It's a credit to Hiroshima's recovery that if the city were to change its name, you'd never guess its harrowing past. A few miles from Miyajima, one of Japan's most beautiful destinations, the city is lined with shopping malls, its inhabitants play baseball and football, it hosts festivals throughout the year and is famed for its oysters and okonomiyaki (Japanese omelette). In some ways, it's a very normal place.

But however much the city has moved on, Hiroshimans are determined no one should ever forget what happened here. A visit to the city's Peace Memorial Park tells the story, in excruciating detail, of the day the bomb fell, devastating the city and killing 140,000 people.

A Unesco world heritage site, the park is as distressing as it is informative – a strange mix for one of the country's leading tourist attractions. And with tickets costing ¥50 (42p), it is also one of the cheapest.

It's not an easy place to be. Little is left to the imagination: over 90% of the buildings in Hiroshima – many of them wooden – were damaged when the bomb was dropped on 6 August 1945. Little survived the blast, or the subsequent fires, but what is now called the A-Bomb Dome held its shape. Originally an exhibition hall, this now stands in a corner of the park like a charred scarecrow, a small example of how the city looked immediately after the explosion.

Code-named Little Boy, the Hiroshima bomb (three days later, Fat Man was dropped on Nagasaki) was only the second nuclear device ever detonated. When its creator, J Robert Oppenheimer, stood in the desert watching the infamous Trinity Test he was reminded of the Hindu verse: "I am become death, destroyer of worlds." His colleague, test director Kenneth Bainbridge less poetically but no less accurately remarked: "Now we are all sons of bitches." So unknown was the bomb's potential to kill and maim, that Hiroshima had been deliberately spared by the extensive allied fire-bombing campaign for several months. They wanted an accurate measure of how their device could devastate an urban setting.

The exhibits go well beyond recounting the devastation of that August day, going into terrifying detail about bombs that the former Soviet Union and the US have since tested in Siberia and the Pacific. Many of those devices were several thousand times stronger than the Little Boy.

The numbers involved and the potential future devastation are difficult to comprehend. To put it in context, the museum warns of a "nuclear winter", an apocalyptic state in which the effects of an atomic war block out the sun and slowly kill the planet. Disneyland, this is not.

Naturally, by far the most distressing section of the museum depicts Little Boy's arrival – a child's scorched tricycle, a series of grizzly photographs showing what happens to human bodies when they face weaponised nuclear power, a step onto which the shadow of a man has been scorched. He was sitting waiting for a bank to open when the sky exploded.

Meanwhile, downstairs there is a collection of letters. Each mayor of Hiroshima has written a letter of protest every time a world leader carries out a nuclear test. Hundreds of these ignored appeals line the walls of the museum. The most recent was written to Barack Obama last June highlighting the Fukushima nuclear disaster.

Back outside the museum in the centre of the park is an eternal flame in memory of the thousands of people who died on that day, and months later through radiation poisoning. The light will be extinguished when the last nuclear missile is decommissioned, but for now, the mayor of Hiroshima keeps writing.

Elsewhere, perhaps the saddest section of the park is dedicated to the children who died such as Sadako Sasaki, a schoolgirl who survived the blast only to be eaten away by leukaemia a decade later. While in hospital, she started folding paper cranes, trying to reach 1,000, a number said to grant a wish from the gods. However, the illness claimed her when she had completed 644 (other sources say she actually made it to 1,000). When she had passed away, Sasaki's family and classmates completed the chain and buried it with her. This practice continues for anyone afflicted, and Sasaki's statue at Children's Peace Monument is often besieged with origami.

More than a million people visit the Memorial Park a year, and it is impossible to come away without being deeply affected. Most are teenagers on school trips. Watching them shuffle around the museum, it struck me that perhaps the reason Japanese people often make the peace sign when posing for a photo is that they know its value more than most.

• Hiroshima Peace Memorial website

• Jamie Lafferty is a travel writer who ordinarily blogs at idoneaholiday.blogspot.com and tweets at @megaheid. He travelled to Japan as part of the Travel Volunteer Project.

For more information go to the Japan National Tourism Organisation's website: jnto.go.jp/eng