The collapse was caused by snow piling up on the roof, the Emergencies Ministry said. The area where the roof collapsed is part of the power plant's original structure and not the so-called "sarcophagus" built after the accident.
No one was hurt in the incident and the radiation level in the so-called exclusion zone around the plant had not changed, the power station's management said.
“There is no threat to the lives or health of the population,” Russia’s Emergencies Ministry said early on Wednesday while confirming the report from the plant.
The Chernobyl disaster took place on April 26, 1986, when one of its four nuclear reactors exploded. Two workers were killed in the explosion and 28 other rescuers and staff died of radiation exposure in the next months. Tens of thousands of people were evacuated.
The Soviet leadership was slow to admit the scale of the accident and order an evacuation. Radioactive contamination spread as far as northern Sweden and the UK.
A new "sarcophagus," which will cover the existing one built after the accident, is currently under construction, as engineers fear that the older one may soon start crumbling, possibly leading to a radiation leak.
The new arch-shaped sarcophagus is expected to contain the unit safely for about one hundred years.
The Chernobyl nuclear power plant has set up a committee that will conduct a probe into the collapse. The investigation is expected to last for two weeks, the plant’s press service said.
“The committee has been tasked with analyzing the situation that led to the building structures' collapse within a 14-day period and with introducing proposals to overcome the consequences,” the statement said.