Terumi Tanaka, 78, holds his purple health pamphlet while sitting next to fellow bomb survivor Mikiso Iwasa in Tokyo. (Chico Harlan/THE WASHINGTON POST)

Before leaving the house Thursday morning, Terumi Tanaka grabbed his briefcase, which contained a gently worn purple pamphlet summarizing what could go wrong.

The pamphlet lists his date of birth, his current address and his blood type. It allows him to receive free tests, whenever he wants, for diseases such as liver cancer and leukemia. It has lots of blank charts, too — space for doctors’ notes if things ever go bad, because if you carry one of these booklets, you never really know.

Like thousands of others in Japan, Tanaka carries the pamphlet — a government-issued “Atomic Bomb Victim Health Handbook” — because he is a survivor of the atomic bomb that fell on Nagasaki on Aug. 9, 1945, when he was 13.

In 1957, Japan issued the pamphlets to those who had survived the blasts at Nagasaki and Hiroshima three days earlier, absorbing all the beta particles and gamma rays and neutrons, becoming case studies for the harm they could cause.

Amid a nuclear disaster at the Fukushima Daiichi power plant, millions in Japan are worrying about the particles in the air, where they might spread and what might happen if they come too close. At least 20 million Japanese are old enough to remember the A-bomb attacks, and the worry resonates in particular with survivors, who have spent decades grappling with the inherent uncertainty of radiation exposure.

Just a few months before Tanaka received his pamphlet, a friend from Nagasaki died of leukemia. Both Tanaka and his friend had been about 2.2. miles from the hypocenter, which means the only difference between death and survival is something Tanaka — 78 years old and totally healthy — cannot explain.

But for the 54 years since receiving it, Tanaka has kept the pamphlet, a little bigger than a passport, close at hand, a persistent reminder of the uncertainty he faces.

“Everybody should realize,” Tanaka said, holding the aging booklet, “that this becomes the most important thing in your life.”

On Thursday, at the coastal Fukushima power plant, Japan’s Self-Defense Forces tried to douse water on a series of radioactive units that have refused to cool. They did this using helicopters, armed with 7.5-ton payloads of water; more conventional methods had become too dangerous. A television camera from 20 miles away showed the scene unfolding, the power plant shimmering in its obscene heat.

In his Tokyo home, Tanaka watched on television, and he leaned closer to the screen to follow the helicopters — fluttering specks, like insects.

It looked like a long shot, Tanaka thought.

Like radiation itself, nuclear power is a part of Japan’s legacy. Short on natural resources, this island nation depends on its 54 nuclear reactors for 30 percent of its energy supply. In the first decade after the United States dropped atomic bombs on Hiroshima and Nagasaki, occupying U.S. forces banned press reports and many studies on the effects of radiation, and that’s the primary reason, as Tanaka emphasizes, that Japan took so long to promote health care for blast survivors. But there’s also this: Japan, in 1954, launched its own nuclear power research program. It wanted domestic support for the initiative.

“We aimed for an energy that is 100 percent safe,” Tanaka said, “but 100 percent never exists.”

Tokyo is 150 miles south of the Fukushima Daiichi complex, but diplomats and foreigners are leaving fast. Japan says all areas beyond a 19-mile radius of the plant are safe, but the United States, South Korea and Australia disagree, urging citizens within 50 miles to evacuate.

Tanaka chairs a Tokyo-based organization for A-bomb survivors, and when he arrived at work Thursday, he greeted Mikiso Iwasa, a Hiroshima survivor and colleague. They talked about the nuclear emergency, and they agreed that nuclear power had again been underestimated.

“I’m worried,” Iwasa said.

“I’m worried, too,” Tanaka said.

On that day in August 1945, Tanaka was on the second floor of his house. His mother and sister were downstairs. A fierce light pierced the room — “You became white,” is how Tanaka remembers it — and four or five seconds later, a jolt of pressure blew out the windows and knocked over furniture.

During those four or five seconds, Tanaka had run down the stairs and huddled in a corner. His mother and sister had scrambled outside, diving under the front porch. None was seriously injured.

But after that exposure to radiation, Tanaka dedicated his life to learning more. He read research as fast as it emerged. He studied the effect of radiation on blood cells, and its likelihood of causing birth defects. Tanaka was terrified about having a child. He felt as if something inside of him — a part of his DNA — “was broken.”

He married a woman from Nagasaki, who was not in the city at the time of the bombing, and their first child was stillborn. Tanaka blamed himself, although a doctor said the child had been choked by the umbilical cord. So they tried again, and Tanaka’s wife gave birth to a boy with eczema. Tanaka, again, blamed himself.

In subsequent years, Tanaka intermittently mailed his purple pamphlet to the government, which inserted updated pages containing information about an expanding range of illnesses. But Tanaka’s pages, as a sign of his health, remain blank.

Iwasa, 82, has a book filled with doctors’ ink. Unlike Tanaka, he was sick for months after the bombing, and his skin developed purple leopard spots. As an adult he has had asthma, high blood pressure and bronchitis. He developed skin cancer in 1996. It came back in 1999. He developed prostate cancer in 2007.

But in a way, both Tanaka and Iwasa think about their health in the same manner. Although they try not to, they think about the problems that might yet be hiding.

“Radiation will definitely hurt your cells,” Tanaka said, “but that doesn’t mean it will happen immediately. A victim, I think, is bound to become sick.”

Special correspondent Akiko Yamamoto contributed to this report.