Analysis Of No.4 Fukushima Reactor Pose The Greatest Danger

An explosion and fire at the pool containing spent fuel rods at the No. 4 reactor of the quake-stricken Fukushima No. 1 nuclear power plant Tuesday could prove more serious than explosions at two other reactors.

The fire at the pool could potentially pave the way for another explosion that could destroy the building that houses the reactor and release a large amount of highly radioactive materials into the atmosphere.
At the No. 4 reactor, the pool that contains spent fuel rods is located outside the nuclear pressure vessel and the containment vessel.
The only thing that separates the pool from the outside world is the concrete building that houses the reactor.
The spent fuel rods used at the reactor were stored in the pool, which sits next to the reactor.
The spent fuel rods require constant cooling with water because the residual heat is so high it is capable of producing the equivalent of tons of steam per hour.
The supply of water stopped after batteries used to run the water pumps ran out, apparently leaving the remaining water in the pool to evaporate due to heat.
As a result, the spent fuels were exposed and generated hydrogen from the alloy covering the spent fuel rods.
The hydrogen reacted with oxygen, causing the explosion.
To reduce the danger, water needs to be quickly pumped into the pool to cool the fuel rods.
The No. 4 reactor had been shut down before the quake for safety checks.
That would make it easier for workers to fix the cooling problem than the No. 1 and No.3 reactors.
But a highly radioactive substance is believed to have already been discharged into the atmosphere after the fire damaged the building.
If so, repairs could be extremely dangerous for workers, given the level of possible radiation exposure.
While experts say a catastrophic hydrogen explosion at the building is not likely, it must be prevented at all costs to keep the highly concentrated radioactive substances from spewing into the atmosphere.
The explosion Tuesday that may have damaged a device at the No. 2 reactor of the Fukushima No. 1 nuclear power plant could hamper the crucial task of securing water to cool the core, experts said.
“It all depends upon how to secure water from now,” said Hideo Kobayashi, a visiting professor of fracture mechanism at Yokohama National University.
He said if the device, known as a suppression pool, was damaged, concerns about radiation exposure could prevent workers from pumping seawater into the reactor’s pressure vessel to cool the core and bring the reactor under control.
The explosion rocked the No. 2 reactor at the plant in Fukushima Prefecture on Tuesday morning, apparently damaging the suppression pool, which is connected to the reactor’s containment vessel.


The suppression pool reduces pressure within the containment vessel by turning steam into water.
Keiji Kanda, a professor emeritus of nuclear material protection at Kyoto University, said if the pressure vessel’s structure stayed secure in the explosion, major damage to the highly radioactive nuclear fuel rods was unlikely, even if the suppression pool was knocked out.
“If the outer building that houses the reactor remains sound, the release of radioactive material into the air will be limited,” Kanda said. “It is urgent to prevent the situation from deteriorating further.”
Kobayashi, however, noted that the blast at the No. 2 reactor was “quite different from what hit the No. 1 and No. 3 reactors, where hydrogen explosions occurred in the buildings that house them.”
“Damage to the suppression pool means that part of the containment vessel has been broken,” he said.
Kobayashi said if water contaminated with high levels of radioactive substances should leak from the containment vessel, workers’ access to the reactor would be blocked due to risks of exposure.
Keiji Miyazaki, a professor emeritus of reactor engineering at Osaka University, said a steam explosion may have occurred at the suppression pool.
A steam explosion is a sudden transformation of water into vapor that occurs when part of a molten core structure touches water.
The cooling system at the No. 2 reactor failed Monday, causing the reactor’s water levels to fall. Subsequent work to pump in seawater was suspended when the pump’s fuel ran out, temporarily exposing all fuel rods measuring about 4 meters to high-temperature, high-pressure steam.
“There is a possibility that the bottom of the pressure vessel fell down to hit the suppression pool that contains water, triggering a steam explosion,” Miyazaki said.
If that was the case, the situation is more serious because it means the pressure vessel has also been damaged.

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