Setsuko Thurlow was a 13-year-old schoolgirl when the first bomb fell.

It was August 6, 1945, shortly after breakfast, and she and her classmates were starting their first day of work at an army headquarters, a wooden building about a mile from the centre of the blast.

She was on the second floor, not far from the window, when she saw a bluish-white flash "like a magnesium flare" through the glass.

Later, she would remember the sensation of floating in the air, waking in total darkness, and hearing the faint cries for help from her classmates near her. Some called for their mother, others for God.

Setsuko was one of the lucky ones. She owes her life to an unknown man who rescued her from underneath some fallen timber.

Seventy years have passed since the United States shocked the world by dropping atomic bombs on the Japanese cities of Hiroshima and Nagasaki.

Today, Setsuko is a hibakusha, a survivor, one of a frail and increasingly diminishing group who have pledged their lives to use their personal stories to ensure no such atrocity happens again.

The average hibakusha is about 80 years old. Setsuko herself, a grandmother, is 84, and all have made their warning clear that time is running out for them to see nuclear weapons completely disappear in their lifetime.

"The survivors are getting old and passing away, leaving a smaller number of us," says Setsuko, who has spent this week in Edinburgh sharing her story with school pupils and politicians.

"I have travelled to Scotland to warn against the renewal of Trident. I want to touch hearts and minds. Depending on nuclear weapons as a solution to a problem is nonsense. It is illusion and delusion."

While she has often said that it is painful for her to share her memories and painful too for others to hear them, Setsuko says she feels she has a powerful commitment to tell the story of Hiroshima and the devastation she witnessed.

"I was rescued by a stranger," she says. "I was freed. I owe my life to a lot of people and I have dedicated it to them by telling the story of that day."

On that August morning, a man she has never been able to find since had lifted the fallen timbers that had pinned her to the ground and pushed her towards an opening as the ruins burned around her.

By the time Setsuko escaped to the nearby hills with a few other surviving girls, the rest of her classmates had burned alive.

"My whole city, my loved ones, my classmates, my schoolmates, whole families. They were destroyed," she says.

In the immediate aftermath, Setsuka says she did not see the infamous mushroom cloud from the atomic bomb because she was in it.

In a talk to the Physicians for Global Survival in 2003, she described the silence in the cloud, broken only by the groans of the dying.

"Although it was morning, it looked like twilight because of the dust and smoke in the air," she said.

"Streams of stunned people were slowly shuffling from the city centre toward nearby hills. They were naked or tattered, burned, blackened and swollen. Eyes were swollen shut and some had eyeballs hanging out their sockets.

"They were bleeding, ghostly figures like a slow-motion image from an old silent movie."

The injured, she recalled, held their hands above the level of their hearts to lessen the throbbing pain of their burns and had "strips of skin and flesh" that "hung like ribbons from their bones."

Setsuka and the other survivors tore their clothes and drenched them in water from a nearby river in order to give moisture to the dying who were begging them for a drink.

"No doctor came, no nurse," Setsuka recalled. By the time night fell, she said, she only felt numbed by the sights of the day as she sat with the rest of the survivors on the hillside and watched her entire city burn.

More than 140,000 people were killed in the Hiroshima bomb attack. In Nagasaki, a further 70,000 people lost their lives.

Those who were near the epicentre of each explosion, which reached close to one million degrees, were incinerated and vaporized without a trace.

Those who were terribly burned lived for several days or weeks. Setsuka's sister and her four-year-old son Eiji were left unrecognisable apart from her voice and a unique hair-pin in her hair.

They held on for several days in agony without access to medical care before passing away.

The final deadly legacy of the atomic blasts would last longer. Like many of those exposed to the radiation from the blasts, Setsuka's uncle and aunt noticed purple spots on their bodies a few weeks later, followed by nausea, loss of hair and fever before they too lost their lives.

"For me, it happened many years later once I had become a grandmother," says Setsuka.

"My granddaughter was diagnosed with a brain tumour and on that day, I finally thought the bomb had caught me."

Fearing the cancer was an inherited result of her exposure to the atomic radiation, Setsuka made frantic calls to medical professionals, even to the atomic bomb hospital in Hiroshima, desperately trying to find out if it could have have been the cause.

"They told me to relax, that there were no other known cases of third generation illness," says Setsuka. "But always you are scared."

"In the aftermath of the bombing, I saw a lot of human suffering," she says. "But I also saw the strength of human survival and resilience.

"I saw a lot of people helping the injured and the dying. And those images strengthen my spirit. I wanted to be like them, I want to be a contributing member to society, a 'helping' person."

Setsuka became a social worker, helping individuals, children and families.

"And they are all linked to the nuclear weapons argument - as all could be destroyed again," she warns. "I saw horrible things, but it has made me appreciate the preciousness of each and every human life.

"We were created by something up there and we are all members of the same family. We have to learn to live together in harmony. Not by killing."

"I think in the UK, you people have a serious decision to make whether to replace Trident or not," adds Setsuka.

"Replacing it will cost what? £100bn? All that money when the young doctors are struggling and people are wanting further education and better healthcare?

"I'm an outsider who cannot tell you what to do but as someone who has seen the terrible consequences of what one nuclear bomb did for me and my city and my family, please, think once agai n of the preciousness of human life - because that's where resources should be spent."