The fate of Japan's Fukushima prefecture, where hundreds of thousands of people have been evacuated, appears to lie somewhere between the outcomes at Three Mile Island and Chernobyl. There were no evacuations during the Three Mile Island accident, which released about 50,000 curies of radioactive gas. Today you can picnic outside the gates of the plant without fear of lingering radiation.
Chernobyl, by comparison, was a nuclear volcano, churning millions of curies of radiation into the sky. Twenty-five years later, only official workers are allowed within 30 kilometers of the entombed reactor at Chernobyl, and radiation levels inside the zone exceed normal background radiation by factors of 100.
Decisions about when to return to Fukushima and how to mitigate any leftover fallout depend on the results of ongoing work to stabilize the pressure inside the reactor cores of units 1 through 3 at the Fukushima Dai-Ichi nuclear plant, and to cool the fuel rods stored in the spent fuel pools. Until then the amount of radiation falling on the land and sea around the complex, and far beyond it, will continue to accumulate with every puff of steam and cloud of smoke issuing from the wrecked reactor buildings.
At best, evacuees from the exclusion zone won't be able to return to their homes for months, according to Arnold Gundersen, a nuclear consultant and chair of the Vermont Yankee Public Oversight Panel, which oversees the nuclear power plant of the same name.
Gundersen, however, isn't expecting the best. "I think there'll be local contamination off the site, certainly out two or three or four miles, that will make that portion of the exclusion zone uninhabitable for 20 years," he says.
In a worse case, the spent fuel rods catch fire, releasing millions of curies of volatile radioisotopes directly into the atmosphere. Wind would carry a radioactive plume over Japan, and rain or snow would bring the fallout down to earth and deposit it in highly localized patterns. That's what happened during the Chernobyl disaster. Some abandoned villages inside Chernobyl's 30-kilometer exclusion zone are radioactively cleaner today than populated villages outside the zone. The map of hot spots shows not concentric circles but oddly shaped blobs.
If conditions aren't so bad, decisions about a return will likely be as political as they are scientific, because no regulatory body has determined a "safe" level of radiation. Troublingly, the Japanese ban on milk, agricultural products, and drinking water in the four prefectures nearest to the Dai-Ichi plant suggest that radioactive fallout in levels potentially hazardous to human health has already contaminated areas far outside the exclusion zone.
Authorities have revealed few details about the particular radionuclides they've found in the food and water, and the situation changes daily. On Wednesday, Tokyo officials announced contamination of tap water by iodine-131 and advised parents not to give tap water to infants. Radioactive iodine is linked to thyroid cancer. It's not the only radioactive component in the fallout. Cesium-137 is also present. According to Robert Alvarez, a senior scholar for nuclear policy at the Institute for Policy Studies, there's enough cesium-137 in the spent fuel pool of unit 4 at Fukushima Dai-Ichi to equal all the cesium-137 released from Chernobyl's shattered reactor core. Cesium-137 has a long half-life—30 years as opposed to eight days for iodine-131—and it persists in the environment at dangerous levels for many decades. Because it mimics potassium, it accumulates in muscle tissue as well as in plants, and has the ability to chemically bind to concrete, making it difficult to remove. If the spent nuclear fuel in unit 4's cooling pool caught fire, some Japanese communities could, depending on weather conditions, become so saturated with cesium-137 that they would have to be abandoned for generations.
In a stabilized situation, the radiation levels that triggered bans of spinach and milk and other foods will likely diminish mostly because of radioactive decay, and as radioactive particles are washed by rain from plants and into the soil. Once contaminants such as cesium-137 and strontium-90 get into the soil, though, plants absorb them, and they enter the food chain. Chernobyl, an extreme case that Fukushima may not mimic, pumped so much fallout across the northern hemisphere that many European countries still have limits on the consumption of sheep, reindeer, and hogs.
Short of scraping off the top layer of soil, there is no way to get rid of this kind of fallout. Entire villages near Chernobyl were bulldozed into trenches to keep radioactive contaminants from spreading. In any event, Japanese regulators will be forced to make a range of hard decisions about how much radiation is safe for people to eat or be exposed to in their homes, playgrounds, and workplaces.
There is some comfort, but it is chilly indeed. Fukushima is already the second-worst nuclear accident in history, but it doesn't yet begin to approach the total radiation released at Chernobyl. For such an event to occur, said Michael W. Golay, a professor of nuclear science and engineering at MIT, "it would have to involve all the nuclear fuel at a multitude of sites within the overall power plant location, and with failure of everything the people there are trying to accomplish. I don't really see a prospect for things occurring in such an awful form."
The bottom line: Fukushima residents face years of uncertainty about their land's habitability. Politics may trump science in determining a return.