office in Tokyo would be recorded via teleconference counselling plant manager Yoshida: "I think using seawater in a hasty way would be wasteful, as materials will be corroded. Can we agree we have the option of waiting as long as possible in order to use fresh water?"

Yoshida's hackles rose at this interference from head office. "We don't have the option of using fresh water," snapped the plant manager. He then decided to lay his cards on the table. "You know that using fresh water means we'll be able to use the reactor later because it won't be damaged by salt, right? [But] when a massive supply of cooling water is needed like it is now, it is very hard to stick to just fresh water. I don't think there's any other way but to use seawater."

At 2.50pm, there was no fresh water left, and injection for cooling was stopped.

By now, Sato had been trapped inside the bunker for 24 hours. He was desperate to see his wife and daughter. Suddenly, the ground began shaking. Everyone, including Sato, believed it was yet another aftershock. But the rumbling was followed by a roar. It reminded him of a clap of thunder.

"Did Reactor 1 just explode?" asked Yoshida.

It could have been a hydrogen explosion. But Yoshida immediately feared the worst: what if the reactor core itself had exploded? That would scatter nuclear fuel all over Fukushima Dai-ichi, releasing more than enough radiation to kill everyone sheltering inside the bunker.

All around Takashi Sato, plant workers were starting to express their deepest fears. Some were thinking of making a run for it. But if they did, they could have been exposed to a fatal dose of radiation. Best for now to stay inside the bunker.

Eventually, it was confirmed the blast did come from the Reactor 1 building. But it appeared the containment vessel was still intact. Only the building itself was damaged, so most likely it was a hydrogen explosion. Then word was passed around. Radiation had been released, so no one in the building was to go into the rooms closest to Reactor 1. "Stay on the other side of the bunker, please."

Yoshida knew he had to get cooling water injected into Reactor 1 as fast as possible. But he also had to stop Reactor 2 and Reactor 3 from exploding as well. He ordered preparations to get underway for opening the vents at both reactors.

As he did, Japan's national broadcaster, NHK, was making a special announcement on TV. Residents of Fukushima were being advised "to stay inside, close doors and windows and turn off air conditioning". They were also told to cover their mouths with masks, towels or handkerchiefs. And not to drink tap water.

The only one who didn't appear confused at this point was Yoshida. He had just taken a call from Ichiro Takekuro, TEPCO's government liaison, telling him to stop the seawater injections, which he had started.

Showing the maverick streak that had endeared him to his colleagues inside the bunker, Yoshida disobeyed Takekuro's order and continued injecting seawater into Reactor 1. As plant manager, Yoshida had the authority to ignore, overrule and defy head office. He was in the bunker. He was in control. He had hundreds of people's lives in his hands. "Suspending the seawater could have meant death [for those at the plant]," he later revealed. Already, he felt they'd cheated death several times.

As the discussions went on, Sato was sitting on the floor nearby, having his dinner. He hadn't eaten since breakfast and was starving. He poured some boiling water into the bag and waited for his instant rice to soften and swell.

It had been a morale-zapping day. It had started with a visit by a pissed-off prime minister, a meddlesome politician whose interference only served to confuse and slow down the emergency response. But then the vent had happened, and Sato had thought his nightmare was over. He had started thinking of being reunited with his wife and his seven-year-old daughter.

Then he remembered. Tomorrow was Nao's birthday. "My beautiful girl will turn eight and here I am stuck in this building, just metres away from a leaking nuclear reactor."

Sato knew the hydrogen explosion in the Reactor 1 building eight hours earlier had set work back, and there were now fears that Reactor 3 could explode, too. He rolled down onto his back and pulled the thin blue blanket over himself. The floor was rock hard, but he had to try to get some sleep.

The words crackled over the phone line from Fukushima Dai-ichi. The usually unflappable Yoshida was in a panic. Fear was evident in his voice. "Mr Komori, the water level! The water level has fallen!"

Yoshida was talking about Reactor 3. The water had dropped below measurable levels, leaving the 548 fuel assemblies fully exposed. The Fukushima Dai-ichi manager was reporting the emergency to TEPCO's managing executive officer, Akio Komori, at company headquarters in Tokyo.

"The fact the pressure is rising in the dry well means that - like Reactor 1 - a hydrogen explosion is now a possibility," warned Yoshida.

He would later complain to his colleagues in the plant's earthquake-proof building that he was fast running out of tricks to stop Reactor 3 boiling over and blowing up. "We have neither water, nor ideas," he muttered.

Then, at 11.01am, Yoshida, Sato and everyone else in the bunker felt a rumble. The boom that followed was terrifying. Sato put his head in his hands. "We are finished," he thought. All around him, he could hear the muttering of men who believed they were damned.

The situation was spiralling out of control, and Yoshida was now worried about a chain reaction. The explosion at Reactor 3 had damaged the electrical circuits for opening the venting valves at Reactor 2, meaning pressure would continue to build inside its containment vessel. Now Reactor 2 could blow up, setting off a Chernobyl-type cataclysm. The plant manager got back on the line to Tokyo HQ. "It might not be the best time to raise this, but with these two explosions, the site is in shock. We are doing what we can, but morale is slipping," he explained.

And it was about to slip further. At 1.25pm, Yoshida feared the Core Isolation Cooling system at Reactor 2 had stopped. Head office in Tokyo wanted the problem fixed, no matter the sacrifice.

Yasuhiro Ishii's orders had come through from the top - the prime minister's office. Commander Ishii and his team of firefighters from Tokyo's Hyper Rescue Squad, an elite unit specialising in nuclear, biological and chemical disasters, was to proceed to the Fukushima Dai-ichi Nuclear Plant to spray water on the Reactor 3 fuel pool.

Commander Ishii was a veteran of the 1995 Kobe earthquake. A tall, wiry man with short hair, grey sideburns and a ready smile, Yasuhiro Ishii had been a firefighter for 30 years. He knew immediately the crisis at Fukushima Dai-ichi would be the greatest challenge of his career, and the direst threat to his nation since the end of the World War II.

Men over 40 were picked first, and, despite their protests, members of the squad in their early 20s were told to stay behind.

The chosen few started packing their truck. It was called a Special Disaster Rescue Vehicle, and its entire body was insulated with lead. It had air intakes with filters that prevented radiation getting inside. As an added precaution, it was also pressurised to stop hazardous particles from slipping in. Commander Ishii had picked the six men who would come with him in the truck.

Twelve hours later, they drove through the gates of Fukushima Dai-ichi. Inside the 3.5-square kilometre site, it looked like a war zone, one involving plenty of high explosives. Buildings had been shattered, and pieces of concrete wall and steel reinforcing were spread across the compound. Crumpled and overturned cars were wedged into weird places. The tsunami had also washed in trees, which lay across roads. "This just can't be true," thought Commander Ishii as he and his men crept through the plant inside their sealed disaster-rescue truck. They took a radiation reading. It was unbelievably high.

Their initial plan was to park their truck by the sea so they could suck up as much water as they needed. Inching forward in their vehicle, they then wanted to roll out and connect each 350-metre length of hose so it stretched about a kilometre from the coast to Reactor 3. Then they would begin pumping water into the fuel pool 50 metres above. But this turned out to be impossible. The site was strewn with concrete debris and timber. The tsunami had also uncovered manhole and ditch covers, leaving yawning holes in the road. Other sections of the roads inside the plant had caved in. They couldn't get the truck anywhere near their position point by the sea. They would have to work outside the sealed safety of the truck, out in the radiation.

They managed to find a new location between Reactor 3 and Reactor 2, and so they hopped out of the truck to begin unrolling the hoses. The men had to connect up a kilometre of hosing to a Super Pumper fire engine, then shoot jets of water into the pool.

"We were wearing multi-layer suits," recalls Ishii. "For more than 10 hours, we couldn't eat or drink. We couldn't go to the toilet. Our communications were cut and there were no lights. But we were absolutely determined to do it. You can achieve extraordinary things in emergencies."

After seven hours inside the plant, the Hyper Rescue Squad finally hooked up the hoses. Commander Ishii and his men began spraying water into Reactor 3's spent nuclear fuel pool. They were pumping three tonnes of seawater into it every minute. After a few hours of spraying, the men retreated to the front gate of the plant. It was 12.45am, and they had been awake for more than 24 hours. But they still couldn't take off their face masks or layers of protective suits; not until they were well away from Fukushima Dai-ichi.

But Ishii's squad had little time to rest. After reaching their base 30 kilometres from the nuclear plant the seven firefighters wolfed down a quick snack and curled up where they could. Two hours later they were woken, and told to prepare for their second and final shift at Fukushima Dai-ichi. This time they knew what they were heading into. "It was incredibly silent," remembers Ishii. "In fact, I'd say it was frighteningly silent."

On their final shift inside the nuclear plant, the Hyper Rescue Squad trained TEPCO workers how to keep the pumps operating and how to spray water into the Reactor 3 fuel pool. Despite the soaring radiation levels, the risk of falling debris and the horrible working conditions, Ishii and his men had succeeded in their mission. The commander felt he had helped to save Japan.

Nearly a year and a half after the meltdowns at Fukushima Dai-ichi, Masao Yoshida broke his silence. In a video message, the manager of the nuclear plant at the time of the disaster confessed he thought he and his fellow workers would never make it out alive. It was Yoshida's only appearance since the meltdowns. He had preferred not to comment about what had happened until all the official investigations were completed and their reports were released. And the plant manager was also in hospital being treated for cancer of the oesophagus.

In his message, Yoshida repeatedly praised the courage of his workers. "It was clear from the beginning we couldn't run. Reactors 5 and 6 would have also melted down if people hadn't stayed on site. My colleagues went out there again and again. The level of radiation on the ground was terrible, yet they gave everything they had. Pushing their physical limits, they would go out and risk their lives, come back in, then go out to do it again."

Yoshida is now regarded as a national hero by many. He was the man who ignored orders from his TEPCO superiors to stop pumping seawater into one of the stricken reactors, and he was the man who refused to be pushed about by pesky politicians like Naoto Kan. And he was a hero for staring death in the face to save his country from an even worse nuclear nightmare.

The plant manager dismissed any suggestion, such as the one from the former prime minister, Kan, that an evacuation of Fukushima Dai-ichi was contemplated. At least, it wasn't contemplated by him. "I never said to headquarters anything about pulling people out - it never occurred to me ... There was no way we were going to leave the plant," Yoshida insisted.

One of those fighting to the end alongside Yoshida was his deputy and friend of 30 years, Atsufumi Yoshizawa. They are both proud members of the "Fukushima 50", men who had invoked the spirit of the doomed soldiers of Iwo Jima during World War II - forgotten and exhausted defenders fighting to the bitter end inside their bunker.

Yoshizawa is circumspect with his perception of how Japan regards the Fukushima 50: "We have two faces. On one hand, we are heroes who risked our lives. On the other, we are employees of TEPCO, who caused the accident. It is natural for the people forced to evacuate because of the radiation to believe their plight is because we did not work hard enough. But I think the men who stayed behind were wonderful, and I am proud."

Fukushima by Mark Willacy, published by Pan Macmillan, is available on July 1.

Lead-in photograph by AAP.

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