Two systems are being put in place at the damaged Fukushima Daiichi plant in Japan that should significantly improve water management at the site. Tokyo Electric Power Company (Tepco) is seeking approval before full operation of the impermeable seawall and subdrain system starts.
|Construction of the steel impermeable seawall (Image: Tepco)
Tepco said that it has completed work on restoring the subdrain system at the plant and has almost completed construction of a deep steel wall along the seawall in front of the damaged reactor buildings. It said the system is "ready for approval from the regulator and the other important stakeholders."
The subdrain system at the plant was damaged in the March 2011 accident that followed an earthquake and tsunami. It has now been restored and rebuilt. Once back in operation, the system will capture 500 to 700 tonnes of groundwater per day, enabling it to be pumped up and treated.
The system will lower the groundwater level around the reactor buildings, which is currently up to 4 metres higher than the level of the water inside the buildings' basements.
The water collected by the subdrain system will be treated in a purification system that Tepco claims will "clean the water to a safer level than any international or Japanese safety standard." The company said that any of the water discharged "would meet the same stringent criteria, and be subject to the same monitoring requirements, as the water currently being discharged as part of the groundwater bypass system." However, Tepco noted that no final decision has yet been made on the ultimate disposition of this water and that no discharge would be made without the prior agreement of the relevant ministries and local fishermen.
Tepco is also nearing completion of an impermeable wall on the east side of the site to prevent groundwater from flowing out to sea. This steel pipe sheet pile wall extends 30 metres below the seabed. The groundwater that will collect at this wall has not entered the reactor buildings and is therefore not nearly as contaminated as the water that has collected in the basements. This water will be treated along with the water collected from the subdrain system.
|The subdrain and seawall will be used along with the existing groundwater bypass system to further significantly reduce the flow of water into the sea (Image: Tepco)
"The two systems aim to further control the amount of radioactive material released from the site," Tepco said. It estimates that the systems will halve the amount of contaminated water accumulating at the site to some 200 tonnes per day. The company also estimates that the subdrain system and seawall will reduce the amount of radioactive material reaching the sea to one-fortieth of the current amount.
Tepco is also building an 'ice wall' around Fukushima Daiichi units 1 to 4. Construction of this began in June with freezing of the soil scheduled to start by the end of March 2015. The ice wall will direct uncontaminated groundwater around the reactor buildings, rather than through them. This in turn will significantly reduce the amount of water that will be pumped up by the subdrain as well as the amount reaching the seawall.
Tepco's chief decommissioning officer Naohiro Masuda said, "These two systems are important components of our larger strategy to improve water management at Fukushima. Along with the other multi-layered measures - which include sophisticated water treatment systems, a groundwater bypass system and development of the 'ice wall' around the perimeter of the four reactor units - I believe the two systems will contribute to further improving water management at the site."
Researched and written
by World Nuclear News