Workers will then break the shaft's ceiling and insert the polymer in a different spot from where they tried to place the concrete, they said.  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 plant, which was heavily damaged in the earthquake and resulting tsunami last month.  Radiation levels in the shaft have been measured at more than 1,000 millisieverts per hour, which is more than 330 times the dose an average resident of an industrialized country naturally receives in a year. Radioactivity above the shaft was measured at 250 millisieverts per hour, said Tokyo Electric, the plant's owner.  Tokyo Electric said it is discussing other methods to use should the polymer fail, but it hasn't identified what those other methods may be.  The discovery of the leak comes after a feverish effort in recent days to explain a sharp spike in contamination in 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.  Officials announced Thursday, based on samples taken the previous afternoon 330 meters (361 yards) off the plant, that seawater showed levels of iodine-131 measuring 4,385 times above the standard and cesium-137 at 527 times beyond normal. Experts say the latter radioactive isotope may be a greater concern because it persists longer, since it takes 30 years to lose half its radiation -- compared to an eight-day half-life for the iodine-131 isotope.  While officials know the radioactive water is coming from the cracked pipe, they don't yet know where it originates.
A first attempt to plug a cracked concrete shaft that is leaking highly radioactive water into the ocean off Japan failed Saturday, so officials are now exploring alternatives, spokesmen for Tokyo