The Yomiuri Shimbun
Tokyo Electric Power Co. is considering alternative methods, including the construction of improvised systems, to cool reactors at the crippled Fukushima No. 1 nuclear power plant.
Ongoing operations--which involve pouring water directly into the reactor cores to lower temperatures inside the pressure vessels of the Nos. 1 to 3 reactors to below 100 C--have not worked as expected, and restoration of the reactors' existing cooling systems is not likely to happen soon.
The discouraging outlook has prompted TEPCO to begin exploring new options.
The original cooling systems worked by circulating water inside the pressure vessels. After absorbing heat in the pressure vessels, water was extracted and cooled by heat exchangers before being returned to the pressure vessels.
The damaged systems are now unable to circulate water, so new water must be continually added to the pressure vessels to control temperatures.
However, because the water sources used in the continual injections are positioned lower than those incorporated in the original systems, it is unclear whether fuel rods in the reactors are being entirely covered with water.
In addition, water contaminated with radioactive substances has apparently leaked from the pressure vessels, which means TEPCO cannot increase the amount of water being injected.
On March 23, new water was being channeled into the No. 1 reactor at a rate of 300 liters per minute. The current rate is about one-third this amount.
The surface temperature of the No. 1 reactor pressure vessel is still above 200 C. The temperature of nuclear fuel inside the vessel is much higher.
A TEPCO official said: "We're currently removing heat that's being generated by the fuel. We haven't been able to remove the huge amount of heat that accumulated while the reactors were in operation."
TEPCO's immediate goal is to restore the power supply to the reactors and the existing cooling systems.
But key elements of those systems are located in the basement floors of the reactor buildings, areas that are contaminated with radioactive materials, and this is forcing TEPCO to consider other options.
One proposal is to build makeshift cooling systems. TEPCO would have to access pipes connected to the pressure vessels at some point outside the reactor buildings where radiation levels are low. From there, the company could connect the pipes to new heat exchangers and pumps.
Another option under consideration is trying to lower the temperatures of the pressure vessels from the outside, by filling the containment vessels that surround them with water or some other coolant. The success of this plan would depend on whether the containment vessels have been damaged.
"It might be wise to set up new cooling systems," said Michio Ishikawa, the top adviser of the Japan Nuclear Technology Institute. "If the current conditions continue, the amount of radioactive substances being discharged from the reactors might increase. Therefore, all possible options need to be considered."
(Apr. 8, 2011)
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