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Figure 2 shows the typical rate at which heat from a shut-down reactor core boils away water when the cooling systems are not functio ning. The vertical axis shows the boil-off rate in gallons per minute. The horizontal axis shows the time, in days, since the reactor was shut down.
Even a week after being shut down, the heat from a reactor core boils water at a rate of nearly 60 gallons per minute. The boil-off rate declines with time while the rate of radioactive release decreases.
The reactor core resides in the lower portion of the reactor pressure vessel (see Figure 3).
The normal water level inside the reactor vessel is roughly 196 inches, or 16 feet, above the top of the reactor core. The nuclear fuel is protected against damage caused by overheating as long as it remains covered with water.
A rough rule of thumb is that it takes 200 gallons of water to raise or lower the level inside the reactor pressure vessel by 1 inch.
For the boil-off rate of 60 gallons per minute a week after shut down, it takes around 200 seconds for the water level inside the reactor pressure vessel to drop an inch. If the reactor cooling system fails one week after the reactor was shut down with the water level inside the reactor vessel is at its normal level,
it would take approximately 11 hours for boil-off to reduce the water level down to the top of the reactor core.
Without cooling, the boil-off would continue to drop the water level below the top of the nuclear fuel in the reactor core. As nuclear fuel was uncovered by water, it would begin heating up. As the fuel temperature increased to 1,800°F, a chemical reaction between the metal cladding of the fuel rods and the steam flowing past would generate large quantities of hydrogen. If the heat-up continued past 2,200°F, the exposed fuel would begin to melt.
This process appears to have produced the hydrogen that leaked from the reactor vessel and caused the explosions in the reactor buildings of Units 1 and 3. In addition, Japanese Electric Company officials have said they expect that some of the fuel has been uncovered long enough to have melted.
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