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Thursday, Sep. 29, 2011
Reactor temperatures said near cold shutdown
Tokyo Electric Power Co. reported Wednesday that the bottom of all three crippled reactors' pressure vessels at its Fukushima No. 1 nuclear plant were now below 100 degrees — a collective first since the crisis erupted after the March 11 temblor-tsunami catastrophe.
The temperature of reactor 2, which had remained over 100 degrees, had fallen to 99.40 as of 5 p.m. Wednesday.
The temperature for reactor 1 has been under 100 degrees since late July, and the same condition was reached for reactor 3 on Sept. 5.
Lowering the temperature of the bottom of the pressure vessels below 100 degrees is a key condition for achieving their cold shutdown, which the government and Tepco seek to establish by year's end.
"Although the temperature (of reactor 2) has been showing a falling tendency overall, the temperature of the bottom of the pressure vessel has gone up and down, so it is still too early to (make a definitive conclusion)," Tepco spokesman Junichi Matsumoto told an evening press conference at the utility's headquarters.
Tepco has been employing a sprinkler system approach, using pipes to spread an increasing amount of coolant water over the melted core of reactor 2.
As of 5 p.m. Wednesday, the temperatures for reactors 1 to 3 were hovering between 78 and 79 degrees.
In regular reactor operations, a cold shutdown means the reactor-core coolant temperatures are under 100 degrees.
But because reactors 1, 2 and 3 are damaged and their fuel rod cores melted down, the government redefined the concept of their cold shutdown. One condition is to cool the temperature of the bottom of the pressure vessel below 100 and another is to keep the radiation leakage from the three reactors under 1 millisievert per year around the plant.
On Sept. 20, Tepco estimated the annual radiation level around the plant at 0.4 millisievert, although it said measurements were needed at more locations.
This means Tepco may have achieved its cold shutdown goal, but Matsumoto admitted further monitoring is needed because the Nuclear and Industrial Safety Agency is expected to take more time to evaluate whether this status can be declared.
Even if a cold shutdown is achieved, the government and Tepco are still unsure of the extent of the melted fuel cores and of the reactor damage.
Thus experts say there is still a long way to go before an end to the nuclear crisis can be declared, even if the cold shutdown is achieved.