Radiation readings at the troubled Fukushima No. 1 nuclear plant have consistently followed a downward path through Friday morning, according to Nuclear and Industrial Safety Agency measurements taken roughly 1 kilometer west of the plant's No. 2 reactor.

The agency also said Friday the radiation reading at 5 a.m. Friday came to 279.4 microsievert per hour, compared with 292.2 microsievert per hour at 8:40 p.m. Thursday, shortly after the Self-Defense Forces discharged water from fire trucks in an attempt to cool an apparently overheating spent fuel pool.

In an unprecedented move, up to 64 tons of water was discharged by helicopters and fire trucks of the SDF as well as a water cannon truck of the Metropolitan Police Department into the pool at the No. 3 unit of Tokyo Electric Power Co.'s Fukushima plant Thursday.

The mission was set to continue Friday as part of the effort to avert any massive release of radioactive materials into the air from the pool, while efforts to restore the lost cooling function by reconnecting electricity to the plant through outside power lines have accelerated, according to the government.

The spent fuel pools at the power station lost their cooling function after the quake and tsunami struck last Friday. It is also no longer possible to monitor the water level and temperature of the pools of the No. 1 to 4 units.

Among the six reactors at the Fukushima No. 1 Nuclear Power Station, the No. 1, No. 2 and No. 3 reactors that were operating at the time of the quake halted automatically, but their cores are believed to have partially melted as they lost their cooling functions after the quake.

The buildings housing the No. 1, No. 3 and No. 4 reactors have been severely damaged by apparent hydrogen blasts, and the No. 2 reactor's containment vessel suffered damage to its pressure-suppression chamber at the bottom.

The government has set the exclusion zone covering areas within a 20 kilometer radius of the plant, and urged people within 20-30 km to stay indoors.