Question:
Answer:
Your question is very broad, and difficult to answer without more specificity, however I will attempt to address some main points and hope that I answer the ones that are of interest to you.
This events that affected the power plant are two-fold, first earthquake damage and then tsunami damage. While information continually changes, these two events caused sufficient damage (at magnitude 9.0, it would be shocking if they didn't cause damage) to affect both primary safety systems and multiple redundant backup safety systems, such as diesel generators, battery reserves, and containment, structure, and cooling systems.
Let it be known, that none of these reactors failed in their emergency response during the first hours to first day (approx.). All 3 units that were online successfully carried out a SCRAM, thus shutting down the nuclear chain reaction. These reactors are designed to carry out a SCRAM both under manual command and by automatic trips. Furthermore a SCRAM is to take between 4 and 18 seconds, depending on the model and specific conditions. BWR reactors have control rods that insert from the bottom, rather than the top so there is no gravity drop, but other safety considerations have been made making up for this.
The cooling systems which are normally not needed as cooling is being achieved by the mechanism responsible for converting heat into steam which is used in to drive a turbo-generator. In an emergency however, the reactor has been shutdown, but the fuel remains very hot, and will increase in temperature if not actively cooled, due mostly to decay products generating heat as a byproduct. The cooling systems, 2 high pressure (High Pressure Coolant Injection System & the Automatic Depressurization System) work when the reactor is still at high pressure (for that reactor model) and 2 low pressure (Low Pressure Coolant Injection System & Core Spray System) work when the pressure is below a certain set point. It is important to note that immediately after a SCRAM, that the first cooling system to activate would be the HPCI, which requires no power to run, but it has limited volume and can only replace a certain amount of the cores operating volume. Recirculation pumps are expected to be working either in primary or auxiliary mode, and feedwater is assumed to available.
Again the devastation was so severe that what happened was a disconnect from the loop that allows for coolant water to be cooled (these reactors, due to size use a unique ocean heatsink, as opposed to the concave towers we are accustomed to in the U.S.), subsequently this caused a LOHS, or Loss Of Heat Sink. Even if power had been restored, there was no immediate way to restore the damaged loops and regain a path for the thermal energy dissipation.
The spent fuel pool, depending on how long since fuel had been placed in it, would take a considerable amount of time to drive of it's store of water, but it like a core, must be actively cooled. This was not happening for the same reasons previously described.
As a staying measure, extremely corrosive seawater was used to cool the core(s) and perhaps the spent fuel pool as well. These reactors will NEVER run again as the chemistry of seawater (a highly mineralized solution) reacting with essentially all the parts of the reactor render it unusable. Furthermore, while it may have seemed like a good idea, it actually speeds up the rate at which any undamaged fuel cladding is degraded. Make no mistake, a partial meltdown had already occurred, but using this type of water is damaging fittings, welds, valves, and most importantly, cladding that contains the fuel elements.
This water is now, as expected saturated with dissolved fission products, activation products, and will require treatment to remove the radionuclides. This is extremely expensive! An option is to hold the water for decay (1 year is more than sufficient) and then release the water far enough from populated areas in a very slow rate, establishing a progressive dilution. The more radioactive an isotope is, the shorter it's half-life, thus, holding for a 1 year will assure the most dangerous products have gone through many half-lives, and that the remaining products can be more easily disposed of or at least more easily handled.
That is the situation so far. Again, I hope I have addressed most of the key points.
