Tokyo Electric Power Co. (TEPCO), the operator of the troubled nuclear power plants in northeastern Japan, said that high-pressure streams of water shot Thursday evening by plant workers effectively cooled down an overheating spent fuel pool.

However, radiation levels detected around the facility have not been reduced. The latest figure indicate that after the water sprat maneuver was completed, radiation level at about 3,600 microsievert per hour, unchanged from before.

Five fire-trucks shot 30 tons of water at the No. 3 reactor building of the Fukushima No. 1 nuclear power plant, in an effort to cool down the pool that contains spent nuclear fuel rods outside the reactor wall.

Earlier on Thursday, the radiation level around the power plant’s administration building climbed to 4,000 microsievert per hour from 3,700 after two government helicopters dropped tons of water earlier in the day.

Among the six reactors at the Fukushima No. 1 nuclear plant, the buildings that house the reactors have been destroyed by hydrogen blasts at the No. 1, No. 3 and No. 4 reactors, while the No. 2 reactor’s containment vessel suffered damage in its pressure-suppression chamber at the bottom.

The No. 1, No. 2 and No. 3 reactors were operating at the time of the earthquake, but the automatically halted – it is believed their cores partially melted as they lost cooling functions after the quake.