CORVALLIS, Ore. - The Japanese response to the Fukushima nuclear accident was heroic at first and energetic in the two years since then, experts say, and is now reaching a point in many areas where science and social concerns may diverge - the question becomes, how clean is clean enough?

Considerable work still remains to be done at and near the reactor complex where the most serious damage and radioactive contamination took place, following the tsunami and reactor accident that began on March 11, 2011.

But through sustained and well-managed cleanup efforts in many other areas, enormous progress has been made in the past two years, said Kathryn Higley, professor and head of the Department of Nuclear Engineering and Radiation Health Physics at Oregon State University.

"I was recently standing on top of one of the heavily damaged reactors at the Dai-ichi nuclear power station, and even there it was surprising how moderate the radiation levels are now," said Higley, who toured the region last month. She also met with local experts and has been involved in international efforts to assist in the response to the accident since it occurred.

"This incident occurred in the midst of an enormous geological disaster and the response to contain it was heroic from the beginning," Higley said. "And in the aggressive cleanup efforts afterward, they've made tremendous strides and have learned a lot about what decontamination procedures are most effective. Certainly challenges remain, but they are working through them."

Many of the approaches have been basic, Higley said, like removing grass and vegetation, sometimes a little topsoil, washing buildings, carefully measuring the levels of cesium and other radioactive contaminants to ensure they are at safe levels. Radiation can be monitored by sophisticated instruments at levels that are far below anything that will pose a health threat. It's considerably higher, for instance, across many areas of the Rocky Mountains than in other parts of North America.

The government is subcontracting cleanup in some of the less-affected areas and handling the most heavily contaminated sections itself. And higher levels of radioactive contaminants have been detected in some nearby fish and other marine species that tend to bioaccumulate the toxins. But the dose implications are modest, Higley said.

A question that local Japanese residents and policy makers are already confronting, Higley said, is at what point to conclude that any remaining contaminants or radiation no longer pose a health threat, what areas still need more work, and how much more expenditure of money and resources is warranted. In many places this gets to a discussion of natural background levels of radiation, and what constitutes safe versus risky levels.

"In science we have a pretty good understanding of when radiation exposure is too high," Higley said. "It's much more difficult to say how low is low enough. We live in a world of radiation that comes naturally from the sun, our food, soils, rocks, and the foundations of our homes. We also receive it from industrial activities and medical tests.

"The issue of how low is low enough that people in Japan are facing right now often becomes more of a social and political question than a scientific one," she said.

Most researchers have already concluded that the health impacts from the Fukushima incident will be modest, with the greatest potential for effects on power plant employees who directly worked to contain the accident. Those workers will have a higher chance of getting cancer, but even that might not be detectable, studies suggest.

There continue to be wide areas near Fukushima with minute levels of contamination and higher radiation levels than they used to have. But at the same time, those levels are less than some other areas of the world with naturally high radiation levels due to local geology, such as Kerala, India, home to millions of people.

Higley said it's also worth noting that in this cleanup effort the Japanese are learning a great deal about how to most effectively decontaminate buildings and urban areas. It's information that could be of considerable value if any place in the world were ever attacked with a "dirty bomb" by terrorists, she said.

"We're also going to be learning things for years about the environmental cycling of radioactive contaminants," Higley said. "Near Fukushima we have an entire landscape that has been affected, and studies of it in the future will help us better understand movement of radioactive materials in the world we live in."

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Kathy Higley, 541-737-0675