The fuel rods have an uranium core and a zirconium cladding which cannot be exposed to air so you need another 16 feet of water above the reactor to lift the fuel rods vertically and drop them into the storage pool. Next you need a crane above the water with space above and below. Finally you need a roof. As you can see each design step leads to a higher structure. It’s also very rigid due the structural components and has a lot of mass at the top. This has a lot of inertia. You have just constructed the worst type of building for an earthquake zone – tall, rigid and high inertia which means that as the bottom moves laterally the top will tend to not move and tremendous strain develops in the structure. This is bad enough but it gets worse. Over time the cumulative effects of damage caused by neutron irradiation to metal components include swelling (volume increase), irradiation hardening, and irradiation embrittlement (the influence of irradiation hardening on fracture toughness). The Fukushima reactors were built in the seventies. This means the primary containment vessel which is made of metal is at risk of rupture even under normal pressure. In this design the vessel is only removed on the decommissioning of the plant.
Unfortunately GE wanted to reduce the cost of construction in order to compete with its competitors so they made a pressure suppression design using a torus half filled with water. At higher pressure the water in the torus would condense the steam so lower pressures (1000 psi rather than 2000 psi) would exist in the primary containment. Lower pressure meant thinner metal containment and concrete secondary containment which in turn reduced the construction costs.
People have made comparisons with Chernobyl.
Chernobyl had 200 tons of enriched uranium and Fukushima has 1800 tons if you include all of the reactors and storage pools. I find it ironic that 福島市, or Fukushima-shi means "good-fortune Island”. Dr. Gerhard Wotawa of the Austrian Institute said the iodine 131 released from Fukushima in the first three-four days was about 20 percent of that released from Chernobyl during a ten-day period based on measurements made at monitoring stations in Japan and the United States.
For Caesium-137, the figure could amount to some 50 percent of the amount released at Chernobyl. Pouring sea water onto the rods has several drawbacks. The cold water causes the zirconium cladding on the rods to crack if they are hot releasing radioactive uranium and fission products. The salt from the evaporating sea water coats the rods and acts as a thermal insulation increasing their temperature. The salt coating also reduces water flow through the reactor increasing the temperature. If the zirconium cladding gets too hot then it reacts with the water producing hydrogen which can explode and the zirconium can ignite with the oxygen to melt the uranium inside.
Types of radioactive isotopes released from Chernobyl versus days
P.S. A little later, I will publish a post on the social and cultural ramifications of nuclear exposure in Japan.