Some 45 per cent of 1,080 children under 15 from the Japanese Fukushima Prefecture have tested positive for thyroid exposure to radiation, a nuclear watchdog report says.

The screening was performed by Japan's Nuclear Safety Commission between March 26 and March 30 in the wake of the Fukushima Daiichi nuclear disaster, the independent body said on Tuesday.

Among those who tested positive, the degree of exposure was measured at 0.04 microsieverts per hour or less in the majority of cases. The maximum measured was 0.1 microsieverts per hour in a one-year-old. The government-established threshold for further investigation is 0.2 microsieverts per hour, so no emergency probe will be undertaken, officials said.