Tons of highly radioactive water have flooded basements at the crippled Fukushima No. 1 nuclear power plant after pumps that should not have been in use were mistakenly turned on.

Plant operator Tokyo Electric Power Co. announced April 14 that 200 tons of contaminated water have flooded the basements of buildings on the plant site.

Although TEPCO officials said there were no channels for the contaminated water to leak out of the flooded basements, the Nuclear Regulation Authority ordered the utility to continue to monitor the situation and collect the leakage as soon as possible.

TEPCO officials said the water was highly contaminated because it was used to cool reactors at the Fukushima No. 1 plant. The water contains several tens of millions of becquerels per liter of radioactive cesium because it has yet to be purified using a system that removes radioactive materials.

The contaminated water has been collected beneath a group of buildings that house the central waste processing facilities.

Workers noticed something amiss on April 10, as the water levels in buildings that should have been pumping out water were found to be rising.

Workers discovered April 12 that four pumps that should not have been used were in operation. Those pumps are normally used to reverse the flow of water or to send the water to other destinations when problems arise in pumping the water to the original destination.

All the temporary pumps were halted around 5 p.m. on April 13. However, by that time about 200 tons of contaminated water had flooded the basement floors where water should not be accumulating.

The pumps have to be manually operated to send the water to other destinations. TEPCO will check to see if workers mistakenly operated the pumps.