The media are awash in stories about the Japanese nuclear reactors. Some of this material, especially that here at dKos, is useful and reliable background information. However, there's one huge shortcoming in the coverage: nobody is telling you the bottom line. The headlines scream with information that might portend doom and death -- or might not. I think it would help to cut through all the speculation and zero in on the bottom line numbers that determine life and death in this issue.
That bottom line is simple: how much radioactivity has been released to the environment? Radioactivity is the ONLY threat that nuclear reactors pose to the general public in an accident. Nobody will be suffocated, lacerated, burned, mashed, crushed, or drowned by this accident. As far as the public is concerned, there is just one threat: the increased probability of dying from cancer as a result of exposure to radiation.
We have a rough rule of thumb to guide us in this is: exposure to 100 milliSieverts of radiation will increase your chances of getting cancer (most likely leukemia) by about 1%. This applies to the entire population. Therefore, if 1 million people are each exposed to 100 milliSieverts of radiation, we would expect about 10,000 of them to die of cancer within 30 years because of that exposure. Although there is some debate about this, I accept the linearity hypothesis, which says that the effect is linear across all exposures: if 1 million people are exposed to an extra 1 milliSievert of radiation, then we can expect 100 of them to die within 30 years because of that exposure.
So, the bottom-line question is: how many milliSieverts has the population of Japan been exposed to because of this accident? We cannot directly measure this number, but we have some very good indicators of its magnitude. The best of these is the total dose at the fence line. This is the total dose that a person would receive if they were standing at the outer fence of the plant property. That number can be estimated from this graph:
New York Times graph
You can carry out an "eyeball integration" to estimate the total fence line dose; I read it as about 100 milliSieverts over the course of the accident. Further away, that total dose drops off rapidly, for three reasons: first, a goodly portion of the radionuclide releases are isotopes with short half-lives. They may be radioactive two minutes after they escape the reactor, but an hour later, when they start reaching people's homes in the safety zone, some of their radioactivity has died out. Second, there's a natural dilution effect with distance from the plant. Third, the general weather pattern is from west to east, so much of the radioactivity is being carried out to sea, not towards land.
Putting all these factors together is still complicated. For simplification, I'm going to concentrate on the 140,000 people who are in the six-mile deep band just outside the 12-mile exclusion zone. Here comes a REALLY quick-and-dirty calculation, assuming that: 1) The fence-line reading is taken 100 meters away from the source; 2) the radionuclides are diffused evenly as they spread. Then at a distance of 20 km, their density will add up to a total integrated dose of about 1 microSievert. A dose this large applied to 140,000 people yields about 0.03 additional cancer deaths due to this accident.
(I must confess that I find this number surprisingly low. I cannot rule out the chance of a stupid mistake, nor will I defend my assumptions as right on the money, but I do believe that this calculation is basically sound and just about as good as we can get given the paucity of hard data. If you'd like to present an improvement to the calculation, please do so.)
So there you have it: the bottom line is that this accident, so far, has imposed 0.03 deaths upon the general public. The deaths among the workers will be much higher, of course. Moreover, if the spent fuel rods in the spent fuel pools catch fire again and release significant amounts of radioactivity, then the total public casualties will go up. But the hard, solid fact is that the number of people who will die because of this accident is tiny compared with so many other dangers we face in modern life.
Keep that in mind as you peruse the screaming and moaning about this accident.