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Decommissioning Plan of Fukushima Daiichi Nuclear Power > Plan & Action > Contaminated water processing > Measures for Water Leakage > Questions on the "Ice Wall"

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Questions on the "Land-side Impermeable Wall (Frozen Soil Wall)"

1. PURPOSE

Q: Why is TEPCO building a land-side impermeable wall (frozen soil wall)?
A: The purpose of the wall – which will be formed by freezing the soil – is to prevent groundwater from entering the four damaged reactor buildings. This will prevent the water from becoming contaminated inside those buildings.
A video explaining about the land-side impermeable wall (frozen soil wall) can be seen here (video).

Q: Why is that important?
A:Once the water becomes contaminated it has to be cleaned and stored on site, at least until decisions are made on disposition of the water. Currently, as the result of groundwater flowing downhill from the mountains to the sea, contaminated water is accumulating at the rate of 400 tons a day, and the cleaning and storage requirements for such a volume of water are very great. The land-side impermeable wall (frozen soil wall) is part of a cluster of strategies to reduce the rate of accumulation of contaminated water.

Q:What are some of the other strategies?
A:The other strategies include a bypass system for some groundwater, the subdrain system (prompt report), and various methods to minimize the impact of rainwater that runs along the surface.

2. HOW IT WORKS

Q: How will the land-side impermeable wall (frozen soil wall) block groundwater?
A: The wall is actually a wall of frozen soil that will block the flow of groundwater, so that the water flows around the perimeter of the four generating units rather than into the buildings.

Q: Does the wall actually consist of ice?
A: No. Unfortunately, that is the name that so many people have given it that we, too, are using it for simplicity. The wall will use refrigerated coolant, running through pipes that have been placed vertically in the ground, to freeze the surrounding soil. Of course, there is some water moisture in the soil that will freeze as well, but it is not really a “wall of ice” as some people might imagine it.

Q: Why was this method chosen over a more conventional physical barrier?
A:There are many underground pipes and other structures associated with each of the four generating units. Building a watertight physical structure around all those obstacles would be nearly impossible and, even if it could be done, the contruction would be more complex, time consuming, and disruptive. It likely would also generate much more potentially contaminated excavated soil that would need to be safely disposed of.

Q: Is this an experimental technology, or has it been proven elsewhere?
A: This technology has been used successfully on many construction sites in Japan and elsewhere since at least the 1960s, especially those involving road and railway tunnels, to keep groundwater out during construction. Some projects are on as large a scale as Fukushima Daiichi, such as the one at a mine at Cigar Lake, Saskatchewan, Canada. This will be the first time the technology has been used specifically for the purpose of keeping groundwater out of buildings, and it will also be the first time the frozen barrier will be kept in place for multiple years.

Q:Has the technology been successfully tested at Fukushima before building the full-scale wall?
A: Yes. Between March 14 and the end of FY 2014, the technology is used to freeze a small area of soil at Fukushima Daiichi, and the demonstration so far was successful.

Q:Didn't I read that the project suffered a setback when it failed at Fukushima Daiichi?
A: An entirely unrelated effort, to freeze contaminated water inside the trenches, used a different technology and was unsuccessful. Freezing moving water inside a trench is entirely different from using circulating coolant to freeze the surrounding soil. One had nothing to do with the other and the ice wall is moving forward on schedule. A video explaining the difference between freezing the trenches and the land-side impermeable wall (frozen soil wall) can be seen here (video).Detailed explanation material can be obtained here (document).

3. HOW IT WILL BE CONSTRUCTED

Q:How will the wall be constructed?
A:The best way to understand how the wall will be constructed is to watch this brief animation here (video).Pipes designed to carry a chilled liquid will be sunk vertically into the ground at specific intervals, taking care to avoid existing underground pipes and other obstacles. A refrigeration plant will be constructed that will chill the liquid coolant to under minus 30 Centigrade, and the coolant will be circulated through the pipes, freezing the soil and keeping it frozen. The principle is similar to that used to freeze ice skating rinks, except that the pipes will be standing straight up, and they will freeze the soil instead of water.
Q:How can you be sure to avoid underground obstacles?
A:As with any kind of excavation and construction, we will take a variety of precautions such as examination of drawings, test bores, and field evaluations. In some instances it may be necessary to adjust placement of the coolant pipes, and in some instances even penetrate existing water pipes using a watertight seal.

<Coolant Pipe Installation Piercing Underground Obstacles>
Penetrating buried objects


<Coolant Pipe Installation Surrounding Underground Obstacles>
Install freezing pipes along the axis of object


Q: How many workers will be involved, and what precautions are being taken for their safety?
A:Approximately 360 people are working on the project daily, usually for 3-4 hours at a time. Workers wear appropriate protective gear, are tested for radiation upon entering and exiting the site, and are subject to exposure limits set by the government to protect human health. In addition, steps are being taken both to remove sources of radiation from the construction site, such as the removal of surface soil and contaminated debris, and to shield workers from radiation sources that cannot be removed.
Q: What is the current status of construction, and when will the ice wall become operational?
A:The project is progressing on time. 236 holes out of 1571 have been drilled as of August ,2014.

4. OPERATING THE BARRIER

Q: How long will the land-side impermeable wall (frozen soil wall) remain in place?
A:We plan for the barrier to be in place for six years. This will provide time to drain and clean the contaminated water from the buildings and make them watertight. The goal is to achieve this by 2020, by which time TEPCO plans to stop all the water flowing into the buildings at the Dai-Ichi site.
Q: What happens if the refrigeration system fails, due to mechanical failure or any other reason, such as earthquake?,
A:The wall is designed to withstand various stresses including earthquakes, and various backup systems, such as emergency power, will be in place to guard against failure of the refrigeration system. Still, if the system is down for any reason, the frozen barrier would remain functional for approximately two months. It is likely that any necessary repairs or maintenance could be concluded within that time frame.
Q: Will diversion of groundwater around the reactor buildings lead to subsidence and destabilization of the structures?
A:As the water level under the buildings lowers, various measures are being taken to prevent the outflow of contaminated water. For more detail on how this will be accomplished, please refer to page 7 of the material here (document).Evaluations have been performed on the potential for subsidence and this is not expected to be a problem.
According to TEPCO's calculation, a likely subsidence would be limited to maximum a few millimeters, so that would not affect the building or the surrounding environment. In addition, the calculated maximum slant assuming a "nonequivalent subsidence" or "unequal settlement" which may occur is an insignificant level. These evaluations have been confirmed by specialists at Japan's Nuclear Regulation Authority.


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