The walkway workers used to reach the No. 1 and No. 2 reactors at the Fukushima No. 1 nuclear power plant was destroyed by the March 11 tsunami. (Nuclear and Industrial Safety Agency)
Another, possibly more serious, venting of radioactive materials into the atmosphere remained a possibility late Thursday as workers struggled to bring an unstable reactor under control at Fukushima No. 1 power plant.
According to officials, the No. 1 reactor at the plant was exhibiting unstable temperatures and pressures within the core.
In the worst-case scenario, steam containing radioactive materials would have to be released, according to officials of Tokyo Electric Power Co., the operator of the Fukushima plant, and the Nuclear and Industrial Safety Agency (NISA).
White smoke believed to be steam was confirmed coming from the storage pool for spent fuel rods of the No. 1 reactor Thursday morning.
Seawater is being pumped into the No. 1 reactor core to cool the decay heat of the fuel rods.
However, on Tuesday, the temperature in the core at one time rose to about 400 degrees, about 100 degrees over the design limit. While exceeding the limit will not lead immediately to damage to the core, the amount of water pumped in was increased from early Wednesday as a precaution.
While the temperature fell to about 218 degrees by Thursday afternoon, pressure within the containment vessel began to rise.
Pressure within the pressure container may have risen after the pumped-in seawater turned to steam and that may have led to steam moving into the containment vessel.
According to NISA officials, the temperature and pressure within the No. 1 reactor appeared to have stabilized Friday. NISA officials said the temperature of the No. 1 reactor core was 204.5 degrees as of 6:10 a.m. Friday.
In the worst-case scenario, the containment vessel may become damaged by the steam pressure and that could lead to the release of large amounts of radioactive materials.
To avoid that possibility, TEPCO officials are considering ventilation to lower the pressure by releasing steam into the outside atmosphere.
While that is a measure that TEPCO officials want to avoid because the steam contains radioactive materials, it has been used a number of times after the Great East Japan Earthquake on March 11 damaged the Fukushima No. 1 plant.
According to NISA officials, the No. 1 reactor was ventilated once on March 12, while the No. 2 reactor was ventilated on March 13. The No. 3 reactor was ventilated three times, on March 12, 13 and 14.
However, those ventilations were conducted in a way to reduce the effects on the environment.
The steam was first sent to the suppression pool under the pressure container and made to pass under water in the pool before being released.
That process is believed to reduce the level of radioactive iodine released in the steam to about 1 percent of the original level.
If the pressure cannot be reduced by that process, TEPCO officials would be forced to turn to dry ventilation in which the steam does not pass under water. That would mean a much larger release of radioactive materials.
An attempt to reduce pressure within the No. 2 reactor was made on March 15, but it is unclear if steam was actually released.
Because there is the possibility that part of the containment vessel of the No. 2 reactor may have been damaged on March 15, there is the possibility steam was ventilated without passing under water.
An increase in radiation levels due to such ventilations could affect the work to restore functions in other reactors if workers have to be evacuated.
Because the steam is released from a ventilation tower about 120 meters high, TEPCO officials will also have to take into consideration wind direction before deciding to release the steam.
A NISA official said Thursday about releasing steam from the No. 1 reactor, "We will make a decision after taking into consideration the pressure, temperature, the pumping of seawater and the status of the core. In the end, the prime minister will have to give his approval."
Meanwhile, other experts said that even in the best-case scenario, it would take at least one month to bring the No. 1 to No. 3 reactors to a cold shutdown in which the temperature of the cooling water falls under 100 degrees.
To achieve a cold shutdown requires two pumping systems to work, one to circulate water into the core and the other to cool the heated water by using seawater.
However, there is a strong possibility that the pumps in the No. 1 and No. 3 reactors are damaged. In that case, because sufficient water could not be supplied, seawater may have to be pumped in for several months.
Radioactive materials would be released during that period.
According to TEPCO sources, a greater degree of precision control would be required the larger the pump was. Moreover, the pumps must also be cooled at the same time. The pipes to the cores have many valves.
Even if sensors could be used to let workers know the status within the core, a source said, "Properly operating the cooling mechanism would be many times more difficult than pumping in water."
Workers have already run into problems trying to confirm if the pumps actually work.
On Wednesday, high radiation levels were detected at the No. 2 reactor, forcing workers to evacuate.
On Thursday, two workers at the No. 3 reactor had to be taken to the hospital after being exposed to radiation.
Following the exposure to radiation by the workers, consideration was being given to processing the water that contained high levels of radiation as well as changing the location where power cables would be installed.
Moreover, if the hydrogen explosions at the No. 1 and No. 3 reactor which damaged the outer building also damaged the pumps, the pumps would have to be replaced.
Even if replacement pumps were available, installing them under high radiation levels would be a difficult task.
When an accident occurred in 1979 at the Three Mile Island plant in the United States, it took about three weeks after the accident to achieve a cold shutdown.
Because of the damage to the four reactors at the Fukushima plant from the tsunami and hydrogen explosions, Hiroaki Koide, an associate professor at Kyoto University's Research Reactor Institute, said, "The conditions are much more difficult. Achieving a cold shutdown in one month would be very optimistic."
Fuel rods in the core continue to give off decay heat from the elements that have decayed after nuclear fission.
According to calculations by Koide, decay heat has decreased to minimal amounts compared to when the reactor was actually operating, but the fuel rods still have about 6,000 kilowatts of energy in the No. 2 and No. 3 reactors. The energy will only gradually reduce to about 3,000 kilowatts after six months and to 2,000 kilowatts after one year.
Koide said, "To achieve cold shutdown will require dissipating the heat from the core into the ocean by using the two pumping systems, but the work will be time-consuming because of an environment of high radiation levels."
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