MARK WILLACY: It was the seismic rupture that spawned a tsunami up to 40 metres high, and the biggest nuclear crisis in a generation. Five years on, the fallout continues. Tonight we go on a journey into the heart of this ongoing crisis...

MARK WILLACY [TO CAMERA]: I'm just metres away from the main reactor buildings here at the Fukushima nuclear plant.

MARK WILLACY: What's happening? Going high. They don't want to go any further?

MARK WILLACY: ... and we reveal the frightening enormity of the clean-up job - just how long it might take, how much it might cost and how dangerous it still is.

GREGORY JACZKO, FORMER CHAIRMAN, US NUCLEAR REGULATORY COMMISSION: This really is unchartered territory. Nobody really knows where the fuel is at this point, and this fuel is still very radioactive and will be for a long time. There's no playbook - they're making this up as they go along.

MARK WILLACY: It's a stunning landscape of verdant forest, sheer mountains and meandering streams, a side to Fukushima few outsiders get to experience, and after the nuclear fallout drifted and settled here, few would now want to.

MARK WILLACY: Today I have come back to check whether the much publicised decontamination of this poisoned landscape has reached into Fukushima's remote mountains.

MARK WILLACY: Well, we're a few kilometres into the no-go zone here near the village of Namie, and the higher you climb up into the mountains the higher the reading on the Geiger Counter. So it would suggest that a lot of radiation is still in the mountains, in areas that are very, very difficult to clean up.

MARK WILLACY: Nestled in the valleys, or hidden away in the mountains, are hundreds of hamlets like this - irradiated, abandoned, and now overgrown. It's doubtful anyone will return, because there's been no attempt at decontamination here. It's like a science-fiction movie, in which everyone has simply vanished. But the man I am on my way to visit doesn't fear the fallout. Having refused to leave his Fukushima farm just a few kilometres from the melted reactors, Naoto Matsumura is often described as the most contaminated person in Japan.

NAOTO MATSUMARA: In the beginning I felt extremely lonely. But now I'm used to it and I'm not lonely at all.

MARK WILLACY: Helping Naoto Matsumura combat the loneliness is his collection of stray animals. Today, like every day, he's back inside the nuclear no-go zone in the village of Tomioka to feed some abandoned cattle that can never be sold or leave the zone.

NAOTO MATSUMARA: To be honest, I don't know who most of these cows belong to. At the time of the meltdowns their owners fled.

MARK WILLACY: Tomioka is where Naoto Matsumura was born. The empty streets, the abandoned shops, the train station to nowhere, none of it bothers him, neither does the fact that nature is slowly reclaiming what people have left behind. It's spring, so Matsumura takes me to a nearby village once renowned for its natural beauty.

This is the village of Yonamori, inside the nuclear no-go zone. And this is its famous Sakura Dori - Cherry Blossom Street. These blooms have come out five times since the Fukushima nuclear disaster, and it's been five years since anyone has sat here to eat, drink, celebrate the annual cherry blossoms.

While we marvel at the blossoms, the loudspeakers in this empty village crackle into life with a daily announcement from the safety affairs office. "In 30 minutes the no-go zone will close, please leave the area immediately", it says. But not even these abundant, beautiful blooms can distract from the truth of this contaminated landscape. All over the polluted zone thousands of workers are busy scraping up the topsoil into large black bags, and only from the air can you appreciate the sheer scale of this operation. So far more than ten million of these bags have been filled. They're then stacked at thousands of separate locations across the contamination zone. Despite the efforts so far, more than eleven hundred square kilometres of Fukushima's forests, mountains and villages remain uninhabitable. Dozens of communities still haven't been touched.

More than 160,000 people were forced to evacuate during and after the Fukushima nuclear meltdowns. Now, five years on, 100,000 still have not returned. Some remain trapped in tiny temporary apartments, while others say they'll never return to these contaminated communities. Most of those forced to flee the radiation will forever remain nuclear refugees, according to the man who was Prime Minister during the disaster, Naoto Kan.

NAOTO KAN, [Former Prime Minister, Japan]: There are mothers with small children who say they don't want to return, because even after decontamination the level of radiation is higher than before the accident.

MARK WILLACY: But a few have come back.

AZUMA HASHIMOTO: I believe I was the first to return to live here. I wanted to come back first so that Naraha residents could follow me.

MARK WILLACY: It was nearly five years before Azuma and Yumiko Hashimoto were allowed to return to their decontaminated home in Naraha town. Mrs. Hashimoto's family has lived in this house for eight generations, but that looks like where the line will end because her daughter is refusing to return with her one-year old son. In fact, Naraha is now a village without children, because no-one with a family wants to come back.

YUMIKO HASHIMOTO: It's sad when you can't hear the voices of children. It's silent at night time. We eat dinner, take a bath and there is no conversation so we just go to bed.

MARK WILLACY: The silence is particularly haunting for Azuma Hashimoto, because for years he was in charge of disaster prevention at both of Fukushima's nuclear plants.

AZUMA HASHIMOTO: I never imagined that such a thing would happen. So now I feel the towns that flourished from nuclear power have been betrayed. I can't accept that.

MARK WILLACY: Foreign Correspondent has been invited by the plant operator TEPCO to spend a day touring the sprawling facility and to meet the man in charge of decontaminating and decommissioning the Fukushima plant, Naohiro Masuda.

MARK WILLACY: Has anything like this ever been attempted before?

NAOHIRO MASUDA, [CHIEF FUKUSHIMA DECOMMISSIONING OFFICER]: There has never been an accident at a nuclear plant like the one at Fukushima where three reactors had meltdowns. We are currently working on a timetable to decommission the reactors over the next 30 to 40 years.

MARK WILLACY: The former prime minister, now a fierce opponent of nuclear power disagrees.

NAOTO KAN: I think it will take longer than that. This is a major accident, which has never happened anywhere in the world. I think 40 years is an optimistic view.

MARK WILLACY: For the initial part of our tour of the nuclear plant we are kitted out in light protection gear, and our first stop is a reminder of the colossal task facing TEPCO.

MARK WILLACY: So this is the tank area, so here we've got over a thousand tanks on this site. TEPCO is removing about 62 nuclear substances from the water. The only one they can't remove is tritium. But so far they're taking at least 62 elements out, but still there's about a thousand tanks on site that they've got to deal with.

Tritium goes directly into the soft tissues and organs of the human body, potentially increasing the risk of cancer. The site has nearly reached its capacity, with more than half a million tonnes of contaminated water, much of it pumped in to keep the melted reactor fuel from heating up again.

NAOHIRO MASUDA: If we leave it stored there's a danger it will leak into the sea. So it's important that we treat it.

MARK WILLACY: But on top of that, every day 150 tonnes of groundwater flows into the plant, and some believe this poses the biggest threat of all.

GREGORY JACZKO: What concerns me is the volume of water that exists at the site. This water contamination problem is not under control, and it's not really controllable. There really isn't any way to stop it.

MARK WILLACY: Particle physicist Gregory Jaczko was the chairman of the United States atomic watchdog, when Fukushima melted down. He was getting real-time information as the disaster unfolded. He warns that the task of keeping three melted reactors stable and then cleaning them up will take decades.

GREGORY JACZKO: It's a very, very difficult situation. There is no simple solution, there is no silver bullet that is going to put a stop to everything and make this just go away overnight.

MARK WILLACY: But there is one problem TEPCO can make go away, and that's getting rid of the millions of disposable protection suits and hazard masks used by the workers on site. And to do that, the company has built an incinerator several storeys high to begin burning the backlog - enough to fill 28 Olympic swimming pools.

MARK WILLACY: Okay. Well, the heat here's really intense as you'd imagine because this is the incinerator, the furnace, where they're burning all this irradiated protective gear. Now every day six and a half thousand workers are on this site and sometimes they have multiple changes of clothing. So there's a lot of gear to burn because a lot of it is unsafe, it's irradiated.

MARK WILLACY: Those six and a half thousand thousand workers have come from all over Japan. Their radiation exposure is closely monitored. But last year the allowable level was more than doubled. The country's nuclear watchdog said the step had to be taken to allow workers to stay onsite longer, in a bid to keep the crisis at Fukushima "containable".

MARK WILLACY: Well, we spent several hours around the plant already. But we're going closer to the reactor buildings now, which means we've got to put on more heavy duty protective gear. (ZIPS UP SUIT) We're all set to go.

For this part of our tour we are accompanied by five minders, because we are heading to the buildings housing the melted reactors and there are restrictions. TEPCO is worried about possible nuclear terrorism, and won't allow us to film certain security sites.

Reactor 1, Reactor 2, Reactor 3 with all the rubble over there. Be very careful.

The radiation spikes the closer we go to the reactors, where deep inside lies the melted nuclear fuel and TEPCO's greatest challenge, according to the man with possibly the toughest job in Japan, Decommissioning Chief, Naohiro Masuda.

NAOHIRO MASUDA: This is a job we've never done and there is no textbook. It's a huge challenge to balance carrying out the work while limiting the radiation exposure for the workers. It's the most difficult problem.

MARK WILLACY: I'm just metres away from the main reactor buildings here at the Fukushima nuclear plant. Behind me, Reactor 3. Now, we saw what happened there, there was a hydrogen explosion right after the nuclear fuel melted. Next to it, Reactor 2. It's still a problem today. There was no hydrogen explosion, but what happened inside there no-one really knows because the radiation is so high no-one to this day has been able to get inside. And there is Reactor 1, and it could present particular problems for TEPCO because that is where probably the worst meltdown occurred. They don't know where the nuclear fuel is and it could take TEPCO several years to even work that out.

NAOHIRO MASUDA: We haven't actually seen where the melted fuel fell, so it's important to find it as soon as possible.

MARK WILLACY: For the first time, Foreign Correspondent can reveal just how vast the amount of melted nuclear fuel is, the three molten blobs that lie somewhere deep within each of these buildings.

NAOHIRO MASUDA: It's estimated that 200 tonnes of debris lies within each unit so in total about 600 tonnes of melted debris fuel and a mixture of concrete and other metals are likely to be here.

TEPCO EMPLOYEE: This area has very high exposure.

MARK WILLACY: Very high exposure?

TEPCO EMPLOYEE: Yeah.

MARK WILLACY: So it's creeping up, the closer we get to the buildings - the reactor buildings? We can't stay here? Okay.

MARK WILLACY: And the most daunting task - one the nuclear industry has never faced - is getting the melted fuel out. TEPCO admits the technology it needs hasn't been invented yet.

GREGORY JACZKO, FORMER CHAIRMAN, US NUCLEAR REGULATORY COMMISSION: They've sent in some robots. All the robots have been disabled because of the high radiation fields. It may be possible that we're never able to remove the fuel. You may just wind up having to leave it there and somehow entomb it as it is. I mean that's certainly a possibility. There is no playbook, they're making this up as they go along and that's in a lot of ways the best they can do.

MARK WILLACY: Back on the bus, we head closer to the base of the reactor buildings. We hop off at Reactor 4 and start to move down the line. Immediately the radiation level begins to rise.

[Alarm goes off]

MARK WILLACY: What's happening? Okay, so we're just between Reactor 4 and Reactor 3, but the radiation level has gone up to a point where our TEPCO guides are not comfortable going any further, so we'll head back.

After a day inside the nuclear plant it is time to strip off our gear and submit to routine testing. It's a process the workers here go through every day, and it's a reminder of the decades-long task ahead of TEPCO.

NAOHIRO MASUDA, [CHIEF FUKUSHIMA DECOMMISSIONING OFFICER]: We don't believe TEPCO can do this on its own. We now have experts from around the world looking at the current status, checking on what we are doing and making decisions with us.

NAOTO KAN: [FORMER PRIME MINISTER, JAPAN] So far the government is paying $70 billion to support TEPCO. But that is not enough. It will probably cost more than $240 billion.

MARK WILLACY: For others, the cost of the disasters can never be measured in dollar terms.

MARK WILLACY: Well, we've just crossed into the no-go zone just a couple of kilometres north of the nuclear plant. We're heading to Norio Kimura's house, or what remains off it. His is one of the more tragic stories to come out of the tsunami. Not only did he lose his father and his wife, but he also lost his little daughter Yuna. And every now and then he comes back to Fukushima and he still searches for her remains, because she's the only person still listed as missing from Okuma town just north of the plant.

I first met Norio Kimura several years ago. Today he's again out searching for any trace of little Yuna. He and his volunteers are only allowed into the no-go zone for five hours at a time. Soon Kimura is approached by some police who ask us not to film them or what they have found. After a few minutes he returns with a bag.

Oh wow!

It contains some family photos the police have found in the piles of debris, and for Kimura something else far more precious.

NORIO KIMURA: This is Yuna's T-shirt from her kindergarten.

MARK WILLACY: How do you feel when you see Yuna's shirt like this?

NORIO KIMURA: Yuna is not really here, but it's proof of her existence and that she was here. To feel that is the biggest joy for me now. It's a happy moment.

MARK WILLACY: We take the photos and Yuna's top to a nearby temple where Norio Kimura keeps everything he has found belonging to his dead family. His father's body was found inland not long after the tsunami. His wife's remains were found floating off the Fukushima coast weeks later. At this shrine he adds the latest finds, including Yuna's kindergarten top. Deep down, he knows his youngest daughter will never be found.

NORIO KIMURA: The probability of finding her is quite low and very difficult. I know that. But if I stop searching, Yuna will never be found. I can't say that there is no hope, so I will continue on.

MARK WILLACY: Norio Kimura's community inside the fall-out zone will never be rebuilt. In fact, it's destined to be the site of a dump for contaminated waste and soil. But further north, outside the zone, there's hope. I've retuned to Rikuzentakata, which was wiped out by a 13-metre high tsunami. I arrived here in 2011 right after the waves to be greeted by scenes of devastation and death.

No survivors are being pulled out of here, just hundreds and hundreds of bodies.

MARK WILLACY: Five years on I'm back in this exact same spot. And in the end, one in every ten of Rikuzentakata's residents would die in the tsunami. The debris is all gone now, replaced by five million cubic tonnes of earth, scraped off a nearby mountain and put in the city centre. That's in the hope that this community can be raised by up to 13 metres to protect itself from future tsunamis.

MARK WILLACY: Rikuzentakata is a fishing town, and Yoshiharu Yoshida is one of its shrewdest sea dogs. And when the earth shuddered five years ago, the fisherman was one of those who jumped on their boats and headed straight out into the Pacific, riding over the tsunami as it rolled towards the coast.

YOSHIHARU YOSHIDA, [FISHERMAN]: After one night out on the boat I came back after dawn. The sea was full of debris. I came back here, but it was so full of rubble I couldn't get in. So I went back out.

MARK WILLACY: As well as the earthworks to raise the town, authorities are building towering seawalls, some more than five storeys high. They're part of a 400 kilometre chain of gigantic tsunami defences being built along the coast of north east Japan. But fisherman Yoshiharu Yoshida scoffs at the idea that concrete walls can repel the raw power of a tsunami.

YOSHIHARU YOSHIDA: It probably won't be able to prevent it if a tsunami like the last one comes. In another town the people questioned why the sea wall had to be so high, because they couldn't see the sea at all. In fact, it may cause fear.

MARK WILLACY: While some communities outside the zone rebuild, others in the contaminated areas continue to crumble and wither. Few spots symbolise both the natural and nuclear disasters of 2011 better than the Ukedo Primary School. Here, the tsunami smashed through the bottom floor, while the top floor remained untouched, except by the radioactive fall-out from the nuclear plant just a few kilometres away. The 80 children of this school survived the waves, but they've never been back. Some former supporters of atomic energy now believe the risks are just too great, including those who had to deal directly with the fall-out at the time.

NAOTO KAN [FORMER PRIME MINISTER, JAPAN]: It's become clear that nuclear power is more dangerous than other energy sources. In the past, they said it was cheap - but it was only cheap if the cost of accidents, or the cost of disposing of spent fuel and nuclear waste was not included.

MARK WILLACY: You were the Prime Minister of Japan at the time. How close did this country come to all-out disaster?

NAOTO KAN: We were extremely close. If all the reactors had had a meltdown, there was a risk that half or all of Japan could have been destroyed. So in a way, the accident took us to the brink of destruction.

GREGORY JACZKO, FORMER CHAIRMAN, US NUCLEAR REGULATORY COMMISSION: You have to now accept that in all nuclear power plants, wherever they are in the world, that there's a chance you can have this kind of a very catastrophic accident and you can release a significant amount of radiation and have a decade-long clean-up effort on your hands. And that's the reality of nuclear power.

MARK WILLACY: My journey ends on a beach I first visited after the disasters. Then I went in with police in radiation protection gear to search this area for bodies left behind by the tsunami. The beach is empty now, silent except for the waves. But just down the shore, hidden behind this outcrop, is the Fukushima nuclear plant, its stacks visible in the distance. The waves that crashed over this coast five years ago caused the costliest natural disaster in human history, but the nuclear drama continues to play out inside this secluded facility, and will do for decades to come.