“Sixty years ago, when I was 10,” recalls Sakue Shimohira in the opening moments of “The Last Atomic Bomb,” a documentary film by Robert Richter opening today at the Pioneer Theater, “my mother and big sister were burned to a crisp.”

Ms. Shimohira is one of the 250,000 remaining Hibakusha (literally “people exposed to the bomb”), Japanese survivors of the atomic blasts in Hiroshima and Nagasaki that forced the Imperial armed forces to surrender and ended World War II. Mr. Richter and his collaborator, Kathleen Sullivan, are both fervent supporters of nuclear disarmament. In Ms. Shimohira’s straightforward and detailed recollections of the gruesome aftermath of the Nagasaki blast, they have found perhaps the single most persuasive and qualified voice to argue their cause.

The U.S. Air Force’s original target for the second A-bomb was Kokura, a small coastal city with a large military presence, but weather forecasts were poor and both date and destination were changed. The port town of Nagasaki hosted a major Japanese naval base, as well as the Mitsubishi steel works and military ordnance works, and looked to have clearer weather, so Kokura was spared. But when the planes arrived, the bomb detonated almost two miles off target. The 21-kiloton destructive yield, almost twice the destructive power of the Hiroshima bomb, was blunted somewhat by the hills defining the Urakami Valley area of the city.

Having heard rumors of “a new bomb at Hiroshima” that just a few days earlier had leveled that city a moment after an air raid was sounded, the citizens of Nagasaki (including Japan’s heaviest concentration of Roman Catholics) knew to wait in shelters indefinitely. But the wait and the heavy black protective clothing that the civil defense authority instructed those in the shelters to wear despite the August heat were no more help against the blast’s wide spectrum of deadly radiation than the bomb shelters themselves.

Estimates of the number of civilians killed instantly ran between 40,000 and 70,000. The death toll during the following year was far greater.

Mr. Richter is a veteran of Edward R. Murrow-era CBS news and has multiple PBS documentaries to his credit, including a lucid 1988 deconstruction of the Kennedy assassination,”Who Shot President Kennedy?” Not surprisingly, “The Last Atomic Bomb,” with its blend of archival footage, interviews with scientists, and explicative animations showing the destructive path that the Nagasaki airburst’s 7000-degree, 600-mph winds took, follows the “NOVA” style sheet. But the film very sensibly eschews conventional PBS-brand narration. No contemporary voice-over pro or sympathetic celebrity could possibly compete with the graceful, horrible power of Ms. Shimohira and other Hibakusha eyewitnesses.

“You may not want to hear this, it’s a little unpleasant to hear,” Ms. Shimohira matter-of-factly cautions before describing the symptoms of radiation sickness that became a part of every Nagasaki resident’s life in the years after the sky went white on the morning of August 9, 1945. As a counterpoint, “The Last Atomic Bomb” offers the voice of various period newsreels and educational films trumpeting the effectiveness of the second blast (or “the disaster” as one announcer tidily calls it) and the Japanese capitulation that followed.

“Even as we clear the rubble from his devastated cities,” one late 1940s narrator says of the average defeated Nagasaki man in the street, “we’re purging his mind of narrow, fanatical, war-breeding ideas.” His body was another matter. Unfortunately, what military authorities weren’t doing was sharing the scientific data they collected from examinations of the thousands of dead, dying, and grievously ill.

Anxious about Soviet ambitions in a potential arms race and overwhelmed by the seemingly bottomless medical needs of a vanquished enemy’s civilian population, Japan’s occupiers, the film tells us, squelched information about the symptoms, causes, and potential treatments for the horrendous ills resulting from direct exposure to the bomb and the lingering fallout contamination that followed. Among the Japanese, false rumors of contagiousness forced some of the most ill and desperate to eat out of trash cans and lie in the street well after reconstruction began.

“The radiation kept chasing us,” says Ms. Shimohira, who unsuccessfully sought to help her younger sister survive a virtual purge of white blood cells from her immune system and the pain and shame of foul, maggot-infested wounds that she ultimately couldn’t endure.

“The Last Atomic Bomb” also unflinchingly documents the residual prejudice that still surrounds A-bomb survivors. Even second- and third-generation Hibakusha are sometimes deemed genetically unfit for marriage by traditionally minded non-bomb survivors.

“If you encounter an atomic bomb once,” Ms. Shimohira offers, “you will never be at peace again.” In the latter part of her life, that restlessness and spiritual identification with the cause of her ordeal has led Ms. Shimohira into the role of spokeswoman, educator, and activist on behalf of global disarmament. Most of the final half hour of “The Last Atomic Bomb” spotlights a globetrotting turn the 70-year-old took in 2005 to deliver letters to nuclear club heads of state Tony Blair, Jacques Chirac, and George Bush. She urged them to disarm or to at least visit Nagasaki and know the potential devastation they have the power to unleash a thousandfold.

Her quest isn’t exactly a huge success, and the film stretches past a reasonable running time as a few politicians patronizingly say what you’d expect them to say to an exquisitely poised septuagenarian Japanese woman whose capacity for dignity and almost biblical endurance appears limitless.

What is far more effective documentary material is a brief meeting Ms. Shimohira shares with a French Auschwitz survivor. Though speaking through translators, these two strangers have an instant rapport. The Frenchman describes his personal hell, and Ms. Shimohira listens attentively before cutting to the chase. “What happened to your family at the time?” she asks gently. They wind up talking not of politics, wars, or the future. Instead, they compare notes on the equally dire fates of the people they loved the most, each survivor at peace, for a moment anyway, in the company of one of the few people alive who might understand the colossal loss and pain each has shouldered for more than a half century.