Construction has started of a wall of frozen soil at the Fukushima Daiichi nuclear power plant to prevent groundwater entering the reactor buildings. The ice wall is expected to take nine months to complete.
|Drilling of the first of 1550 holes has started at Fukushima Daiichi (Image: Tepco)
Tokyo Electric Power Company (Tepco) began work to build the underground ice wall today, having received approval last week from Japan's Nuclear Regulation Authority to proceed.
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.
Tepco plans to drill holes some 30-35 metres into the ground and insert 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 will be placed in the ground to create a 1.5km-long ice wall around units 1 to 4.
A Tepco official was cited by media sources as saying, "We plan to end all construction work in March 2015 before starting trial operations."
The construction of the ice wall is estimated to cost some ¥32 billion ($313 million). It was mandated by the government as part of a concerted effort to bring the contaminated water situation at Fukushima Daiichi under control.
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.
Tepco recently started diverting groundwater around the reactor buildings at Fukushima Daiichi by pumping it out of the ground before it reaches the plant and then releasing it into the sea. The groundwater bypass system could reduce the ingress of water to the basements by 100 tonnes per day and therefore reduce the total volume of water Tepco must decontaminate. As well as the bypass, an impermeable underground wall has also been built between the reactors and the sea. Together with the ice wall, these measures should virtually eliminate the movement of groundwater.
Researched and written
by World Nuclear News