Here's why the risk of a nuclear accident in Ukraine has 'significantly increased'

The head of the International Atomic Energy Agency warns that the risk of a nuclear accident at the Zaporizhzhia nuclear power plant has increased.

The director general of the IAEA said in a recorded statement that the shelling around the nuclear plant should stop.

The plant's last operating reactor may have to be shut down due to the crisis. The current nuclear crisis could get worse if that happened. This is how.

Nuclear plants need electricity

Europe's largest nuclear plant can produce thousands ofmegawatts of electricity. The plant is dependent on the same electricity grid for power.

The power is used to keep the plant running. Nuclear power plants need water and electricity in order to function safely.

The conflict has caused the four high-voltage lines that connect the nuclear complex to the electricity grid to be cut. The last 750kV line was severed.

There was a fire on the site that caused the backup line to be disconnected. The power lines were being targeted in a press conference by Grossi.

He told reporters after he returned from the plant that those who have military aims know how to hit where it hurts.

Zaporizhzhia has been making its own power, but that's a limited solution

The nuclear plant has been using "islanding operation mode" since it lost its connection to the grid. The rest of the facility is powered by low levels of electricity produced by the Unit 6 reactor.

A nuclear engineer who was not authorized to speak publicly said that the reactor at Zaporizhzhia was designed to operate during startup.

He says it can't be done for a long time. The problem is not with the reactor itself, but the turbine, generators, and other systems that are designed to run at higher power levels than islanding mode provides.

The strain on the Ukrainian operators is adding to the problem. Most of the plant's staff live in the nearby town of Enerhodar. The water, sewage and electrical supplies have all been disrupted by the same fighting that damaged the lines.

Operators and their families are at risk because of the shelling.

Shutting down the last reactor will trigger emergency generators

It's more likely that Ukrainian authorities will power down the last reactor. That could make the crisis worse.

Nuclear reactor are more similar to grills than gas stove. They remain hot even after they're shut off. There is still water in the core.

Emergency diesel generators will be used to keep the reactor cool after it shuts down. Emergency generators are used to cool a nuclear reactor. The Nuclear Regulatory Commission requires U.S. plants to use emergency diesel generators.

Steven Nesbit is a nuclear engineer and member of the American Nuclear Society's rapid response task force. The Turkey Point Nuclear Plant in Florida had to use diesel power for days after Hurricane Andrew in 1992.

If the generators run out of fuel, a meltdown could occur

There are more than a dozen emergency generators standing by. The agency says the plant normally has a 10-day reserve of diesel fuel, but currently has 2,250 tonnes of fuel.

If the generators are damaged or the fuel is low, there is a chance of a disaster.

That doesn't mean there would be a catastrophe like Chernobyl. A mix of design flaws and operator error made it impossible to replicate the Chernobyl disaster.

The reactor at Zaporizhzhia has been shut down for a while to allow the nuclear fuel to cool.

In the worst case scenario, the reactor at Zaporizhzhia is a modern design surrounded by atainment building. Three to four feet of reinforced concrete is designed to resist very high internal pressures.

Any radioactive material can be held in it.

The world's nuclear agency isn't interested in testing this. Grossi wants all sides to put a safety zone in place.

He says there is no time left.