The three men, all in their 30s, might be any construction workers knocking back Sapporos at a tiny izakaya, or neighborhood bar. Around them is the friendly clutter of any small, working-class drinking place in Japan. Fading calendar portraits of a favorite singer fill every last inch of wall space not given over to bright posters for ocean resorts, photos of kimonoed actresses striking classical poses, or plaques on which celebrities have inscribed their autographs. There’s even a framed snapshot of the old-broad proprietress next to the celebrated tough-guy director and TV star Beat Takeshi.

One patron is missing many of his teeth and has the intense, staring eyes of the slightly too ferocious guerrilla in a samurai movie. Clad in a blue tracksuit and tennis shoes, 34-year-old Hideyaki Kusumoto leans forward and addresses practically the only foreigner to be seen in the town—a small, trim man, dressed in a summery pink sweater and Stabilicore running shoes, with a deep tan and gray appearing along the sides of his thin reddish hair.

“Doctor, I know about the workers in Chernobyl who’ve suffered. Am I at risk?”

“It takes about 30 or 40 years to get cancer from radiation, except among children,” the visiting American says in the calm, clarifying tones of a seasoned physician. “So if workers from Chernobyl have cancer now, it’s probably not because of radiation.”

“But I’m working in a place where the radiation is really high,” says a colleague in thick black glasses, 30-year-old Masaya Ishikawa. “I work two hours a day. I get 1.67 millisieverts every two hours.”

He pulls from his wallet a sheaf of tiny white receipts on which his daily radiation dose at Japan’s Fukushima Daiichi nuclear power plant, crippled by the earthquake and tsunami of last March, is given to him every day. “I know that cancer will appear only 30 or 40 years later, but what about other diseases? What’s the maximum exposure I should get? At wh...