The United Nations nuclear watchdog said on Monday that the remaining members of the one shift of technical staff who had been on duty at Chernobyl’s radioactive waste facilities since Russian forces seized the site last month had been relieved.

For more than three weeks, the Ukraine facilities, located next to the now-defunct power plant that was the site of the world’s worst nuclear disaster in 1986, were run by a single shift of personnel who happened to be on duty when Russian forces took control on Feb. 24. They were all unable to leave until Sunday.

The International Atomic Energy Agency (IAEA) has been saying for weeks that the situation, which meant that the staff on duty were exhausted and working under extreme pressure, posed a growing risk to the site’s safety and that they should be rotated out.

“With the exception of thirteen staff members who declined to rotate,” the IAEA said in a statement. “Ukraine’s regulatory authority said about half of the outgoing shift of technical staff left the site of the 1986 accident yesterday and the rest followed today.”

The Ukrainian regulator said most of the Ukrainian guards who have also been there since it was seized remained at the site, the IAEA added. The agency said last week that there were 211 technical staff and guards at the site, without breaking that number down further.

The departing technical staff have been replaced by Ukrainian colleagues who like them are based in the nearby town of Slavutych, the IAEA said, citing the Ukrainian regulator.

“The new work shift … includes two supervisors instead of the usual one to ensure that there is back-up available on the site, the regulator said,” according to the IAEA statement.

The IAEA added, also citing the Ukrainian regulator, that an agreement had been reached on how to organize staff rotations.