Power plant workers had been trying to fill the shaft with fresh concrete, but that did not change the amount of water coming out of the crack, the spokesmen said at a news conference on April 2.

According to the spokespmen, their "plan B" is to use polymers to stop the leak. A Tokyo Electric expert will visit the site Sunday morning and decide what polymer to use before the work begins.

Water from the 2-meter-deep, concrete-lined basin has been seen escaping into the ocean through a roughly 20-centimeter (8-inch) crack, the company said earlier Saturday. The shaft lies behind the turbine plant of the No. 2 reactor at the Fukushima Daiichi nuclear power plant, which was heavily damaged in the earthquake and subsequent tsunami last month.

The discovery of the leak comes after a feverish effort in recent days to explain a sharp spike in contamination of seawater measured just off the plant. Tokyo Electric said the shaft lies next to the water intake for the plant's steam condenser, at the end of a long channel that has been filling with radioactive water for several days.

The continued injection of tonnes of water into the reactor cores and spent nuclear fuel pools shows that the race to prevent further explosions or widespread release of radiation into the atmosphere remains far from over.

All these efforts come just more than three weeks after a 9.0-magnitude earthquake and following tsunami struck northeast Japan, effectively wiping out some communities and leading to the deaths of nearly 12,000 people, and leaving more than 15,400 missing, according to Japan's National Police Agency.

By April 2, concerns seem to have abated somewhat about the airborne radiation that led to the ordered evacuation of 78,000 people and another 62,000 living within a 20-30 km radius being told to stay indoors.