Unlike an operating nuclear plant, which can use some of the electricity it produces to power its operation, the Chernobyl plant is completely dependent on outside sources of electricity.
Problems can arise when the power is cut, as Ukrainian officials and the International Atomic Energy Agency say it has been by Russian troops.
The scene of the worst nuclear disaster in history, when one of the four reactor exploded and burned 36 years ago, is mostly powered by diesel generators. A former employee of the plant said that some equipment was functioning on battery power, and that the plant's radiation monitoring systems had been affected.
The I.E.A.E said there was no critical impact on safety at the complex. If all the backups failed and Chernobyl was left without power, what would happen?
The first two reactor of the Chernobyl plant were completed in the late 1970s. The third and fourth units were operating by 1983, but the one that was destroyed three years later was not.
More than two dozen people died from exposure to high levels of radiation, the result of an ill-advised and ill-executed test. Many more people were exposed to the radioactive particles from the burning reactor core, and they suffered long-term effects.
The exclusion zone around the plant remains largely off limits, despite the fact that it was declared the most contaminated part of Ukraine and Belarus. Villages and a city were abandoned permanently.
The last of the plant's three reactor were shut down in 2000. The nuclear fuel and equipment that generated power have been removed.
There is no risk of a core meltdown at the plant if the power goes out and the water goes out. The backup power systems were wiped out when an earthquake and tsunami hit Japan in 2011.
Chernobyl has a lot of nuclear waste on site.
The fuel inside a reactor is eventually used up. In the nuclear power industry, the fuel that was removed from the four Chernobyl reactors is stored in pools of water that cools it down as it decays. When fuel is removed from a reactor, there is a lot of decay and therefore a lot of heat, so plants need power to run pumps that circulate the storage water, removing excess heat in the process.
The fuel in the storage tanks could catch fire if the water in the tanks got so hot that it boiled off. That was one of the risks in the disaster.
The I.A.E.A. says that the used fuel assemblies at Chernobyl are safe because they are old.
The heat load of the spent fuel storage pool and the volume of cooling water contained in the pool is sufficient to maintain effective heat removal without the need for electrical supply, according to the agency.
Before the Russian invasion of Ukraine, workers had begun transferring fuel to a long-term dry storage facility. When the fuel has cooled enough, it is ready for dry storage.
The ruins of the destroyed reactor are the main source of nuclear waste. 200 tons of fuel remain in a lava-like mix with molten concrete, sand and chemicals that were dumped on the reactor during the disaster.
This highly radioactive mixture was found throughout the remains of the reactor, having flowed through doorways and drain pipes and down stairwells and other parts of the structure before it hardened. Some of the fuel-filled materials are only studied by boring into them in places that are completely out of reach.
There is no cooling system for a loss of power in the chaotic remains of the destroyed Chernobyl reactor.
In recent years there have been episodes in which nuclear reactions have started spontaneously in pockets of these fuel-based materials, leading to spikes in radiation levels. Someday, they will have to be dealt with.
Without monitoring humidity and radiation, workers wouldn't know if there was a new episode. The former employee with knowledge of the plant said that the systems that helped control the humidity had stopped working.
The destroyed reactor has been covered with a large arched structure in order to confine the waste and protect against any release of radiation. The work of removing waste to long-term storage is expected to take decades.
The facility only received an operating license from the authorities in Ukraine last year. Large cranes and other specialized equipment allow crews to work safely. Most of the work could not be done without a power supply.
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