Children hide under a table at a junior high school as a strong aftershock jolted Ishinomaki, northeastern Japan, on Thursday. (AP)

Tokyo, March 24: Even as the Japanese authorities began distributing bottled water for the estimated 80,000 children less than a year old in the Tokyo area, city authorities said today that levels of a radioactive isotope found in water samples from one purification plant had fallen by more than half in the last 24 hours, now testing below the country’s stringent maximum for infants.

“Continued monitoring of the situation is essential,” a Tokyo government statement said, as workers tried to stave off meltdowns and radioactive emissions from a stricken nuclear plant about 225km north of Tokyo. Two workers were hospitalised with radiation burns today, and a third was also injured.

Yesterday, the capital and two neighbouring prefectures were warned that tap water was testing at levels of iodine 131 that Japan considers unsafe for infants, though not adults. The isotope can accumulate in the thyroid and cause cancer.

Japan, with its painful legacy of atomic attack, maintains standards far more conservative than international agencies. The warning set off widespread anxiety, and a run on bottled water emptied store shelves. This morning, the authorities were considering importing bottled water.

It was not clear why the levels of iodine 131 had fallen. The authorities said that frequent rain in recent days may have washed radiation into Tokyo’s watershed, which lies almost entirely north and northeast of the city.

Halting progress was reported in efforts to regain control of the six reactors at the nuclear plant, Fukushima Daiichi, where cooling systems were knocked out in the earthquake and tsunami on March 11. An official of Tokyo Electric Power said workers had managed to restore lighting in the central control room of the No. 1 unit, an important step toward restarting the reactor’s cooling system. The temperature in the reactor pressure vessel has been showing a worrisome increase, and Yukio Edano, the chief cabinet secretary, said that efforts were being focused on resolving the problem.

Officials of Japan’s Nuclear and Industrial Safety Agency said that the Nos. 1 and No. 4 units were giving off white smoke, but that it was not interrupting repair work.

Edano said the three injured workers had suffered radiation burns on their legs while dragging an electrical cable through contaminated water in the effort to restore a crucial pump at Unit No. 3. Two were taken to Fukushima Medical University Hospital and were expected to be transferred to the National Institute of Radiological Science in Chiba City, east of Tokyo. He did not elaborate on the status of the third worker.

Hiro Hasegawa, a Tokyo Electric Power Company spokesman, said that the third man had not been hospitalised, but that he could not comment further.