Lots is being said about the reactors involved in the Japan disaster. A lot of bad information is being circulated. Let me try to explain a little bit about what I believe has happened (based on 16 years of experience in PWR and BWR reactors).
The initial earthquake initiated a shutdown in the 5 reactors at 2 sites. In a BWR the primary cooling reaction is from steam generation that is used to drive the turbines, the steam is then cooled down via a secondary colling circuit (in this case seawater) through a condensor and then it is pumped back into the reactor to do it all again. After the shutdown, where the rods are pushed up into the reactor from the bottom via hydraulics and latched into position with massive spring loaded latches that require an electrical override to release, the tsunami hit and crippled the secondary cooling loop. Now the reactor couldn't cool down.
When a atom like Uranium splits, it creates two nearly equal fission products, all of which are usually radioactive. Essentually you get Krypton, Xenon, Iodine, Cesium and other isotopes in the same weight range. These fission products generate heat in a natural decay cycle that lasts several days for the most active and many thousands of years for the less reactive isotopes. Most of the heat is generated within 24-36 hours after shutdown. This decay heat is what is causing the problems, not a prompt-criticality or a runaway nuclear accident.
The only way to cool the reactor without the secondary cooling loops is by venting steam. Venting steam usually will go into the containment building which has cooling of its own to keep the pressure from getting too great. However, if the venting steam isn't balanced by injection of reactor coolant, then the level gets too low and the rods heat up to much. Zirconium is usualluy used for containing the nuclear fuel in rods. When Zirconium gets too hot it will cause water to dissociate into H2 and O2, which was then vented into the containment structure. All that was needed was a source of ignition to cause this explosive mixture to explode. This is what destroyed the contanment building, not a nuclear explosion. The reactor containment building is intact. There is no reactor fire as in Chernobyl neither could there be since they are flooding the reacotr with seawater now.
The prevailing winds are from the south and east this time of year in Japan, that means China, Cambodia, Vietnam, Russia and other countries on the Asia subcontinent get the joy of any fallout. The chance of much, if any, contamination getting into the jetstream is minimal.
The initial earthquake initiated a shutdown in the 5 reactors at 2 sites. In a BWR the primary cooling reaction is from steam generation that is used to drive the turbines, the steam is then cooled down via a secondary colling circuit (in this case seawater) through a condensor and then it is pumped back into the reactor to do it all again. After the shutdown, where the rods are pushed up into the reactor from the bottom via hydraulics and latched into position with massive spring loaded latches that require an electrical override to release, the tsunami hit and crippled the secondary cooling loop. Now the reactor couldn't cool down.
When a atom like Uranium splits, it creates two nearly equal fission products, all of which are usually radioactive. Essentually you get Krypton, Xenon, Iodine, Cesium and other isotopes in the same weight range. These fission products generate heat in a natural decay cycle that lasts several days for the most active and many thousands of years for the less reactive isotopes. Most of the heat is generated within 24-36 hours after shutdown. This decay heat is what is causing the problems, not a prompt-criticality or a runaway nuclear accident.
The only way to cool the reactor without the secondary cooling loops is by venting steam. Venting steam usually will go into the containment building which has cooling of its own to keep the pressure from getting too great. However, if the venting steam isn't balanced by injection of reactor coolant, then the level gets too low and the rods heat up to much. Zirconium is usualluy used for containing the nuclear fuel in rods. When Zirconium gets too hot it will cause water to dissociate into H2 and O2, which was then vented into the containment structure. All that was needed was a source of ignition to cause this explosive mixture to explode. This is what destroyed the contanment building, not a nuclear explosion. The reactor containment building is intact. There is no reactor fire as in Chernobyl neither could there be since they are flooding the reacotr with seawater now.
The prevailing winds are from the south and east this time of year in Japan, that means China, Cambodia, Vietnam, Russia and other countries on the Asia subcontinent get the joy of any fallout. The chance of much, if any, contamination getting into the jetstream is minimal.