Still, experts said that the design was successful in some respects. When countless news reports suggested that containment buildings at several of the reactors had "blown up," the plant was in fact behaving as it should have. The explosions now appear to have been confined to corrugated metal structures that aren't designed to capture a meltdown. When hydrogen gas was released after built-in pressure relief valves were triggered, one of the corrugated buildings reportedly exploded, but the primary concrete containment structures remained intact. If the hydrogen gas had not been released in that way, the outcome could have been much worse, engineers said.
Although the degree of radiation exposure to workers is not yet clear, experts believe that residents of the area were exposed to only a few hundred milli-rems (a rem is a measure of biological damage to tissue). According to the 1982 book, Nuclear Power: Both Sides, by Michio Kaku, a few hundred millirems (less than a rem) could be interpreted as a relatively small amount of exposure. The book suggests that 1,000 rems would kill a person a few days after exposure, 500 rems would kill half of the exposed population within a few weeks, 200-400 would cause radiation sickness and hemorrhaging, and 50 rems would cause no immediate visible effects, but could induce long-term damage. Half a rem -- which is what most residents may have received -- is not considered a high risk.
The effects closer to the reactors are still unknown. At the height of the emergency, the Nuclear Energy Institute reported a dose rate of 40 rems per hour close to the plant. It's not yet known, however, whether plant employees were subjected to those levels for a significant amount of time.
Terry suggested that the accident is far worse than Three Mile Island but not nearly as bad as Chernobyl. "At Chenobyl, you had a carbon-moderated reactor, and all of the carbon went straight up, causing a chimney effect," he said. "It pushed the radioactivity into the upper atmosphere, where it got caught up in the jet stream and was measured as far away as Sweden."
The ultimate effects of the Fukushima accident are not yet fully understood, and may not be until years from now. In the meantime, engineers are saying the effects have been very serious, but not catastrophic, at least in terms of the effect on the public.
"You could say there was exposure that might cause cancer 30, 40 or 50 years from now," Stubbins said. "On the other hand, 20,000 people got killed immediately by the tsunami. You've got to put it in perspective."