Tokyo Electric Power Company submitted the application to the Nuclear Regulation Authority on Monday, in order to gain permission to freeze the 7 meter-wide section on the mountain side.

Workers started circulating coolant in buried pipes to form the 1.5 kilometer-long wall in March of last year. They hope to keep groundwater from entering the crippled buildings and being contaminated with radioactive substances.

The utility company has been taking a cautious approach, expanding the frozen area little by little to gradually reduce the groundwater inflow.

That's because of the regulator's concern that freezing the entire area could lead to a sharp drop in the groundwater level outside the reactor buildings, which could cause the tainted water to leak out.

But company officials now say they are sure that water will not leak out. And they say the barrier is already working to decrease the flow of water. The daily amount of groundwater flowing into the buildings is now 100 tons. When the project started, it was 400 tons.

The officials say that the completion of the ice wall will further reduce the amount.