President Volodymyr Zelensky of Ukraine called the damage “significant” but said there were no signs of radiation leaks. A Kremlin spokesman denied that Russia had carried out the strike.

Ukraine said a Russian drone crashed into the containment structure, starting a fire, but so far radiation levels have stayed constant.

The outer shell that was struck is a protective cover built in 2016 over a heavy concrete containment structure. Soon after the 1986 disaster, the inner layer (in which there was no indication of a breach on Friday morning), was placed on the plant’s fourth reactor. The containment shells seek to prevent radiation leaks.