This morning, the Japanese government officially upgraded Fukushima on the International Nuclear Events Scale to a 7, or “Major accident”. The new rating is the highest on the scale, and puts Fukushima on a par with the worst nuclear accident in history—Chernobyl. Understandably, the press has made quite a big deal out of new rating, but the reality is that Fukushima is a very different accident than Chernobyl.

The maps on the right do show a passing similarity between the two accidents. The top one shows overflight measurements of radiation in the region around Fukushima. The bottom one shows Caesium-137 measurements from Chernobyl. The data are not completely analogous, but I think the maps give a sense of how both accidents have contaminated large areas and created regional hotspots that will require additional evacuations.

UPDATE: Several people have pointed out that there are no scales on the maps provided. A colleague of mine pointed me towards this site, which (sort of) compares the two events.

But there are very important differences between Fukushima and Chernobyl. The biggest, in my mind at least, is the timescale over which the accident occurred. When Chernobyl’s reactor number 4 exploded in 1986, it scattered debris over a wide area and sent radioactive fallout high into the atmosphere. Entire villages near the reactor had to be evacuated in a matter of hours, and many residents had to leave personal effects behind. A fire burned at the site until 5 May, spewing tones of radioactive material over 200,000 square kilometres. By November, workers had successfully completed a concrete sarcophagus around the core, effectively sealing it off. In the short period following the explosion, the accident spewed some 14 million terabecquerels of radiation into the environment.

The Fukushima accident has unfolded much more slowly. The damaged reactors exploded over a period of days, and after a modest initial release, radiation has fallen off. So far, the reactors have spread about half-a-million terabecquerels into the air. I haven’t been able to find hard data on the first month after Chernobyl, but I’m willing to bet my lunch that it put out a lot more in that period.

The problem is that Fukushima’s slow bleed of radiation is going to continue for a good period of time to come. Reactors are normally kept cool by recirculated water, but at Fukushima, the circulation system has been heavily damaged, and the only solution is to simply dump tons of water onto the cores. The water absorbs radioactive isotopes like caesium-137, and itself becomes a big waste problem. Moreover pictures from as recently as 10 April show steam continuing to rise from the reactors.

This seepage in the form of water, dust and steam is creating slow-motion Chernobyl at Fukushima Daiichi. Yesterday the government announced evacuations from several villages outside the current exclusion zone. Unlike the dramatic evacuations from the Chernobyl reactors, these will take place slowly over a matter of a month or so. That’s because radiation levels in the region are low enough to be safe in the near term, but not in the long-term. As ground-contamination data continues to be collected, I suspect we’ll see more localized evacuations over the coming weeks and possibly even months.

Time is important, especially in nuclear accidents. Fukushima’s slower burn makes it easier for officials to respond to radiation risks and protect the population from contaminated food, water and air. Yet the slow release of radiation into the environment could still have great long-term economic and environmental consequences. As we’ve just written, the clean-up of Fukushima will probably resemble Chernobyl in many ways, and could take longer.

For full coverage of the Fukushima disaster, go to Nature’s news special.

For a selection of our coverage in Japanese, see Nature Asia Pacific.

Credit: US NNSA/UNSCEAR