There is some good news and some bad news.
The good news is that some of the most horrible of the radioisotopes have decayed away. This means that the reactors are now less able to harm the general public. A further release of radioactivity from the reactors would still be bad if it occurs, but if it does then each day which goes by will make it less harmful.
As Japan’s nuclear safety authority has started to advise the public from the 20 km area around the reactor site to take stable iodine (KI, potassium iodide) I assume that some of the iodine from inside the reactors has escaped. I do not have any idea currently of how much iodine has escaped. But just becuase some iodine has escaped does not mean it has all got out of the reactors, even at Chernobyl much of the radioactive iodine failed to escape from the plant.
It is also important to bear in mind that a serious accident in a used fuel pond will not release any of these shortlived radioactive iodines. So if something occurs in one of the ponds then at least you can not be exposed to the shortlived iodines.
The isotopes which caused the greatest harm at Chernobyl to the general public were the shortlived radioactive iodines.
Iodine has a series of shortlived radioisotopes I-131 (half life 8 days), I-132 (2 hours), I-133 (20 hours), I-134 (53 minutes), I-135 (6.6 hours), I-136 (83 seconds) and some more neutron rich ones which are even more shortlived.
Due to the very short half lives of I-134 and I-136 these isotopes have decayed away totally now, I do not think it is likely that the general public could ever be exposed to these isotopes. The plants in Japan were held in a safe state by the back up power for about ten hours after their were shutdown. During this time the amounts of these very short lived iodines would have decayed to almost zero.
As one week has gone by since the reactors were shut down the amount of I-135 will have decayed to a very low level. The 25 half lives will have lowered the I-135 by a factor of 33554432 (33.6 x 10^6) which means to me that this isotope has gone from the cores.
The I-133 will also have decayed a lot, now after one week it has declined by a factor of more than 256, each day which goes by more than halfs the amount of this radioactive iodine so rapidly this isotope is vanishing from the cores.
The I-131 still has the potential to be a problem, it has a half life of 8 days so the reactors still contain about half as much of this isotope as they did at the moment of shutdown.
While I-132 has a short half life, it is constantly formed by the beta decay of Te-132 (half life 3.2 days) so the half life to watch here is that of the tellurium parent. The good news is that we have had over seven days since the reactors were shut down. Each week of time lowers the threat posed by both Te-132 and I-132 by a factor of 4.
Whatever happens now that so much time has gone by between the shutdown of the reactors and now means that even if something horrible happened in the reactors now then the iodine release would be smaller than what it could have been on day one.
I want to enpower you, rather than letting you sit there and stew as a passive person I want to give you the ability to do radioactive decay calculations. Rather than having to wait for someone else to do it, if you have a pocket calculator you will be able to do it for yourself.
If you want to make your own calculations on the decay of the radioisotopes then go to http://atom.kaeri.re.kr/, here you can get the half life of almost any isotope. The only isotopes missing from the data here are super exotic ones which are only seen in a few rare research labs which are devoted to the study of super shortlived isotopes.
Use the following maths
Activity now (A) = Activity at zero time (Ao) e– lambda t
Lambda = ln 2 / half life = 0.693 / half life
t is the time in the same units as the half life.
To make it more easy for you I have calculated lambda (decay constants) for the iodines
I-131, lambda = 0.086 days^-1
I-132, lambda = 7.25 days^-1
Te-132, lambda = 0.216 days^-1
I-133, lambda = 0.80 days^-1
I-134, lambda = 19 days^-1
I-135, lambda = 2.5 days^-1
I-136, lambda = 718 days^-1
The bad news is that the radioactivity of the fuel in the ponds will not decay away as quickly as the iodine in the reactor cores. The other bit of bad news is that late this week the level of the accident has gone up from 4 to 5 on the international nuclear accident scale. This means that the accident has changed from an “Accident with local consequences” to an “Accident with wider consequences“. This puts the accident in the same level of accident as the Windscale reactor fire and the Three Mile Island core melt.