Walk with C. Norman Coleman into the cramped office he shares at the Department of Health and Human Services and laugh as he puts on a hat with a stuffed groundhog attached. “What’s on the calendar?” he says, joking. “It doesn’t matter. Every day is the same.”

That’s not really true, at least not for Coleman, but it’s a good thing that the 66-year-old scientist and physician has a strong sense of humor. His days are immersed in two subjects that spark primal fear in most people: terrorism and cancer.

An authority in radiation oncology at the National Cancer Institute, Coleman applied the science of how radiation affects the human body to lead the effort to develop a blueprint for how the United States would handle the health consequences of a radiological or nuclear event.

“After 9/11, a few of us began to ask the question, ‘What does our specialty know that can help the country if there is a nuclear incident?’ ” he said, noting that before the 2001 terrorist attacks, the United States simply was not prepared for such an event.

It was for this work that he was selected as a finalist for the 2011 Samuel J. Heyman Service to America Medals, which honor the federal workforce by highlighting those who have made significant contributions to the country.

The team Coleman led starting in 2004 developed an extensive plan that has addressed the many complexities of a federal response, including:

●How to communicate to the affected population.

●How to ration scarce resources and quickly decide who gets what treatment when there are mass casualties.

●What levels of radiation exposure to the human body pose a danger.

●What treatments would be best to increase chances of survival.

And the team created a preparedness tool kit to assist health-care providers and planners involved in treating, handling and caring for radiation victims in a crisis. It’s at remm.nlm.gov.

Because of his expertise, Coleman was called to Japan in March to help with the response to the massive leak of radiation from reactors at the Fukushima Daiichi nuclear plant after an earthquake and tsunami. When he received the call, he was happy to help, he said, even though his family was nervous because it was unclear how dangerous the radiation exposure was for people in Japan.

Getting to Japan was easy. It was, he recalled, “one of the few times you could get plenty of seats in coach.”

Once there, he used radiation data to assess risks to human health, provided guidance on treatment for contamination, and met with U.S. Embassy staff and other Americans to provide information.

“People were scared,” he said. “You can’t see or feel radiation, but you can measure it, and we were able to give some comfort to people by explaining the situation. We told them what they should worry about and what they didn’t have to worry about.”

Fueled by an insatiable curiosity — “I can’t stand not understanding” is his animating principle — Coleman has fused his work as an internist, cancer researcher and nuclear fallout planner into a highly unusual career.

Coleman was born in Brooklyn, N.Y., and raised in New Jersey. He got good grades when he was young without trying — he earned A’s in subject matter but D’s for effort in elementary school. He started to get serious about academics at the University of Vermont, where he majored in math and realized he was headed for a life in medicine.

He went to Yale Medical School, becoming board-certified in internal medicine, medical oncology and radiation oncology. He was a tenured faculty member at Stanford University’s School of Medicine before joining Harvard Medical School in 1985 as chairman of the Joint Center for Radiation Therapy. In 1999, he moved to the National Cancer Institute, where he created a program to coordinate all radiation oncology activities.

Coleman now divides his working life between a focus on cancer as associate director of the radiation research program at the National Cancer Institute (part of the National Institutes of Health) in Bethesda and a focus on nuclear preparedness as senior medical advisor and chief of the CBRN Team (chemical, biological, radiological and nuclear threats) at Health and Human Services.

Coleman is described by those who work with and around him as ingenious but so humble that you’d never know it if, well, you didn’t know it.

“He comes across as the average Joe that you see at a neighborhood barbecue, but he’s brilliant,” said Health and Human Services spokeswoman Elleen Kane, who has known him for several years.

He and his wife of 41 years, Karolynn, love to travel, especially trekking in the Himalayas, where they have visited Bhutan, Tibet, Nepal and other countries. They have two children. Daughter Gabrielle, 35, is news director at a California NBC station, and as it happens, a curler who competes at the national level; and son Keith, 31, works for Google.

A serious triathlete, Coleman participates in Iron Man competitions, which involve swimming 2.4 miles, cycling 112 miles and running 26 miles. He does it by training every day, a process he finds fun rather than laborious.

One of Coleman’s proudest achievements is a book he wrote, “Understanding Cancer: A Patient’s Guide to Diagnosis, Prognosis and Treatment,” a guide that presents technical details about the disease — diagnosis, treatment and the overall experience — in easily understandable language.

Treating patients as an internist helped him understand the importance of communicating well with cancer patients and their families, which in turn made him realize how important communication would be in the aftermath in a nuclear event and to make that a focus of the blueprint.

“You have to make very complicated things understandable in a non-condescending manner,” he said. “People don’t take things in at first. You have to listen and understand what they heard, and you have to have the same conversation three or four times.”

How does he deal with the fact that his days are filled with treating and planning for nuclear disaster?

“You have to have a sense of humor in this business,” he said.