2011/03/20
More than 4,500 spent fuel rod bundles must be cooled in six reactors at the Fukushima No. 1 nuclear power plant, according to information released by Tokyo Electric Power Co.
A total of 4,546 bundles of fuel rods are stored in the six reactors, with each bundle containing several dozens of fuel rods, which in turn hold hundreds of fuel pellets.
The spent fuel rods release large amounts of heat. And since the cooling systems at the reactors were damaged in last week's Great East Japan Earthquake and subsequent tsunami, there is the danger of damage to the fuel rod bundles from overheating if water in the storage pools evaporates.
The amount of heat being emitted from the No. 4 reactor is especially high. A total of 2 million kilocalories per hour is being emitted, about three times the combined heat coming from the No. 1 to No. 3 reactors.
In addition to 783 spent fuel rod bundles, 548 bundles that have not been completely used are stored at the No. 4 reactor. Those unused fuel rods had been removed from the core in order to replace equipment.
The large amount of heat being released from the No. 4 reactor is due to the fact that the fuel rods not completely used emit more heat than spent fuel rods.
The storage pool at the No. 4 reactor has a capacity of about 1,400 cubic meters. Based on the level of heat being emitted from the fuel rods, a simple calculation shows that the water temperature in the pool rises about 2 degrees every hour.
If the cooling mechanism can be repaired to resume operations, the water temperature can be maintained at under 40 degrees.
But if the tsunami damage makes the cooling mechanism inoperable, it would take about a day for water in the pool to begin boiling due to the heat from the fuel rods. It would take an additional 10 days or so to completely evaporate all the water in the pool.
TEPCO employees said they saw what looked like water in the storage pool of the No. 4 reactor from a helicopter that flew over the plant on Thursday.
The spent fuel rods stored at the other reactors emit much lower levels of heat. The level emitted depends on the capacity of the storage pool and the number of fuel rod bundles stored.
The storage pool at the No. 3 reactor holds about half of the capacity for fuel rod bundles. Efforts to spray water on the No. 3 reactor from firefighting trucks and helicopters have continued since Thursday.
Because the No. 3 reactor only has 514 fuel rod bundles stored in its pool, the amount of heat emitted from the fuel rod is only about 10 percent of the heat from the No. 4 reactor.
However, white smoke has been spewing since Wednesday from the damaged outer building housing the No. 3 reactor, so TEPCO workers have been unable to confirm the condition of the storage pool.
The No. 5 reactor has the second-highest level of heat emitted from spent fuel rods. While the spent fuel rods take up about two-thirds of the pool capacity, the amount of heat emitted is only one-third that from the No. 4 reactor.
Unlike the No. 1 to No. 4 reactors, the pump that circulates water to the storage pool still works in the No. 5 reactor.
The water temperature in the pool was maintained at 66.3 degrees as of 2 p.m. Friday.
However, the cooling mechanism at the No. 5 reactor does not work due to the power outage caused by the earthquake.
Although a restoration of the cooling mechanisms could lower the water temperatures in the storage pools, there is still the question of the cooling mechanisms for the reactor cores.
One problem that TEPCO may be facing is malfunctioning monitors.
Data about water levels in the pressure containers of the cores released by TEPCO indicate that about one-half to one-third of the length of the fuel rods in the core is exposed above water.
However, even though seawater has continued to be pumped into the No. 1 reactor core, the water level data has not changed at all, indicating the monitoring equipment may have malfunctioned.
TEPCO workers have no way of confirming if the equipment is working properly.
Moreover, readings of the pressure within the suppression pool under the containment vessel in the No. 2 reactor have stopped since Tuesday. The suppression pool may have been damaged by an explosion Tuesday morning.
While the reason is unknown, readings of the pressure within the suppression pool in the No. 3 reactor have also stopped since Monday night.
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