The underground frozen wall being built around Japan's crippled Fukushima Daiichi NPP is, as yet, unable to fully prevent groundwater from entering buildings and mixing with contaminated water, local media reported on 3 November. Plant operator, Tokyo Electric Power Co (Tepco) began freezing soil six months ago with the aim of eventually encircling the four damaged reactor buildings with a 1-metre-thick, 1.5km-long barrier of frozen soil to prevent water from flowing through them and carrying radioactive water to the ocean. However, the project has been plagued by delays and technical difficulties.

The NPP is located between mountains and the ocean, and water from the higher ground is constantly flowing into the reactor buildings where it becomes contaminated, and then out to sea. This water is seeping into the basements of the buildings housing reactors 1 to 3, which were damaged in the March 2011 earthquake, tsunami and subsequent meltdowns. There it comes in contact with molten nuclear fuel and other materials, creating a toxic brew containing caesium, strontium and other radioactive substances. With the exception of tritium, most of those substances can be filtered out, but the filtration systems have also been subject to frequent breakdowns and leakages.

The contaminated water is being pumped out and stored in tanks on the premises. Some 1,000 tanks have been filled so far, but these also suffer regular leaks. The storage system is reaching its capacity, which is a major concern, because 400t of newly contaminated water is being created every day.

The ice wall consists of 1,568 30-metre-long pipes set in the ground at 1 metre intervals. The pipes are filled with refrigerant and cooled to minus 30 C. With the icewall in place, Tepco had planned to remove all the contaminated water from the reactor buildings by the end of 2020 but this target is unlikely to be met.

Tepco opted for the ice wall instead of building a concrete barrier to avoid severing underground pipes and electrical wires. But it has proved neither easy nor cheap and has already cost JPY34.5bn ($327m). Work to freeze certain sections of soil began in late March, far later than planned. The wall on the ocean side was completed in October. But in some places, water is flowing so fast that it is hampering the freezing process. Meanwhile, a series of typhoons and heavy rains sharply raised underground temperatures, melting parts of the wall.