Tokyo Electric Power Co. says it has placed tons of ice blocks in underground trenches at the Fukushima No. 1 complex in an attempt to freeze highly toxic water pooled there, a necessary step before a 1.5-km ice wall can be constructed to keep groundwater out.

Tepco is racing to stop the buildup of radioactive cooling water in the trenches.

To keep groundwater from becoming contaminated and spilling into the ocean, Tepco is attempting something unprecedented: to freeze 1.5-km of soil around the basements of the four reactor buildings.

In April, Tepco inserted refrigeration rods in the trenches to try to freeze the water but abandoned the effort after more than three frustrating months.

According to the company, some 11,000 tons of highly toxic water has flowed into the trenches through the No. 2 and No. 3 reactor buildings.

As an additional measure, Tepco started putting 15 tons of ice per day into the trenches late last month. Though there is now 58 tons of ice in the trenches, the utility has "yet to see" whether it will work, a Tepco official said Monday.

The new method was introduced after an increasingly alarmed Nuclear Regulation Authority urged the company last month to take additional steps as soon as possible to handle the radioactive water.

The official said the company will also consider putting blocks of dry ice in the trenches to help freeze the water.