Innovative facility designed to block water from reactor buildings by forming a complete barrier around the four damaged units
Installation of facilities required for constructing the frozen soil wall, designed to block groundwater at vi from entering reactor buildings and thereby becoming contaminated, has been completed, leading to the eventual goal of removing water from within the buildings.
The wall, often referred to as an “ice wall,” uses chilled fluid circulating through pipes in the ground to create a solid barrier to the penetration of water. It is an innovative application of a technology that has previously been used in the construction of tunnels.
Formation of the wall must await approval of Japan’s Nuclear Regulation Authority, which in turn will depend, at least in part, on TEPCO showing a method to ensure that the wall (and other groundwater pumping operations) do not invert the water level difference in any way that would cause contaminated water to flow out of the buildings’ basements.
The concept of the “ice wall” was developed to overcome the challenge posed by the many pipes and other subsurface structures associated with the reactor buildings. If a conventional wall were built around these subsurface obstacles, creating watertight seals would have been extremely difficult. By freezing the soil in a complete perimeter around those underground structures, the goal is to eliminate the need for other kinds of physical barriers by turning the soil itself into a watertight barrier.
The technology is similar to that used to freeze an ice skating rink, except that the pipes carrying the chilled brine are being placed vertically in the ground, to freeze the soil down to bedrock.
The Japanese government agreed to absorb the cost of the wall and the main contractor is Kajima Corporation. Construction began in June 2014 and a test that has circulated the chilling liquid to specific parts of the wall has been underway since April 2015. North, south and west sides of the facility were completed last September by pouring the brine into the pipes, and the remaining pipes on the east side – facing the sea – were placed in the ground last November.
Part of a Larger Strategy
The frozen soil wall is only one part of a multi-layered strategy being employed to manage the flow of groundwater and rainwater at Fukushima Daiichi.
The strategy seeks to prevent water from becoming contaminated, to remove the source of water contamination, and to prevent contaminated water from leaking.
A groundwater bypass system that diverts water from contaminated areas is one example of preventing water from becoming contaminated and a seaside impermeable wall that blocks the flow of groundwater into the sea is another example of preventing contaminated water from leaking.
The strategy to prevent water from becoming contaminated has reduced the average daily inflow of ground water into the buildings from 400 l/day to 150 l/day (though this amount can vary based on weather and other fluctuating factors.) Successful implementation of the frozen soil water is designed to reduce that inflow further, by keeping water out of the reactor buildings.
Click here to watch a video animation of the wall.