Photo/IllutrationRows of tanks holding contaminated groundwater are seen at the Fukushima No. 1 nuclear power plant in February. (Naoko Kawamura)

The “frozen soil wall” erected around the crippled reactor buildings at the Fukushima No. 1 nuclear power plant at huge taxpayer expense appears limited in keeping groundwater from flowing in.

Tokyo Electric Power Co., which operates the plant, said March 1 that 95 tons of radioactive water has been reduced a day on average between December and early February because of the underground barrier.

“Contaminated groundwater was cut in half due to the wall,” a TEPCO official said.

TEPCO estimated that the volume of polluted groundwater would have amounted to about 189 tons if the ice wall had not been in place during that period.

The utility also said the amount of polluted groundwater was reduced by about 400 tons a day now due to combined measures, such as the wall and wells pumping up water, compared with before such measures were taken.

But Toyoshi Fuketa, chairman of the Nuclear Regulation Authority, has insisted that the wells, not the wall, are the “key” to controlling the groundwater, voicing skepticism about the role of the ice wall.

The utility is proceeding with work to reinforce the wells.

The 34.5 billion yen ($322 million) frozen soil wall project began in 2014 to lay out the 1,500-meter-long underground wall around the No. 1-4 reactor buildings.

A large number of pipes were inserted to a depth of 30 meters to circulate liquid with a temperature of minus 30 degrees through them to freeze the surrounding soil.

It was designed to prevent groundwater from flowing into the plant and mixing with highly radioactive water in the basements of the buildings.

TEPCO’s recent assessment of the effectiveness of the frozen soil wall came after temperatures around the structure dropped to below zero following work that began last August to freeze the remaining final section of the wall.

But experts pointed out that the utility’s assessment is based on figures only when there was little rain.

The water volume rose to 1,000 tons or so a day in late October when two typhoons struck the area.

TEPCO believes that the surge at that time is largely attributable to the downpours from the typhoons.

Heavy rain accumulated in the basement after flowing down holes in the ceilings caused by hydrogen explosions during the 2011 triple meltdown.

It costs more than 1 billion yen a year in electricity fees to keep the wall frozen.

The company plans to remove all the groundwater from the buildings by 2020 so that it can begin work to decontaminate the facilities later.

(This article was written by Masanobu Higashiyama and Yusuke Ogawa.)