March 31, 2011 N-plant has long road ahead / Experts: Decommissioning ********* reactors to take decades
- The Yomiuri Shimbun http://www.yomiuri.co.jp/dy/national/T110331006033.htm
Nuclear experts predict it will take decades to complete the decommissioning of the Nos. 1 to 4 reactors at the ************* No. 1 nuclear power plant.
Tokyo Electric Power Co. Chairman Tsunehisa Katsumata said Wednesday the company will decommission the four reactors, but [b]the most pressing task at the moment is how to dispose of the huge quantity of water that has become contaminated with radioactive materials after being used to cool the reactors. Just disposing of this water will take a long time.
An estimated 13,000 tons of contaminated water has accumulated in trenches--tunnels used for maintenance of the reactors. A large quantity of contaminated water also has to be extracted from the basements of the reactors' turbine buildings, although the exact amount is unknown.
If the contaminated water can be removed, it will pave the way to reactivating the reactors' original cooling systems, which can lower the temperature of the reactor cores more efficiently than the methods now being employed.
Currently, however, workers at the plant are stymied by the contaminated water. They cannot even connect power cables outside the plant to the reactors' control systems.
It may be impossible to restore power to the reactor control systems if internal radiation levels are so high workers cannot repair the machinery, or if the contaminated water cannot be removed.
If water continues to leak, external tanks for temporarily storing it may become full. Workers and experts have said new facilities to store the contaminated water must be secured as soon as possible.
If all the contaminated water can be removed, the reactors then must be put in what is called cold shutdown to prevent the further discharge of large quantities of radioactive substances and bring the reactors into a stable state.
Cold shutdown means all control rods have been inserted into the reactors to stop nuclear fission chain reactions, and the coolant water inside the reactors is below 100 C.
Usually the temperature needs to be lowered further to remove fuel rods for regular checks or decommissioning.
"If the original cooling systems can be activated through a power supply from outside the plant and coolant water circulated, cold shutdown can be achieved in a day or two," Prof. Kenichiro Sugiyama of Hokkaido University said.
But it will likely take a few more years for the nuclear fuel rods to be cool enough to be removed from the reactors to decommission them.
On the other hand, if the current method--putting coolant water into the reactors with makeshift pumps--continues to be used, the situation may become more serious.
"Although the nuclear fuel would cool gradually, it would take at least several months to achieve cold shutdown," said Toru Ebisawa, a former associate professor of Kyoto University's Research Reactor Institute.
This would mean using more water, which would increase the amount of contaminated water.
Currently, they need a way to remove and dispose of bulk contaminated water. This exact question has plagued nuclear power in general for its entire history. Japan does not have vast deserts where they can bury nuclear waste. It raises the question of whether the cost of safely
exporting and disposing of nuclear waste is properly considered when making decisions on the ecomnomic viability of nuclear power.
Certainly, disposing of this amount of waste, under these conditions, considering time restraints and the amount of public scrutiny, is an unprecedented challenge.
From the same article, the following reports an enlightening history of the nuclear power plant decommissioning process:
Overall, it will take decades to complete the process of decommissioning the reactors.
The Japan Atomic Power Co.'s Tokai plant in Ibaraki Prefecture was the first commercial nuclear power plant in Japan to begin being decommissioned. The plant ended commercial operations in 1998, and the decommissioning process is scheduled to end in 2021.
In the decommissioning process, fuel rods are removed and workers wait for levels of radioactivity to fall. During that time, power generators and other equipment with low levels of radiation contamination are decommissioned first.
In the final stage, reactors' steel containers and other equipment are cut into pieces and buried deep underground. At the Tokai plant, heat exchangers and other parts are now being removed.
But in the case of the ********* No. 1 nuclear power plant, in which reactors and buildings were damaged, it is doubtful whether the normal process of decommissioning will be possible.
Shojiro Matsuura, former chairman of the Nuclear Safety Commission, said, "This time, much more time and effort will be needed to lower radiation contamination levels. Twenty or 30 years probably won't be enough."
In the 1979 Three Mile Island accident in the United States, it took a month to achieve cold shutdown and six to seven years to remove melted fuel rods.
Fourteen years later, U.S. authorities declared the decommissioning process complete. Part of the nuclear fuel could not be recovered and remained in the reactors.
A long road indeed. Edited, Mar 31st 2011 3:27pm by SmashingtonWho