The installation of the equipment required for forming a wall of frozen soil at the Fukushima Daiichi nuclear power plant to prevent groundwater entering the reactor buildings has been completed. Approval from the Japanese regulator must be sought before the creation of the wall itself can begin.
|The ice wall will surround Fukushima Daiichi units 1-4 (Image: Tepco)
Ice wall technology is already widely used in civil engineering projects, such as the construction of tunnels near waterways. Small-scale tests using the technology have already been completed at the Fukushima Daiichi site. However, the full-scale use of the technology at Fukushima will see the largest ground freezing operation in the world.
Installation of the equipment for forming the ice wall began in June 2014 and a test that has circulated the chilling liquid to specific parts of the wall has been under way since April 2015. The north, south and west sides of the facility were completed last September, while the remaining pipes on the east side facing the sea were placed within the ground in November.
Tokyo Electric Power Company (Tepco) announced yesterday that all the necessary equipment is now in place for the creation of the ice wall. However, the company noted that it must get approval from the Nuclear Regulation Authority before the equipment can be put into operation. The regulator's approval, Tepco said, would partly depend on the company "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 building's basements".
Kajima Corporation, the main contractor for the facility, has drilled holes some 30-35 metres into the ground and inserted pipes through which refrigerant will be then be pumped. This cooling will freeze the soil surrounding the pumps creating an impenetrable barrier around the reactor buildings. In total, some 1550 pipes have been placed in the ground to create a 1.5km-long ice wall around units 1 to 4. The wall is designed to remain effective for up to two months in the event of a loss of power. The Japanese government agreed to pay for construction of the ice wall, estimated to cost some JPY32 billion ($278 million).
Reducing the amount of contaminated water that it must deal with is a priority for Tepco. Groundwater naturally seeps from land to sea, but at the Fukushima Daiichi site it must negotiate the basements of reactors buildings. It is thought that more than 400 tonnes of groundwater enters the basements each day through cable and pipe penetrations as well as small cracks, mixing with the heavily contaminated water previously used to cool the damaged reactor cores.
The ice wall is only one part of a multi-layered strategy being employed to manage the flow of groundwater and rainwater at Fukushima Daiichi, Tepco said. The strategy to prevent water becoming contaminated has reduced the daily inflow of groundwater into the buildings to 150 tonnes per day. "Successful implementation of the frozen soil water is designed to reduce that inflow further, by keeping water out of the reactor buildings," the company said.
Researched and written
by World Nuclear News