Eight workers are scheduled to be the first to enter the reactor 1 building of the crippled Fukushima No. 1 nuclear power plant since it was ripped apart by a hydrogen blast the day after the March 11 tsunami, Tokyo Electric Power Co. said Wednesday.

Expected to enter Thursday, the eight are part of a plan by Tepco to cool the reactor by filling the containment vessel with water.

However, workers must first verify that the water gauge in the pressure vessel and the pressure gauge in the containment vessel are working properly.

At present, Tepco is unsure if the readings are accurate.

Before the workers can enter, however, the air inside must be filtered to remove radiation.

The utility will set up a ventilation system inside Thursday with oxygen cylinders and erect a tentlike structure at the double-door entrance, with higher air pressure than in the reactor building, to prevent radioactive materials from leaking outside.

Once the ventilation is completed, expected by Saturday, Tepco plans to set up air coolers outside the reactor building to cool the water filling the containment vessel of the reactor. An alternative cooling system is needed to replace the one that went down in the March 11 tsunami.

If the new cooling system works as planned, Tepco said the temperature in the reactor will drop to under 100 degrees, achieving cold shutdown, within several days.

On Tuesday, Tepco said it detected high levels of radioactive substances in seabed samples gathered last Friday about 15 to 20 km from the Fukushima No. 1 nuclear plant.

At a location 15 km north of the plant, 1,400 becquerels per kg of cesium-137 and 1,300 becquerels per kg of cesium-134 were detected in the seabed. Both are more than 1,000 times higher than normal. Some 190 becquerels per kg of radioactive iodine was also detected.

A similar amount was also detected 20 km south of the nuclear plant. It is the first time Tepco has analyzed the ground under the sea.

The utility plans to investigate possible effects of radiation on marine products.

It was earlier reported this week that the environmental group Greenpeace has started taking ocean radiation readings outside Japan’s 12-mile territorial waters.