Tokyo Electric Power Company (Tepco) is considering conducting additional work to help freeze the ground around the crippled reactors at its Fukushima Daiichi NPP, the company said on 30 May. Tepco began freezing the soil in late March to build a 1.5km underground ice wall around the four damaged reactors. The aim of the JPY35bn ($312m) wall is to reduce the amount of groundwater flowing into the reactor buildings, where it becomes contaminated with radioactive substances.

Tepco says the ground at multiple locations along the barrier has yet to reach below zero degree Celsius and one checkpoint remain at around 10 degrees. It says those areas contain more gravel and that the accumulated groundwater may be hampering the freezing process. Tepco is studying additional measures, such as pouring chemical compounds to solidify the ground, and will discuss its ideas with Nuclear Regulation Authority. Tepco had hoped to expand the areas to be frozen by the end of May, but say it will examine the timing carefully, taking into consideration the problem facing the project.

In its quarterly report, covering January to the end of March, on conditions and clean-up progress at Fukushima, Tepco said that soil temperatures at more than 90% of the measuring points in a circumference surrounding four reactor buildings were below zero degrees Celsius. Tepco finished installation of 1,549 pipes to create the underground frozen blockade area in early February 2015. The coolant began flowing through the pipes on 31 March after a series of pressure tests.

The ice wall, built by construction company Kajima Corp, is being turned on in sections for tests, and the entire freezing process will take eight months. The wall requires as much electricity as would be needed to power 13,000 Japanese households. Edward Yarmak, president of Arctic Foundations, based in Anchorage, Alaska, which designs and installs ground freezing systems and which made an ice wall for the Oak Ridge reactor site, says the solution should work at Fukushima.

"The refrigeration system has just been turned on, and it takes time to form the wall. First, the soil freezes concentrically around the pipes and when the frozen cylinders are large enough, they coalesce and form a continuous wall. After time, the wall increases in thickness," he said in an email to The Mainichi.

It is uncertain how the water flow has changed, given that 10% of the wall is unsecured, and may allow incidental leakage or significant leakage, depending on conditions. The 90% that is frozen, the report says, includes the "entire seaside line along with preliminary freezing of some areas to the north and on the mountainside". On that mountainside or uphill side of the frozen soil wall, there are areas "where there is significant space between freezing pipes and thus more difficult to freeze," says the report.

Along with the frozen soil and improved ditching for water control around the plant, Tepco is pumping contaminated water inside the facility to a treatment system. In August 2015, additional transfer pumps were installed in each building and more water level gauges were mounted. At that point water levels were "down incrementally". From September 2015, subdrains were employed which "gradually lowered groundwater level(s) and hindered production of contaminated water". On 7 March, the water level inside the reactor building was below the level of the connections to the turbine building, which is seen as a crucial achievement. Since then, "we have verified that the water levels are being stably maintained", Tepco said.

Since early March, the site has been divided into "highly contaminated areas," which require cumbersome protective gear to be worn by workers, and areas where "regular working clothes or special on-site clothing" is allowed. Measures such as spraying areas to keep radioactive dust down has improved worker safety, says Tepco, which claims they have achieved an annual radiation level of 1mSv along the site boundary.

In an interview with Associated Press in late April, Yuichi Okamura, a Tepco general manager and chief architect of project said the amount of water reaching the reactors was "not zero". He explained that, even if the frozen barrier works as envisioned, it will not completely block all water from reaching the damaged reactors because of gaps in the wall and rainfall, creating as much as 50t of contaminated water each day. He noted: "It’s a vicious cycle, like a cat-and-mouse game. We have come up against many unexpected problems."