As renewed shelling intensified fears about a nuclear accident at the Zaporizhzhia power plant, Ukrainian authorities stepped up emergency drills Saturday and rushed to hand out potassium iodide, a drug that can protect people from radiation-induced thyroid cancer, to tens of thousands of people living near the facility.

In a country still haunted by the memory of the 1986 Chernobyl nuclear disaster, officials urged the public not to panic even as complex negotiations to allow for a team of scientists from the International Atomic Energy Agency to visit the Russian-controlled plant in southeastern Ukraine took on added urgency.

The agency, the United Nations’ nuclear watchdog, has assembled a team of experts to travel to Zaporizhzhia — Europe’s largest nuclear power station — as early as this week.