At least 25,000 people evacuated after the Fukushima nuclear accident in Japan will never be able to go home.

A large swathe of land downwind of the four Fukushima Daiichi reactors smashed by a 15-metre tsunami in March 2011 is so contaminated by radioactivity that it will not be officially safe to return for more than 100 years.

Tens of thousands more who have left their homes outside the most contaminated zone will choose never to return because of the dangers.

One is Rumyko Kobayashi, a quietly spoken grandmother of nine, who had to leave her ancestral home in Tomioka.

"Because the radiation is so high, I can't bring my grandchildren home and I do not want to live in a place where you can't see your grandchildren," she said.

With heart-rending dignity, Kobayashi confesses how strongly she feels about what she's lost.

"I am very, very sorry to my ancestors who lived there for a long time as a family, a chain of generations," she said. "I am very sorry to my children and grandchildren because they cannot come back to live in our cherished home. I feel guilty, as if it was me who blew up the nuclear power station."

The explosions, meltdowns and leaks at Fukushima Daiichi triggered by an earthquake and tsunami three and a half years ago have hurt Japan deeply, triggering 2.2 million compensation claims, an £8 billion decontamination budget and dozens of legal suits. It's a hurt that is going to take many decades to heal.

More than 30,000 square kilometres of northern Japan were contaminated by the huge clouds of radioactivity that belched into the air during the accident. More than 80,000 people were forced to evacuate from the areas closest to Fukushima Daiichi, and at least another 80,000 are reckoned to have voluntarily decided to flee their homes.

The official evacuation zone is divided into three different areas. In the least contaminated, furthest away from the nuclear plant, the Japanese government is hoping to allow 32,900 people to return soon.

In the second area there is twice or three times as much contamination, and no immediate plan to lift the ban on living there. But the government is hoping that, after decontamination work and natural radioactive decay, 23,300 people will be allowed home in years to come.

In the third area closest to the nuclear station, radiation levels are so high that experts say it will be more than 120 years before it will be safe for anyone to be allowed back. That means that the 24,700 who used to live there will all be dead before they can go home.

Many of those who may be allowed back won't want to come. A survey of one village in the evacuation zones, Katsurao, found that 60 per cent of residents either didn't want to return home or weren't sure. Families with young children faced an "enormous challenge" because of the "invisible risk" of radiation, said the village mayor, Masahide Matsumoto.

"At least 25,000 people will never be able to return home, and this will have traumatic, prolonged and widespread consequences," said Maria Vitagliano, international programme director for Green Cross International, an environmental Red Cross active in Japan and set up by former Soviet president Mikhail Gorbachev in 1993.

She added: "In Japan the catastrophe is continuing. Three years on, it is impossible to calculate the full dimensions of this disaster and its terrible consequences for the people. This is a catastrophe that will cause untold suffering for decades to come."

More than 1,600 deaths have been attributed to the nuclear accident by the Japanese authorities, mainly amongst elderly evacuees due to acute stress, suicides or shortages of medical care. Any long-term health damage from the radioactive contamination will take many years to show up.

It is a tragic disaster that is getting worse, according to Ikuko Hebiishi, a green councillor in Koriyama city, 34 miles from Fukushima Daiichi. The evacuations have torn families and communities apart, making people sick and depressed, she said.

"The next Fukushima disaster could happen anywhere and anytime in Japan as long as nuclear power plants exist in our country. The co-existence of nuclear power and human beings is totally impossible."

Since the accident, Japan has closed down all 48 of its nuclear reactors for safety checks. But the government is now planning to re-open as many as possible, despite admitting that more than 60 per cent of the Japanese population is opposed to nuclear power.

Two reactors at Sendai in southern Japan were given the green light by the nuclear safety regulator to restart last month, though they face other hurdles before they can actually be turned on. More than 20 other applications to restart reactors are pending.

"We are suffering from a national disaster, but we have to think about balancing better efficiency and safety," said Toshimo Nakagawa, a government MP for Hiroshima. Another government MP, Fumikioki Kobayashi, added: "We can't immediately abandon nuclear power."

A third government MP, Mineyuki Fukuda, did not deny when pressed that all the reactors could be re-opened. "They will have to meet new safety standards, and when they meet those standards, they will be opened," he said. "If they do not meet those standards, they will not be re-opened."

But the government's attitude towards nuclear power infuriates many. "What makes me angry is that they don't regret what happened," said Yoshiko Aoki, who runs a community centre for Fukushima Daiichi evacuees in Koriyama.

"It's the individual villagers who regret it, and that makes me very angry. They are all afraid of the hazards. They don't have a future and don't feel there is a possibility of going home."

The tsunami was not the issue, Aoki argued. "The biggest problem is the nuclear catastrophe - and it is not only our problem. It is a problem for future generations and for the world."

Rob Edwards travelled to Fukushima with Green Cross International