OKUMA, Japan - A costly "ice wall" is failing to keep groundwater from seeping into the stricken Fukushima Dai-ichi nuclear plant, data from operator Tokyo Electric Power Co shows, preventing it from removing radioactive melted fuel at the site seven years after the disaster.

When the ice wall was announced in 2013, Tepco assured skeptics that it would limit the flow of groundwater into the plant's basements, where it mixes with highly radioactive debris from the site's reactors, to "nearly nothing".

However, since the ice wall became fully operational at the end of August, an average of 141 metric tons of water has seeped into the reactor and turbine areas each day, more than the average of 132 tons a day during the prior nine months, analysis of the Tepco data showed.

The groundwater seepage has delayed Tepco's cleanup at the site and may undermine the entire decommissioning process for the plant, which was battered by a tsunami seven years ago this Sunday.

Waves knocked out power and triggered meltdowns at three of the site's six reactors that spewed radiation, forcing 160,000 residents to flee, many of whom have not returned to this once-fertile coast.

Though called an ice wall, Tepco has attempted to create something more like a frozen soil barrier.

Using 34.5 billion yen ($325 million) in public funds, Tepco sunk about 1,500 tubes filled with brine to a depth of 30 meters in a 1.5-kilometer perimeter around four of the plant's reactors. It then cools the brine to minus 30 C.

The aim is to freeze the soil into a solid mass that blocks groundwater flowing from the hills west of the plant to the coast.

However, the continuing seepage has created vast amounts of toxic water that Tepco must pump out, decontaminate and store in tanks at Fukushima that now number 1,000, holding 1 million tons. It says it will run out of space by early 2021.

Flows reduced

The water inflows often fluctuate with rainfall. The dry month of January averaged 83 tons a day, Tepco data showed.

But when a typhoon struck during the last week of October, 866 tons a day poured into the reactors.

Overall, Tepco says a combination of drains, pumps and the ice wall has cut water flows by three-quarters.

It is hard to measure exactly how much the ice wall is contributing, Tepco officials say, but based on computer analysis the utility estimates the barrier is reducing water flows by about 95 tons a day compared to two years ago, before the barrier was operating.

However, a government-commissioned panel on Wednesday offered a mixed assessment of the ice wall, saying it was partially effective but more steps were needed.

Reuters