Two CH-47 helicopters of the Self-Defense Force (SDF) scooped up sea water and released it over the reactors after another chopper checked radiation levels in the air, a week after the magnitude-9 quake and devastating tsunami rocked the country leaving 14,650 people dead or unaccounted for.
Today's mission was part of efforts to cool the storage pools at the No.3 and No.4 reactors, whose cooling systems were not functioning, raising fears that spent fuel rods could melt and release radioactive material outside the building, national broadcaster NHK reported.
The choppers dumped four loads before leaving the site. Yesterday, they were forced to abort a similar operation due to concerns over high radiation levels.
The unprecedented operation is expected to gain force on the ground soon as Tokyo police prepared to spray water with water cannon truck, with the focus of the nuclear crisis shifting to the pools storing spent fuel rods at each of the plant's six reactors, located outside the steel containment vessels for enclosing toxic radioactive substances.
The government's Nuclear and Industrial Safety Agency said the first priority should be pouring water into the pools at the No 3 and No 4 reactors, which may be boiling and are not fully covered by roofs, that would reduce any radiation leaks, Kyodo news agency reported. The two reactors were rocked by explosions earlier this week.
A rise in the temperature, usually at 40C, causes water to reduce and expose the spent nuclear fuel rods, which could heat up further and melt, and discharge highly intense radioactive material in the worse case, experts said.
Also, the US military is poised to operate a Global Hawk unmanned high-altitude reconnaissance aircraft to take images of the inside of the building that houses the No.4 reactor, Japanese government sources were quoted as saying by Kyodo.
Although the No.1, No.2 and No.3 reactors that were operating at the time of the quake halted automatically with jolts, their cores are believed to have melted as they lost cooling functions in an ensuing tsunami.
An estimated 70 per cent of the nuclear fuel rods had been damaged at the No.1 reactor and 33 per cent at the No. 2 reactor, Tokyo Electric Power Company (TEPCO), the operator of the plant, said.
The water level also dropped in the fuel pool at the No.5 reactor, posing the risk of overheating, according to the nuclear safety agency. The No.4, No.5 and No.6 reactors have had fuel rods taken out of their cores for regular checks.