It was early in the day, but already hot. As she wiped sweat from her brow, Chieko Kiriake searched for some shade. As she did so, there was a blinding light – it was like nothing the 15-year-old had ever experienced. It was 08:15 on 6 August 1945.

“It felt like the sun had fallen – and I grew dizzy,” she recalls.

The United States had just dropped an atomic bomb on Chieko’s home city of Hiroshima – the first time a nuclear weapon had ever been used in warfare. While Germany had surrendered in Europe, allied forces fighting in World War Two were still at war with Japan.

Warning: This article contains graphic content that some readers may find upsetting

Chieko was a student, but like many older pupils, had been sent out to work in the factories during the war. She staggered to her school, carrying an injured friend on her back. Many of the students had been badly burnt. She rubbed old oil, found in the home economics classroom, onto their wounds.

“That was the only treatment we could give them. They died one after the next,” says Chieko.

“Us older students who survived were instructed by our teachers to dig a hole in the playground and I cremated [my classmates] with my own hands. I felt so awful for them.”

Chieko is now 94 years old. It is almost 80 years since the atomic bombs were dropped on Hiroshima and Nagasaki and time is running out for the surviving victims – known as hibakusha in Japan – to tell their stories.

Many have lived with health problems, lost loved ones and been discriminated against because of the atomic attack. Now, they are sharing their experiences for a BBC Two film, documenting the past so it can act as a warning for the future.

After the sorrow, new life started to return to her city, says Chieko.

“People said the grass wouldn’t grow for 75 years,” she says, “but by the spring of the next year, the sparrows returned.”

In her lifetime, Chieko says she has been close to death many times but has come to believe she has been kept alive by the power of something great.

The majority of hibakusha alive today were children at the time of the bombings. As the hibakusha – which translates literally as “bomb-affected-people” – have grown older, global conflicts have intensified. To them, the risk of a nuclear escalation feels more real than ever.

“My body trembles and tears overflow,” says 86-year-old Michiko Kodama when she thinks about conflicts around the world today – such as the Russian invasion of Ukraine and the Israel-Gaza war.

“We must not allow the hell of the atomic bombing to be recreated. I feel a sense of crisis.”

Michiko is a vocal campaigner for nuclear disarmament and says she speaks out so the voices of those who have died can be heard – and the testimonies passed on to the next generations.

“I think it is important to hear first-hand accounts of hibakusha who experienced the direct bombing,” she says.

Michiko had been at school – aged seven – when the bomb was dropped on Hiroshima.

“Through the windows of my classroom, there was an intense light speeding towards us. It was yellow, orange, silver.”

She describes how the windows shattered and splintered across the classroom – the debris spraying everywhere “impaling the walls, desk, chairs”.

“The ceiling came crashing down. So I hid my body under the desk.”

After the blast, Michiko looked around the devastated room. In every direction she could see hands and legs trapped.

“I crawled from the classroom to the corridor and my friends were saying, ‘Help me’.”

When her father came to collect her, he carried her home on his back.

Black rain, “like mud”, fell from the sky, says Michiko. It was a mixture of radioactive material and residue from the explosion.

She has never been able to forget the journey home.

“It was a scene from hell,” says Michiko. “The people who were escaping towards us, most of their clothes had completely burned away and their flesh was melting.”

She recalls seeing one girl – all alone – about the same age as her. She was badly burnt.

“But her eyes were wide open,” says Michiko. “That girl’s eyes, they pierce me still. I can’t forget her. Even though 78 years have passed, she is seared into my mind and soul.”

Michiko wouldn’t be alive today if her family had remained in their old home. It was only 350m (0.21 miles) from the spot where the bomb exploded. About 20 days before, her family had moved house, just a few kilometres away – but that saved her life.

Estimates put the number of lost lives in Hiroshima, by the end of 1945, at about 140,000.

In Nagasaki, which was bombed by the US three days later, at least 74,000 were killed.

Sueichi Kido lived just 2km (1.24 miles) from the epicentre of the Nagasaki blast. Aged five at the time, he suffered burns to part of his face. His mother, who received more serious injuries, had protected him from the full impact of the blast.

“We hibakusha have never given up on our mission of preventing the creation of any more hibakusha,” says Sueichi, who is now 83 and recently travelled to New York to give a speech at the United Nations to warn of the dangers of nuclear weapons.

When he woke up after fainting from the impact of the blast, the first thing he remembers seeing was a red oil can. For years he thought it was that oil can that had caused the explosion and surrounding devastation.

His parents didn’t correct him, choosing to shield him from the fact it had been a nuclear attack – but whenever he mentioned it, they would cry.

Not all injuries were instantly visible. In the weeks and months after the blast, many people in both cities began to show symptoms of radiation poisoning – and there were increased levels of leukaemia and cancer.

For years, survivors have faced discrimination in society, particularly when it came to finding a partner.

“‘We do not want hibakusha blood to enter our family line,’ I was told,” says Michiko.

But later, she did marry and had two children.

She lost her mother, father and brothers to cancer. Her daughter died from the disease in 2011.

“I feel lonely, angry and scared, and I wonder if it may be my turn next,” she says.

Another bomb survivor, Kiyomi Iguro, was 19 when the bomb struck Nagasaki. She describes marrying into a distant relative’s family and having a miscarriage – which her mother-in-law attributed to the atomic bomb.

“‘Your future is scary.’ That’s what she told me.”

Kiyomi says she was instructed not to tell her neighbours that she had experienced the atomic bomb.

Since being interviewed for the documentary, Kiyomi has sadly died.

But, until she was 98, she would visit the Peace Park in Nagasaki and ring the bell at 11:02 – the time the bomb hit the city – to wish for peace.

Sueichi went on to teach Japanese history at university. Knowing he was a hibakusha cast a shadow on his identity, he says. But then he realised he was not a normal human being and felt a duty to speak out to save humankind.

“A sense that I was a special person was born in me,” says Sueichi.

It is something the hibakusha all feel that they share – an enduring determination to ensure the past never becomes the present.

Atomic People will be broadcast on Wednesday 31 July on BBC Two and BBC iPlayer.

If you are affected by any of the issues raised in this story, support and advice is available via the BBC Action Line.