Photograph by Gleb Garanich, Reuters

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A radiation sign marks the Chernobyl nuclear plant in 2011. The disasters at Chernobyl and Fukushima are the only two incidents to receive the highest Level 7 rating on the International Nuclear and Radiological Event Scale.

Photograph by Gleb Garanich, Reuters

Fukushima Leak's 'Level 3' Rating: What It Means

Notable nuclear incidents are evaluated on an international scale of 1 to 7.

Japan's Nuclear Regulation Authority announced Wednesday that it officially is reclassifying the radioactive water leak at the troubled Fukushima Daiichi nuclear power plant from Level 1 to Level 3 on the International Nuclear and Radiological Event Scale (INES), denoting it as a "serious incident."

That rating sounds ominous, to be sure. But its real meaning—and the significance of Japanese officials' decision to make a more dire assessment of the crisis—are, like many other ramifications of the ongoing crisis at Fukushima, frustratingly unclear.

"In some respects, it's not that big of an issue what they call it," explained David Lochbaum, a veteran nuclear engineer who works as a safety advocate for the Union of Concerned Scientists. "It's still the same mess." (See related: "Latest Radioactive Leak at Fukushima: How Is It Different?")

Why the Rating Now?

Workers for Tokyo Electric Power Co., which operates the plant, discovered on August 19 that 300 tons (nearly 72,000 gallons) of highly radioactive water apparently had escaped from a holding tank into the ground over the previous month. The water had been used to cool one of Fukushima's damaged reactors, in the wake of the 2011 earthquake and tsunami that caused one of the worst nuclear accidents in history. (See related "Pictures: The Nuclear Cleanup Struggle at Fukushima.")

Samples taken from containment area showed it to be so heavily contaminated with strontium-90, cesium-137, and radioactive substances that a person standing less than two feet away would receive, in an hour's time, a radiation dose equivalent to five times the acceptable exposure for nuclear workers. Within ten hours, an exposed person would develop radiation sickness, with symptoms such as nausea and a drop in white blood cells. (See related photos: "A Rare Look Inside Fukushima Daiichi.")

Lochbaum and other outside observers suggest that the reclassification really may be Japanese regulators' effort to reassure the public that they're taking the problem seriously, even though they've yet to take any action beyond observing TEPCO's effort to find and seal the leak. "We've been asking the government to do more," said Jim Riccio, a nuclear policy analyst for Greenpeace, the international environmental organization, who derided TEPCO's cleanup efforts as a "fiasco." "They've got to get more involved." (See related story: "Can an Ice Wall Stop Radioactive Water Leaks From Fukushima?")

Even the International Atomic Energy Agency, the global body whose experts helped create the INES scale in 1990, urged Japanese officials to explain to the public why they had chosen to sound the alarm about this leak, after choosing not to rate several previous leaks as serious, according to the Japan Times.

How the Scale Works

The INES scale was created in 1990 as a way of communicating the relative seriousness of incidents in which radioactivity is released at nuclear plants. Before that, various countries relied upon their own classification systems, which made it difficult to compare a nuclear mishap that occurred in the U.S. with one in, say, Europe or Japan.  The scale is a pyramid with seven tiers. The lowest is 1, or "anomaly."

In that sort of incident, a nuclear plant might have a one-time incident in which someone is exposed to a small amount of radioactivity—less than ten millisieverts, roughly the amount that a person gets from an abdominal CT scan—or in which a small amount of low-level radioactive material is misplaced or stolen. Probably hundreds of such events occur around the world every year, according to experts.

At the top of the scale is a level 7, denoted as a "major accident," in which a massive release of radiation threatens people over a widespread area and requires nuclear authorities to take measures such as large-scale evacuations to protect the public.

Only two such events have rated a level 7—the original Fukushima accident in 2011 and the 1986 Chernobyl disaster in the former Soviet Union, which received the rating retroactively. The partial meltdown at the Three Mile Island plant in Pennsylvania in 1979, the worst accident in U.S. nuclear power history, retroactively has been rated a 5.

Somewhere in between is level 3, the rating assigned to the recent Fukushima leak. U.S. Nuclear Regulatory Commission spokesman Scott Burnell said that the rating probably was based upon the elevated exposure risk that the leak presents to cleanup workers inside the plant, rather than to the public.

"In broad terms, going from a one to a three means that it has a greater impact at the site," Burnell explained. "Personnel there have to take actions to monitor and if necessary reduce their exposure. But there is no expectation of effects outside the plant."

Will the Rating Change?

Burnell said that he didn't expect the situation at Fukushima to be upgraded in seriousness past level 3, because it involves a leak of radioactive water from the site, rather than a breakdown of another reactor. "If you don't have an operating reactor, you don't have the starting conditions to generate the impact necessary to rate something a 7," he said.

Another example of a Level 3 accident was a 2005 leak at Great Britain's Sellafield nuclear fuel reprocessing plant. In that event, about 20 tons of uranium and about 350 pounds of plutonium spewed onto a plant floor when a pipe ruptured. The material was contained in a sealed area, so there was no release to the outside world. A 1989 accident at a plant in Vanellos, Spain, in which a fire caused safety systems to fail, also retroactively received a 3 rating. The NRC offers these examples of other INES levels.

Kathryn Higley, head of the nuclear engineering and radiation health physics department at Oregon State University, said that while the level 3 rating is far from the worst-case scenario for a nuclear plant, it's clearly a situation for TEPCO and Japanese regulatory officials to be concerned about. "It's one thing to have these radiation levels in areas of the plant that by design are going to be hot," she explains. "This is not by design."

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.