Regaining control of the four stricken reactors at the Fukushima No. 1 nuclear power plant could take months or years, according to nuclear experts.
And, even if the reactor cores can be cooled below 100 degrees, known as the "cold shutdown" stage, decommissioning will take several decades.
The March 11 earthquake and tsunami crippled the cooling system at the Fukushima plant. Since then, the plant's operator, Tokyo Electric Power Co. (TEPCO), has been using fire trucks and electric pumps to try to cool the cores.
In a properly functioning nuclear reactor of the type used at Fukushima, water is pumped into the reactor core and evaporates after contact with the fuel rods. That steam is condensed using seawater and recirculated, maintaining the pressure level within the core.
However, the workers at the Fukushima plant are currently only able to pump water into the system. Pressure within the core increases because of the resulting steam. If that pressure rises excessively, there is a danger of damage to the core pressure containers and containment vessels.
To prevent that, steam containing radioactive materials has been vented into the atmosphere to reduce core pressure. In addition, some of the water pumped into the cores appears to have leaked, raising concern that the sea near the plant may have been contaminated.
A quick restoration of the cooling system is considered essential to preventing further contamination.
External power has been restored to all four reactors and lights have been turned on in the central control rooms. After monitoring equipment is back up and running, the next task for TEPCO will be to identify what parts of the reactors have been damaged.
There is a strong chance that pumps and other cooling equipment would have been damaged. But, with high radiation levels being recorded in much of the nuclear plant site, the time available to workers to repair the cooling system is limited.
If the cooling system cannot be restored, TEPCO will have to continue pumping in water from external pumps. That would likely entail further leaks of radioactive material onto nearby land and into the sea.
Another key challenge will be removing radioactive water from the basements of turbine buildings next to the core buildings. The idea of constructing new storage pools to take the water has been suggested, but TEPCO still faces many months of stopgap measures to deal with the large volumes of water.
Even if a cold shutdown of the four reactors is achieved, a long process of removing radioactive materials from the cores would still lie ahead.
The key question will be what to do with fuel rods that are thought to have been seriously damaged by the malfunctioning cooling system. Officials will have to decide whether to try to remove and process the fuel rods or to simply bury the reactor cores, complete with the fuel, in concrete.
Those rods are likely to have been partially melted and may have been deformed by the cooling problems. There is also a good chance that plutonium and uranium have been released.
If a decision is made to try to remove the rods, the first task would be to clean up the area around the core, which has been contaminated with high levels of radiation, in order to allow workers to get to the core. Officials would also have to work out ways of safely removing the melted fuel rods.
Once the rods were removed, processing the fuel would then pose a major challenge. They would continue to release high levels of radiation for years.
There are currently no facilities in Japan where fuel can be buried deep underground.
There is a facility in Rokkasho, Aomori Prefecture, that can process spent fuel rods for reuse. But an official with Japan Nuclear Fuel Ltd., the operator of the Rokkasho facility, said, "We have never accepted fuel that has been heavily damaged (and which is releasing much greater levels of radiation than normal)."
After the Chernobyl nuclear accident in the former Soviet Union, that plant's No. 4 reactor was encased in concrete.
However, Keiji Miyazaki, professor emeritus of nuclear reactor engineering at Osaka University, warned that heat from the encased fuel could crack the concrete and release radioactive material into the atmosphere.
"(We should) use the decision made after the Three Mile Island accident in the United States to remove the fuel as a reference point. The fuel should be removed regardless of how much time, money and labor is required. We should work with other nations on this," Miyazaki said. "Because four reactors have been damaged, it will take a number of years to remove the fuel."
Tetsuo Itoh, who heads the Kinki University Atomic Energy Research Institute, said: "I believe a stone coffin would be the best way to prevent a further spread of radioactive material. However, we need to consider whether the fuel should be encased as is or whether it should be moved to a different location. That will depend on the extent of damage to the fuel."
Even an undamaged nuclear reactor takes decades to decommission. After the fuel rods are removed, all of the pipes to the core have to be sealed. The inactive reactor then has to be kept airtight for five to 10 years, allowing radiation levels within the core to fall. The core is then dismantled and removed. Finally, the building that houses the core is taken down. In order to stop radiation from leaking into the atmosphere, more contaminated parts of the building have to be removed before parts with low levels of radiation.
The problem at Fukushima is that the shells of the reactor buildings, which would normally be the last parts removed, have been blown apart by hydrogen explosions. Other parts of the buildings, such as the turbine rooms, have been contaminated with highly radioactive water. New structures will probably have to be erected to prevent further leaks.
Hironobu Unesaki, a nuclear engineering professor at the Kyoto University Research Reactor Institute, said, "Looking at past examples of dismantling reactor cores in Japan, it will take a minimum of between 20 and 30 years to complete the work and return the site to (being) an empty lot."
And, even if the reactor cores can be cooled below 100 degrees, known as the "cold shutdown" stage, decommissioning will take several decades.
The March 11 earthquake and tsunami crippled the cooling system at the Fukushima plant. Since then, the plant's operator, Tokyo Electric Power Co. (TEPCO), has been using fire trucks and electric pumps to try to cool the cores.
In a properly functioning nuclear reactor of the type used at Fukushima, water is pumped into the reactor core and evaporates after contact with the fuel rods. That steam is condensed using seawater and recirculated, maintaining the pressure level within the core.
However, the workers at the Fukushima plant are currently only able to pump water into the system. Pressure within the core increases because of the resulting steam. If that pressure rises excessively, there is a danger of damage to the core pressure containers and containment vessels.
To prevent that, steam containing radioactive materials has been vented into the atmosphere to reduce core pressure. In addition, some of the water pumped into the cores appears to have leaked, raising concern that the sea near the plant may have been contaminated.
A quick restoration of the cooling system is considered essential to preventing further contamination.
External power has been restored to all four reactors and lights have been turned on in the central control rooms. After monitoring equipment is back up and running, the next task for TEPCO will be to identify what parts of the reactors have been damaged.
There is a strong chance that pumps and other cooling equipment would have been damaged. But, with high radiation levels being recorded in much of the nuclear plant site, the time available to workers to repair the cooling system is limited.
If the cooling system cannot be restored, TEPCO will have to continue pumping in water from external pumps. That would likely entail further leaks of radioactive material onto nearby land and into the sea.
Another key challenge will be removing radioactive water from the basements of turbine buildings next to the core buildings. The idea of constructing new storage pools to take the water has been suggested, but TEPCO still faces many months of stopgap measures to deal with the large volumes of water.
Even if a cold shutdown of the four reactors is achieved, a long process of removing radioactive materials from the cores would still lie ahead.
The key question will be what to do with fuel rods that are thought to have been seriously damaged by the malfunctioning cooling system. Officials will have to decide whether to try to remove and process the fuel rods or to simply bury the reactor cores, complete with the fuel, in concrete.
Those rods are likely to have been partially melted and may have been deformed by the cooling problems. There is also a good chance that plutonium and uranium have been released.
If a decision is made to try to remove the rods, the first task would be to clean up the area around the core, which has been contaminated with high levels of radiation, in order to allow workers to get to the core. Officials would also have to work out ways of safely removing the melted fuel rods.
Once the rods were removed, processing the fuel would then pose a major challenge. They would continue to release high levels of radiation for years.
There are currently no facilities in Japan where fuel can be buried deep underground.
There is a facility in Rokkasho, Aomori Prefecture, that can process spent fuel rods for reuse. But an official with Japan Nuclear Fuel Ltd., the operator of the Rokkasho facility, said, "We have never accepted fuel that has been heavily damaged (and which is releasing much greater levels of radiation than normal)."
After the Chernobyl nuclear accident in the former Soviet Union, that plant's No. 4 reactor was encased in concrete.
However, Keiji Miyazaki, professor emeritus of nuclear reactor engineering at Osaka University, warned that heat from the encased fuel could crack the concrete and release radioactive material into the atmosphere.
"(We should) use the decision made after the Three Mile Island accident in the United States to remove the fuel as a reference point. The fuel should be removed regardless of how much time, money and labor is required. We should work with other nations on this," Miyazaki said. "Because four reactors have been damaged, it will take a number of years to remove the fuel."
Tetsuo Itoh, who heads the Kinki University Atomic Energy Research Institute, said: "I believe a stone coffin would be the best way to prevent a further spread of radioactive material. However, we need to consider whether the fuel should be encased as is or whether it should be moved to a different location. That will depend on the extent of damage to the fuel."
Even an undamaged nuclear reactor takes decades to decommission. After the fuel rods are removed, all of the pipes to the core have to be sealed. The inactive reactor then has to be kept airtight for five to 10 years, allowing radiation levels within the core to fall. The core is then dismantled and removed. Finally, the building that houses the core is taken down. In order to stop radiation from leaking into the atmosphere, more contaminated parts of the building have to be removed before parts with low levels of radiation.
The problem at Fukushima is that the shells of the reactor buildings, which would normally be the last parts removed, have been blown apart by hydrogen explosions. Other parts of the buildings, such as the turbine rooms, have been contaminated with highly radioactive water. New structures will probably have to be erected to prevent further leaks.
Hironobu Unesaki, a nuclear engineering professor at the Kyoto University Research Reactor Institute, said, "Looking at past examples of dismantling reactor cores in Japan, it will take a minimum of between 20 and 30 years to complete the work and return the site to (being) an empty lot."
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