The capture of Ukraine's Zaporizhzhia nuclear power plant by Russian forces in March immediately sparked fears that the world could face another nuclear disaster on the scale of the Chernobyl explosion almost 40 years ago. The U.S. Embassy in Kyiv was quick to call the shelling of Europe's largest nuclear power plant a "war crime." 

"We survived a night that could have stopped the story, the history of Ukraine, the history of Europe," said Ukrainian President Volodymr Zelenskyy. An explosion at Zaporizhzhia would have equaled "six Chernobyls," he said, referring to the Ukrainian nuclear reactor meltdown of 1986 — widely seen as the most catastrophic nuclear disaster in history, with unparalleled health, economic, and environmental impacts. 

What's driving fears of a disaster at Zaporizhzhia?

Earlier this week, with continued shelling around the plant repeatedly cutting access to the electricity grid, plans were announced to shut down Zaporizhzhia's last working reactor. A stable electricity supply is essential for any nuclear power plant to maintain cooling systems for radioactive fuel. Zaporizhzhia does have diesel generators to switch on when the primary electricity supply is cut, but they are not a long-term solution.

"If the power goes off, we're then reliant on fairly elderly diesel generators to run the safety systems," Hamish de Bretton-Gordon, an expert in chemical, biological, radiological and nuclear weapons, told CBS News. "Once you lose the main power supply, you're almost in a two-engine airplane which loses one engine, and then you're in a bad position."  

"For a nuclear power station in the U.S. or U.K. to have to go on emergency power might happen once or twice in a decade," said de Bretton-Gordon. "So when you've got it happening once or twice a week … the chances of further problems increase exponentially." 

After the shutdown, Zaporizhzhia will enter a "cold state," according to Ukrainian nuclear operator Energoatom.   

But the integrity of the plant has been compromised by weeks of shelling, according to a 52-page report compiled by the United Nations-backed global nuclear watchdog, the International Atomic Energy Agency (IAEA), which sent a team of inspectors to Zaporizhzhia at the beginning of September. 

"Any further escalation affecting the six-reactor plant could lead to a severe nuclear accident with potentially grave radiological consequences for human health and the environment in Ukraine and elsewhere," the IAEA warned. 

The agency's Director General, Rafael Grossi, noted in the report that the Ukrainian technicians who've continued operating the plant under Russian occupation are "under constant high stress and pressure, especially with the limited staff available." He voiced concern that the conditions could "lead to increased human error with implications for nuclear safety."