TOKYO (AP) ― Experts from the International Atomic Energy Agency arrived in the Japanese city of Fukushima on Sunday to observe the massive decontamination effort following the world’s worst nuclear disaster since Chernobyl.

Local doctors also began a long-term survey of children for thyroid abnormalities, a problem associated with radiation exposure. Officials hope to test some 360,000 people who were under the age of 18 when the nuclear crisis began in March, and then provide follow-ups throughout their lifetimes.

The 12-member IAEA group was to visit farms, schools and government offices throughout Fukushima prefecture in northeastern Japan to observe the clean up process. It is the U.N. atomic agency’s second major mission to Japan since the crisis at Fukushima’s Dai-ichi nuclear power plant began.

Nearly 20,000 people were killed when the earthquake and tsunami hit Japan on March 11, and the disaster severely damaged the Fukushima complex. Officials say the plant is now relatively stable, but tens of thousands of people still cannot ― or choose not to ― return to their homes because of the radioactive contamination.