First, I am moved by the destruction and devastation in Japan. My prayers are going out to the folks over there. The loss of life is much greater than the preliminary reports.
In a former life, I worked as a nuclear reactor operator so I think I understand the basics of what is occurring in Japan. Much of what is happening depends on the design of the plant. What is the percentage of enrichment of the fuel? What is the coolant used? (Likely very pure water but nuke plants can run with other types of coolant.) What is the operating temperature and pressure of the plant? What is design and placement of the control rods?
The plants where I worked is in some ways like the typical cooling system in a car. The engine is a heat source, water is pumped thru the engine and is warmed, it passes thru pipes to the radiator and is cooled and then the cooler water returns to the engine to be heated. The whole system is closed with an overflow source to allow for expansion and contraction of the water and this allows for the addition of more water to the system.
If the engine is running, ever just idling, and the water pump breaks, water ceases to circulate in the engine. As it heats—even under pressure—it will turn to steam. In a reactor when this happens the core can become uncovered, the fuel rods will distort and could leak fuel into the reactor. What can happen is an uncontrolled reaction that damages the reactor vessel and/or a steam explosion. The end result is an uncontrolled release of radioactive material.
To prevent such an occurrence, there are emergency generators on the site in case an external source of power is needed to run pumps for cooling during maintenance or emergency. Since the plant is on the coast, a last resort would be salt water. The plant can never be operated again if saltwater is used to cool the reactor compartment.
It appears that this option has been used to cool the reactor. When the water was pumped in and came in contact with the pressure vessel—which contains the nuclear fuel—a brittle fracture resulted, some water flashed to steam and the building which was structurally compromised by the earthquake collapsed. At this point the core is covered and the plant is a multimillion dollar pile of scrap.