Tokyo Electric Power Co. started Tuesday pumping highly radioactive water at its Fukushima No. 1 nuclear plant’s reactor 2 turbine building into a nearby storage facility, a crucial step toward restoring the reactor’s dedicated cooling system, the government nuclear watchdog said.

Because key equipment to activate the cooling system is located in the basement of the turbine building, workers can resume efforts to restore the cooling system once the radioactive water is pumped out.

“We believe there is no other option but to transfer the radioactive water to the storage facility,” said Hidehiko Nishiyama, spokesman for the Nuclear and Industrial Safety Agency.

There is now about 25,000 tons of highly contaminated water in the turbine building and in an underground trench.

Tepco plans to pump 10,000 tons of radioactive water with more than 1,000 millisieverts per hour of radiation to the second-floor basement of the storage facility by around May 14. The storage facility is located about 800 meters from the No. 2 turbine building.

“Although the risk of radioactive leak (by transferring the water out of the turbine building) is not zero, measures have been taken to reduce that risk,” another NISA official said.

Tepco laid as much of the hose as possible inside the turbine buildings of units 3 and 4 in case the radioactive water leaks out, NISA said.

Tepco waterproofed the storage facility and checked whether it could withstand an earthquake similar to the one on March 11, NISA said.

The utility said it will do its best to seal the entrance to the basement to keep seawater out in the event another tsunami strikes.

Tepco is also planning to install a water-purification system to reduce the levels of radiation and salt in the highly contaminated water, NISA said, adding the purified water is expected to be used to cool the fuel rods again.

It is believed water previously pumped in as a stopgap measure to cool the fuel rods leaked into the turbine building through the trench and a storage pit crack and then out into the ocean. That leak has been patched but disposing of the water has proved a big hurdle.

The storage facility seemed an ideal, if temporary, option, but low-level radioactive water was already being stored there. With the approval of NISA, Tepco earlier this month decided to dump the less contaminated water into the sea to make room for the highly radioactive water in the No. 2 turbine building. The move triggered strong criticism from the international community, including South Korea and Russia.

Meanwhile, remote-controlled U.S. robots inspecting the reactor 2 building to check the radiation level detected 4.1 millisieverts per hour just inside the entrance to the ground floor, Tepco said.

High humidity between 94 to 99 percent prevented the robots from checking the radiation level further inside.

Two robots, one with a radiation detector and the other with a video camera, were sent inside the building Monday. But because of the humidity, the camera lens fogged up and couldn’t check the detector beyond the entrance, NISA said.

Separately Monday night, Tepco said it detected high radioactivity coming from the spent fuel pool of the No. 2 unit, indicating that either the fuel rods in the pool were damaged or steam containing radioactive materials that rose from the reactor dissolved in the pool water.

Some 160,000 becquerels per cubic cm of cesium-134, 150,000 becquerels of cesium-137 and 4,100 becquerels of radioactive iodine-131 were detected in a water sample extracted from the storing pool on Saturday.

Normally, these radioactive substances are not present at all.

Information from Kyodo added