How Japan's Accident Stacks Up Next to Chernobyl

Last week's crisis in Tokaimura may rate as Japan's worst ever nuclear disaster, but it doesn't compare to the radioactive tragedies at Chernobyl in the then Soviet republic of Ukraine or even Three Mile Island in the United States. Those catastrophes occurred inside radioactivity-rich nuclear reactors, while the Japanese mishap took place at a uranium conversion plant, where relatively small amounts of fissile material are kept.
Severity of nuclear damage is measured according to a scale that runs from 1 (an anomaly) to 7 (a major accident). Last week's Japanese blunder has been preliminarily rated a 4, while Chernobyl remains the only accident ever to merit a 7. Still, in the dozens of incidents recorded worldwide since 1993, none has merited a rating of more than 3--until now. Just because this isn't Chernobyl doesn't mean it's not serious, says a safety regulator at the International Atomic Energy Agency in Vienna.

Undoubtedly, the 1986 disaster at Chernobyl is the nuclear accident most deeply etched in people's minds. Thirty-one people died immediately when a power surge triggered an explosion, sending radiation spewing out over much of Europe. The nuclear fallout is still devastating parts of Ukraine and Belarus, where cancer afflicts the population and contaminated farmland constrains the economy. Investigations into the accident uncovered rampant flouting of safety regulations and sorely outdated buildings.

But some good has come out of these earlier disasters. The horror of Chernobyl, in particular, was a wake-up call. After 1986, the international community was galvanized to better coordinate emergency efforts, says Gianni Frescura, head of the OECD Nuclear Energy Agency's nuclear safety division in Paris. Countries are now legally obliged to inform neighbors of such accidents, since radioactive emissions don't hew to national boundaries. An international network of nuclear experts was also formed; e-mails have been flying through cyberspace to advise emergency workers in Tokaimura. Many nations, including Japan, have conducted simulations of nuclear accidents in order to iron out evacuation plans and speed mobilization of radioactivity-sniffing robots to the area. Many of the actions that were supposed to be taken in Tokaimura are a legacy of the lessons learned at Chernobyl, says Frescura. Unfortunately, the human error that appears to have caused the Tokaimura accident is also part of the Chernobyl heritage.


Nuclear safety is a hot-button issue in Japan because of a series of recent accidents and, in a few cases, attempts to cover them up. Several mishaps have occurred in Tokaimura, which houses the privately run complex where last week's leak occurred.

A radiation leak at a Tokaimura plant is not reported to government officials until a later investigation unearths evidence

A sodium coolant leak causes a fast-breeder reactor in Tsuruga to shut down. Later it is discovered that the plant operator tried to cover up the incident, even doctoring videotape evidence

A fire breaks out at a nuclear waste reprocessing plant in Tokaimura, and an explosion occurs nearly 10 hours after the original fire had reportedly been put out. Thirty-seven workers are contaminated with radiation

A tritium leak exposes 11 workers to low-level radiation at the Fugen advanced thermal reactor. The operator subsequently admits there had been 11 other unreported leaks during the previous three years

Japan's Science and Technology Agency reveals that 2,000 drums containing stored radioactive waste had been leaking at a Tokaimura reprocessing plant

Fifty-one tons of primary coolant water leak from a cracked pipe at a Tsuruga power station