Efforts to end the release of highly radioactive water at the Fukushima Daiichi nuclear-power plant finally met with some success Tuesday, plant operator Tokyo Electric Power Co. said, with the injection of what it called "liquid glass" gel around a damaged pipe managing to reduce the toxic flow by half.

Workers poured 780 gallons of gel-like sodium silicate onto the rocks supporting the pipe. Authorities said the substance would continue to harden over time and could continue to slow the flow of water.

Workers have tried a variety of methods to reduce the flow, put at several tons of water an hour, since it was discovered coming from a crack in a concrete container near the shoreline Saturday. The water is thought to be from the heavily damaged No. 2 reactor.

The steady flow from the crack is being blamed for a surge in the level of contamination in seawater near the complex. A water sample taken just outside the water intake for the No. 2 unit showed the level of radioactive iodine-131 at 7.5 million times the allowable limit, the most dangerous level of radiation so far detected.

In the latest step the gel-like sodium silicate—a liquid glass, which acts as an adhesive—were poured onto the rocks supporting the pipes running to the ocean. Authorities said the substance would continue to harden over time and could continue to slow the flow of water.