Returning to this diagram, note the orange structure near the top of the building. It runs on tracks. The front track is cut away, but you can see that the back track runs the length of the building. That orange device is used to move the top of the reactor away when the bolts have been removed. Once the Reactor Core is exposed, the orange crane is used to add and remove Fuel Rod Assemblies. The Fuel Rods that are not in use (Expended, but Highly Radioactive) are stored in tanks on the top enclosed floor of the reactor building. You can see the tanks for fuel storage there on the fourth floor of the building.
Those tanks are called the Spent Fuel Pools. Contingent with the melt down in the Reactors, these Spent Fuel Pools (SFP) were allowed to boil dry or boil down so that the fuel assemblies were exposed to atmosphere. UNGOOD!.
The fuel rods are Uranium and Plutonium that is formed into pellets and compressed in Zirconium cladding. The Cladding keeps most the radioactive particulate contained as the rods are in use in the reactor and covered by water. Once out of the reactor they have to cool down from delayed reactions inside the fuel (the pellets) and are allowed to sit in those tanks for up to year. After they have cooled, they can be stored elsewhere and are eventually placed in casks for long term storage.
If the cooling water in the tanks is allowed to boil dry, the rods can heat up to glowing temperatures. When Zirconium gets hot enough, (close to two thousand degrees) it can rapidly oxidize (essentially burn with the oxygen in the air) and breakdown. That ends up releasing the highly radioactive Uranium and Plutonium and Strontium and Cesium and other radioactive particulate to the atmosphere. If we believe many of the experts, a lot of the fuel in the spent fuel pools was released to the atmosphere because of a lack of cooling of those stored rods.
Fukushima was an engineering disaster. They had a 20 foot Quaywall that the sea water could flow right through. An actual viable sea wall to prevent tsunami damage really did not exist. Look at the photographs of the damaged reactor site to see this. The plants were at least slightly elevated above sea level. The Tsunami at sea was measured at 4.4 meters, but you don't design a nuclear reactor plant for the sea height of a tsunami.
The issue that should have been of concern was the inertia caused "run up" of the wave. If you have ever sat on the sea shore you have seen waves run up on the beach. They will frequently run up far higher than the initial height of the wave as it approached the beach. In Japan they have documented run ups of half a hundred feet or more from tsunamis just a few meters high. There was no design of the plant to prevent that run up. I am serious. They did not plan for the run up from a four meter tsunami.
When you consider that the most dangerous thing that can happen to a nuclear reactor plant is a loss of electricity, Fukushima was just waiting for a colossal disaster. They had one effective source of commercial electricity to the plant because the plant was supplied by one transmission tower inland that collapsed during the Earthquake. (San Onofre in San Diego County has/had the same damn problem. I brought it to their attention and hopefully they have corrected it. The commercial electrical power came in from the south - San Diego.)
Total idiocy on the part of the Japanese. Just think, for forty years not one engineer in Japan realized that they had one possible "point of failure" in that one tower. (Is there another word for stupidity beyond stupid?)
The Japanese placed their Emergency Diesel Generators down in the ground in their turbine rooms that were built right next to the sea. When the tsunami came rolling in, the turbine rooms were flooded and the Emergency Diesels were under water and that water was retained there because they were in an underground pool close to sea level. What engineer ever thought of that as a viable design?
From that moment on a total meltdown was assured. No electricity = No pumps = No cooling of the reactors = TOTAL MELTDOWN!
Their electrical connections to other pumps that could have been used to help cool the reactors were all designed in such a manner that they were damaged by sea water. It never occurred to any of the Japanese engineers that such an accident could happen. Most of their important electrical cable runs were built in the floor, and eventually ended up UNDER SEA WATER! Unbelievable!
When they needed to release steam from the reactors, the valve that operated the steam release to atmosphere was found to be designed to fail closed upon loss of electricity. To open it would take a suicide mission to manually operate the valve to release the steam. Anybody assigned that take would have died a horrible painful death from radiation exposure.
Where were the engineers? Where were the inspectors who should have found all of this poor engineering and ordered it corrected before Fukushima 1 was allowed to be put into operation?
Unbelievable!