Explosions at units 1 and 3 occurred due to similar causes. When an incident occurs in a nuclear power plant such as a loss of coolant accident or when power is lost, usually the first response is to depressurize the reactor. This is done by opening pressure relief valves on the reactor vessel. The water/steam mixture will then flow down into the suppression pool, which for this design of a reactor is in the shape of a torus (technical term for the shape of a donut). By blowing the hot steam into the suppression pool some of the steam is condensed to liquid phase, which helps keep the pressure low in the containment.
The pressure in the reactor vessel is reduced by venting the water/steam mixture. It is much easier to pump water into the vessel when it is at a reduced pressure, thus making it easier to keep the fuel cooled. This procedure was well underway after the earthquake. Unfortunately, because of the enormous magnitude of the earthquake, an equally large tsunami was created. This tsunami disabled the onsite diesel generators as well as the electrical switchyard. Without power to run pumps and remove heat, the temperature of the water in the reactor vessel began to rise.
With the water temperature rising in the core, some of the water began to vaporize and eventually uncovered some of the fuel rods. The fuel rods have a layer of cladding material made of a zirconium alloy. If zirconium is hot enough and is in the presence of oxygen (The steam provides the oxygen) then it can undergo a reaction that produces hydrogen gas. Hydrogen at concentrations above 4% is highly flammable when mixed with oxygen; however, not when it is also in the presence of excessive steam.
As time went on, the pressure in the containment rose to a much higher level than usual. The containment represents the largest barrier to the release of radioactive elements to the environment and should not be allowed to fail at any cost. The planned response to an event like this is to vent some of the steam to the atmosphere, just to keep the pressure under control.
Exactly what happened next is not verified; however, the following is very likely the general explanation for the explosion. It was decided to vent the steam through some piping that led to a space above and outside containment, but inside the reactor building. At this point, the steam and hydrogen gas were mixed with the air in the top of the reactor building. This was still not an explosive mixture because large amounts of steam were mixed with the hydrogen and oxygen (from the air). However, the top of this building is significantly colder than inside the containment due to the weather outside. This situation would lead to some of the steam condensing to water, thereby concentrating the hydrogen and air mixture. This likely went on for an extended period of time, and at some point an ignition source (such as a spark from powered equipment) set off the explosion that was seen in units 1 and 3. The top of the reactor building was severely damaged; however, the containment structure showed no signs of damage.
Right after the explosions there were spikes in the radiation levels detected, because there were some radioactive materials in the steam. When the zirconium alloy cladding reacted to make hydrogen, it released some fission products. The vast majority of the radioactive materials in the fuel will remain in the fuel. However, some of the fission products are noble gases (xenon, Xe and krypton, Kr) and will immediately leave the fuel rods when the cladding integrity is compromised. Fortunately, Xe and Kr are not a serious radiological hazard because they are chemically inert and will not react with humans or plants. Additionally, small quantities of iodine (I) and cesium (Cs) can be entrained with the steam. When the steam was vented to the reactor building, the Xe and Kr would have followed as well as some small amounts of I and Cs. Thus, when the roof of the reactor building was damaged , these radionuclides that were in the reactor building would have also been released. This is the reason a sudden spike was seen in radiation levels. These heightened radiation levels quickly decreased. This is because there was no damage to the containment which would increase the quantities of radionuclide released, and because the radionuclides released during the explosion quickly decayed away or dispersed.
Unit 2 explosion
Recent information indicates that unit 2 may have suffered a containment breach. Pressure relief of unit 2 was complicated by a faulty pressure relief valve, which complicated the injection of sea water and the evacuation of the steam and hydrogen. It is reported that the fuel rods were completely exposed twice. More details to follow.
Unit 4 fire
A fire was reported at unit 4 which was in a shutdown state during the earthquake and tsunami for a planned outage. Latest reports indicate that the fire was put out. More details to come.