Power plant workers began filling a cracked concrete shaft with fresh cement to stop highly radioactive water from earthquake-damaged nuclear reactors from draining into the Pacific Ocean, Japanese regulators said Saturday.

Water from the 2-meter deep, concrete-lined basin could be seen escaping into the ocean through a roughly 20-cm (8-inch) crack, the Tokyo Electric Power Company told reporters Saturday afternoon. Electrical conduits run upward through the space, which lies behind the turbine plant of the No. 2 reactor at the Fukushima Daiichi nuclear plant.

Work to fill the shaft with concrete began Saturday, and it was only partly completed Saturday evening, said Hidehiko Nishiyama, a spokesman for Japan’s Nuclear and Industrial Safety Agency.

Radiation levels in the shaft have been measured over 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.

The discovery comes after a feverish search 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.

NISA ordered the utility to start testing water further offshore and to the south, Nishiyama said. Tokyo Electric has now established monitoring posts 15 km (9.5 miles) off the coast, in a line directly offshore, 10 km south and 16 km south.

Highly radioactive water has also been detected in several reactors’ turbine buildings, nearby tunnels and groundwater in the immediate vicinity. But the area around the No. 2 reactor has been of particular concerns, since water in an exposed maintenance tunnel leading from its turbine building showed radiation levels more than 100,000 times above typical levels for nuclear coolants.

A two-day project began Saturday to install a camera in that trench in order to help pinpoint potential leaks, a Tokyo Electric official said.

Spraying was also set to continue this weekend of an experimental new material to lock in radioactive material in and around the nuclear complex — so that it doesn’t seep further into the air, water or ground.

Crews have dispersed about 2,000 liters (more than 500 gallons) of synthetic resin in a 500-square meter locale, according to Tokyo Electric. The aim is to hold the released radioactivity on the ground, so it can’t interfere with the restoration of the cooling systems aimed at preventing the overheating of nuclear fuel rods in reactors and spent fuel pools at the plant.

“You spray it to hold down the loose contamination, and it acts like a super glue,” said Nolan Hertel, a radiation engineering expert at the Georgia Institute of Technology in Atlanta. “You don’t want radiaoactive materials that are loose to get away.”

Meanwhile, Nishiyama said there is a plan to inject hydrogen into the Nos. 1, 2 and 3 reactors.

While there have been no alarms sounded recently about rising pressure levels, this initiative aims to curb the prospect of an explosion caused by a hydrogen building — something that had happened, weeks ago, at all three of those reactors.

Still, this initiative as well as the continued injection of tons of waters into reactor cores and spent nuclear fuel pools shows that the race to prevent further explosions or widespread releases of radiation into the atmosphere remains far from over.

All these efforts come just over three weeks after a 9.0-magnitude earthquake and subsequent tsunami struck northeast Japan, effectively wiping out some communities and leading to the deaths of at least 11,800 people, according to Japan’s National Police Agency.

The Fukushima Daiichi plant was hit hard in the disaster, especially after its primary and back-up systems to cool nuclear fuel in its six reactors and their respective spent nuclear fuel pools failed. Since then, there has been a multifaceted and at times problematic race to prevent explosions (three took place in the days immediately after March 11), the overheating of nuclear fuel and the resulting release of radioactive material into the air, soil and water.

Concerns seem to have abated somewhat, by Saturday, about the airborne radiation that led to the ordered evacuation of 78,000 people, with another 62,000 living within 20-to-30 kilometers being told to stay indoors. An official with the Tokyo Electric Power Company, which runs the power plant, said early Saturday that data from eight new monitoring posts around the plant showed that airborne radiation levels had stabilized, at between .390 and .0019 millisieverts per hour.

On Saturday — after a stop in Rikuzentakata, in Iwate prefecture — Japanese Prime Minister Naoto Kan went to Hironocho, a village in Fukushima prefecture that has served as the operations center for the nuclear crisis effort. The trip, described by the prime minister’s office as aimed at boosting morale among utility company workers and soldiers involved in the effort, put Kan on the edge of the 20-kilometer evacuation zone.

“I appreciate your significant contributions in fighting the invisible enemies in this battle, which will determine the fate of Japan,” Kan said at J-Village, a soccer complex that has become a staging area for the Fukushima Daiichi operation.