Japan did some good things to respond to the Fukishima reactor disaster. Most importantly, they evacuated people close to the reactor promptly. Radiation intensity declines as a function of distance squared from the source. Harm from radiation is roughly proportional to duration time intensity. Thus, the sooner you evacuate people who are closest to the scene, the more you mitigate harm.
At 50 km away from the source, the intensity is 1% of intensity at 5 km away, which is in turn 1% of the intensity at 500 meters away. The intensity at 500km away is about one millionth of the intesity at 500 meters away. The intensity at 5000 km away (the continental U.S. perhaps) is about one hundred millionth as great.
How Great Was The Exposure?
The risk at 50 km away in this case for twenty-four hours on the peak days (March 16/17), 3.6 microSv was at an intensity that was exceed unsafe levels if sustained for a week or two, and about three times as much as the highest exposure of anyone outside the plant received for the entire duration of the Three Mile Island incident and about the same as a half an hour at a typical spot at the Chernobyl plant in 2010. The risk at 300 km away would be safe almost indefinitely.
This intensity of exposure hasn't been sustained, but radiation levels have been elevated.
The exposure at 5km away for the same twenty-four hours (360 microSV), is close to the amount necessary to induce immediate radiation poisoning, would be about seven times safe levels for an entire year in that day alone, is about three hundred times as much as the highest exposure of anyone outside the plant received for the entire duration of the Three Mile Island incident, and is about the same as fifty hours at a typical spot at the Chernobyl plant in 2010.
At 500 meters away from the source in that time period, a four and a half hour exposure is almost inevitably fatal, and thirteen and a half minutes would be enough to induce immediate radiation sickness. At 50 meters away from the source in that time period, a three minute exposure is almost inevitably fatal and eight seconds would be enough to induce immediate radiation sickness. At 5 meters away from the source in that time period, a two second exposure is almost inevitably fatal.
Some plant employees and accident response workers may face deadly doses of radiation (and they were probably acutely aware of that risk and heroically did their jobs anyway), but the general population of Japan isn't at nearly that risk.
Also in the good news department is that the quite low levels of radioactivity found at greater distances from the plant have been mostly in form of radioactive iodine that has a quite rapid half life and won't be a continuing source of high level radioactivity exposure.
The Fukishima accident is still a big deal. It is at least the second worst civilian nuclear power plant disaster in the history of the world. It is far worse that Three Mile Island. Time will tell if this is worse or not as bad as Chernobyl, although it seems likely that it will be not as bad, and a swift response should help as well to mitigate the impact. On the other hand, the harm can only be mitigated if a sustainable way to limit ongoing radiation exposure can be found fairly swiftly. Progress is being made on this front, but it isn't clear that a sustainable solution has been found yet.
While low level nuclear waste rapidly decays to non-dangerous radioactivity levels, high level nuclear waste from the core of reactors continues to be dangerous for much longer time periods.
Even if a solution is found to stop intense radiation from escaping the plant, unless it can be almost perfectly contained, there is a real likelihood that the plant will have to be abandoned and that some radius around them will have to be put off limits as a hot zone. The proximity of the plant to the ocean (it is about 200 meters from the ocean), while good for short term cooling resources, also makes keeping radioactivty materials confined to the plant in the long term harder.
The evacuation has disrupted the lives of hundreds of thousand of people for weeks, the disaster will probably lead to permanent loss of large amounts of real estate and tangible person property for tens of thousands of people (in excess of the losses directly caused by the tsunami and earthquake) probably on the order of a billion dollars worth, will probably kill tens of people from radiation exposure, will probably cause something on the order of a billion dollars of damage to the plant, and will seriously inconvenience hundreds of millions of people. The harm is on the order of magnitude of several billions of dollars, or even tens of billions of dollars depending on remediation costs, etc.
This may still be an acceptable cost if very infrequent compared to coal that kills 30,000 people a year in the United States alone (about the same as the entire toll of the tsunami and earthquake scaled to Japan's population), but it isn't nominal either.