This struck me as puzzling – Fukushima, without at all denying its impact, still seemed less consequential than Chernobyl, also a level 7 accident – until I realized that there is no level 8. Thus, anything between Fukushima and Chernobyl would be classified as level 7 simply because there is nothing higher. A flaw in the classification system? Maybe – but it’s what there is.
NPR has a try at sorting out the differences between Chernobyl and Fukushima:
Though Fukushima and Chernobyl are both level 7 nuclear accidents, the health consequences in Japan to date are much less severe. In part, that's because far more radiation was released at Chernobyl. So far, Fukushima Daiichi has released about one-tenth of the amount of radioactive material that escaped Chernobyl, according to an official from the International Atomic Energy Agency.
A little more:
At Chernobyl, an entire reactor exploded, sending up a massive fire and radioactive plume that dispersed radiation over a wide area. The reactor at the Soviet plant was not surrounded by any containment structure, so radiation escaped freely.
People near Chernobyl were not warned against drinking contaminated milk, and many residents later developed thyroid cancer. Two Chernobyl plant workers died on the night of the accident, and 28 more people died within a few weeks from radiation poisoning. Over the long term, several thousand more people were put at risk for cancer.
That speaks to the Soviet desire to keep things secret, even if the cost was their own population.
I will note, though I don’t want to stress this too hard, that the actual cancer risk from Chernobyl is a highly controversial issue. Why not take a side on it? Because the Soviets did treat the issue irresponsibly and shouldn’t enjoy much room for evading further responsibility now (even if the breakup of the Soviet Union – Chernobyl is in Ukraine - makes that stance problematic.)
Much of the radioactive material already released in Japan has been carried out to sea away from populated areas, thanks to prevailing winds. And the government moved quickly to evacuate people from risky areas and to keep contaminated food out of the stores.
And the Japanese, by contrast, have been quite responsible, with a portion of good luck. There’ll doubtless be more to say about this. One thing really isn’t much like the other, level 7 or no.