A small amount of radioactive cesium has been detected in the urine samples of 141 children in Japan’s Fukushima prefecture, where the crippled nuclear power plant is still leaking radiation, the Kyodo news agency reported on Sunday.

A total of 2,022 infants and young children were surveyed by a Japanese research group.

Three urine samples contained more than 10 becquerels of cesium per kilogram, including a case with 17.5 becquerels. Ten becquerels or less of cesium was found in the samples of the other 138, the Yokohama-based Isotope Research Institute said.

The urine samples also contained about an average 64 becquerels of radioactive potassium.

Japan was hit by a massive 9.0-magnitude quake last year which caused a tsunami, claiming almost 15,000 lives and triggering a number of explosions at the Fukushima nuclear power plant. The tsunami caused a partial meltdown at three of the nuclear plant’s reactors.