TEPCO has undertaken a number of efforts to try to control contaminated water at the site. These have had mixed results with the potential for unintended consequences.
Subdrains & Groundwater Bypass
The subdrains are an interconnected system of sump wells around the reactor buildings. These originally served to act as water control to keep groundwater out of the reactor building basements before the disaster. The groundwater bypass is a series of wells drilled at the crest of the hill above the reactor buildings. Water from both of these locations is pumped up, “treated” for radioactive contamination then released into the port where it can flow out to sea. The treatment process involves filtration systems similar to what the contaminated water from the reactor buildings is run through. More information on this process can be found on this page on the TEPCO website:
Seaside Impermeable Wall
This is the sheet and pipe steel wall TEPCO has been installing alongside the port, in front of the reactor intake systems. The wall currently has an opening in front of unit 4 that is allowing some water to escape into the port. TEPCO claims they will close this hole once the frozen wall and subdrain pumping systems are fully operational. More information can be found on TEPCO’s page:
image | TEPCO
Frozen Wall (Land Side Impermeable Wall)
This 30-meter deep wall of frozen soil is intended to block groundwater intrusion into the reactor building area in the hopes of reducing the amount of contaminated water and leaks to the sea. The boreholes and equipment installation has been completed with most of the wells frozen or in the process of freezing as of August 2015. The true effectiveness of the wall should be understood better over the next year. This project was highly contentious. TEPCO didn’t want to pay for the project, it was eventually funded by the government and the contract given to Kajima Construction. Kajima is a major contractor at the disaster site and already had experience using this technology in large construction projects. As of 2021, the wall has been in use for a number of years. Outside of a few small section failures, the wall has largely been working as intended. It has helped reduce the volume of contaminated water that must be handled on-site.
The plant had a complex system of stormwater handling pipes and canals before the disaster. It was only in 2014 when TEPCO was pushed by NRA to do something about this system. It was realized that this system was allowing contaminated water leaking from storage tanks on the hill to reach the ocean. In 2015 it was discovered that contaminated water was draining off the roof of unit 2 when it rained and this was also flowing out to sea via the K drainage canal. These drainage systems likely created a major pathway for contaminated rainwater and soils to be transported off the plant grounds and out to sea. The years since the initial disaster saw a number of typhoons in addition to large seasonal runoff in spring. The extent of these releases has not been quantified. TEPCO has put in place a number of mitigation systems and monitoring to try to prevent further releases to the sea.
Contaminated Water Tanks
TEPCO initially installed a series of used bolt-together tanks soon after the disaster to collect highly contaminated water as they searched for a more permanent process to deal with the massive amounts of contaminated water being generated daily. These tanks were rated for 3-5 years before the seals would fail. This began to happen even earlier than that, creating a number of major contaminated water leaks at the plant. TEPCO is now in the process of replacing these tanks with welded tanks. As of 2021 most of the bolt-together tanks had been replaced with welded tanks but a few of the old bolt-together tanks still remain 10 years later.