Shelling at the Zaporizhzhia Nuclear Power Plant Is Raising Fears of an Accident. Here’s a Look at the Risks.

KYIV, Ukraine — When Russian forces seized control of Europe’s largest nuclear power plant in early March, a fierce gun battle with Ukrainian troops triggered a blaze that raised worldwide alarm over the risks of a catastrophic radiation leak.

The fire was quickly extinguished. And although a Russian shell hit the No. 1 reactor, its thick walls protected it from damage, the Ukrainian government said at the time.

Now, five months later, repeated shelling inside the Zaporizhzhia Nuclear Power Plant complex over the past seven days has stirred new concerns, with Ukrainian and Western officials warning that the attacks heighten the risk of a nuclear accident.

Each side blames the other for the explosions at the plant.

The Ukrainians have accused the Russians of directing strikes there to cut off energy supplies to other cities and to try to discredit the Ukrainian military in the world’s eyes. The Russians say Ukraine is doing the shelling.

Both sides would suffer if a meltdown occurred and spread radioactive material.

Ukrainian officials have also expressed growing alarm over the working conditions at the facility. More than 10,000 Ukrainian employees are charged with keeping the plant running safely even as Russia has transformed it into a military fortress and engaged in what Ukrainian officials say is a campaign of intimidation and harassment.

Rafael M. Grossi, the head of the International Atomic Energy Agency, said Thursday at a meeting of the U.N. Security Council that the world faced a “grave hour” as the safety of the plant deteriorated and called for a team of international experts to be given access to the plant immediately.

Mr. Grossi said that for now there was “no immediate threat” as a result of the recent shelling but warned that the assessment “could change at any moment.”

The United States has called for the creation of a demilitarized zone around the plant, but Russia has given no indication that it would even consider leaving the facility.

President Volodymyr Zelensky of Ukraine, speaking to a nation that still bears the scars of nuclear catastrophe from the meltdown of the facility at Chernobyl in 1986, said the Kremlin was engaging in “unconcealed nuclear blackmail” and called the situation at the plant “one of the biggest crimes of the terrorist state.”

As world officials warn of the growing risk at the plant, here is a look at the situation and the most pressing concerns.

Shelling continued over the past week.

The Zaporizhzhia plant occupies a spot on the Dnipro River along the front lines of the war between Russia and Ukraine. The Ukrainian Army controls the west bank, while the Russians are entrenched around the plant on the river’s east bank.

For weeks, Ukrainian officials say, Russian forces have been fortifying the outside of the plant and using it as a staging ground for attacks on Ukrainian-controlled territory, calculating that Ukrainian forces will not return fire because of the risk posed by an errant strike. Ukrainian officials said they are mostly not returning fire, and when they do it’s guided, like a drone.

On Aug. 5, shells struck the complex. Shelling has continued over the past week.

After shelling on Thursday, workers at the plant were forced to activate an emergency protection power unit, according to a statement from Energoatom, the Ukrainian agency responsible for running all of Ukraine’s nuclear power plants. It said the plant now stood the risk of operating without proper fire safety standards because of damage to its internal power systems.

Another round of shelling ignited a fire in the area of the plant’s nitrogen-oxygen station but it was put out.

At least one staff member working in the area where dry spent nuclear fuel is stored was injured in yet another episode of shelling.

The most pressing concerns focus on the risk of a meltdown.

While they are designed to withstand a range of risk — from a plane crashing into the facility to natural disasters — no operating nuclear power plant has ever been in the middle of active fighting, and this one was not designed with the threat of cruise missiles in mind.

There are several main concerns.

The concrete shell of the site’s six reactors offer strong protection, as was the case when the No. 1 reactor was struck in March, officials say. More worrying is the chance that a power transformer is hit by shelling, raising the risk of a fire.

Ukrainian officials have accused Russia of hiding dozens of military vehicles with an unknown amount of munitions on the premises of at least two reactors. If a fire were to break out at the power transformers and the electric network was taken offline, that could cause a breakdown of the plant’s cooling system and lead to a catastrophic meltdown, said Edwin Lyman, a nuclear power expert at the Union of Concerned Scientists, a private group in Cambridge, Mass.

He noted that the loss of coolant during the Fukushima accident in Japan in 2011 resulted in three reactors undergoing some degree of core meltdown.

If the cooling is interrupted, Dr. Lyman said, the nuclear fuel could become hot enough to melt in a matter of hours. Eventually, it could melt through the steel reactor vessel and even the outer containment structure, releasing radioactive material.

According to Ukrainian officials, a shell hit a power transformer at the No. 6 reactor at the same time the No. 1 reactor was struck. It did not explode, according to Ukrainian officials.

Dr. Lyman said the threat would decrease in the case of a military strike on the dry spent-fuel storage area next to Zaporizhzhia’s reactors. While used fuel can still be dangerously hot for years, it quickly loses much of its radioactivity, making any breach less threatening — although it if were hit by a shell or missile the radioactive particles would spread in the air.

Workers are facing difficult conditions.

Russian soldiers are detaining workers and subjecting them to brutal interrogations in a search for possible saboteurs, prompting many employees to leave and raising concerns about safety, Ukrainian officials say.

“People are being abducted en masse,” Dmytro Orlov, the exiled mayor of the nearby city of Enerhodar, said during a meeting last month with officials from Energoatom. “The whereabouts of some of them are unknown. The rest are in very difficult conditions: They are being tortured and physically and morally abused.”

A Ukrainian energy official who discussed plant security matters on the condition of anonymity because of the sensitivity of the subject said that at least 100 staff members have been detained in recent weeks. Some who were released bear the scars of torture and 10 employees are still missing, the official said.

Those claims could not be independently confirmed.

Ukraine says Russia is using the nuclear plant as a form of pressure.

Ukrainian officials have said the Russians are using the plant as a form of nuclear blackmail, and that they have shelled the facility to remind the world that they control what happens there. The strikes, they claim, are directed by officials from the Russian nuclear agency, Rosatom, who are on the site and have so far been directed to hit things that are not considered essential to the safe operation of the plant, like the sewage system.

Russia can also disrupt the power supply across Ukraine by reducing the flow of energy from the plant to the Ukrainian grid.

“The Russians understand that energy is a massive tool of power,” R. Scott Kemp, a professor of nuclear science at Massachusetts Institute of Technology, told The New York Times when the Russians first took control of the plant. “It’s a point of tremendous leverage.”

How far radioactive material could spread during a meltdown depends on the circumstances.

Let’s imagine a meltdown occurred and radioactive material spread out from the plant.

Disaster scenarios with nuclear reactors typically are based on local circumstances — how bad is the breach, does the groundwater flow in a specific direction, is the wind blowing and, if so, which way and with what forcefulness over time, steady or variable?

In terms of power output, the six reactors at Zaporizhzhia are roughly the same size as the Chernobyl reactor that in 1986 suffered a meltdown and explosions that destroyed the reactor building. In that case, the breach was extremely bad and the prevailing winds blew the clouds of radioactive debris mostly into Belarus, Ukraine and Russia. Lesser amounts were detected in other parts of Europe.

Dr. Lyman of the Union of Concerned Scientists said that, even if relatively small, the repercussions of a meltdown could involve local contamination, mass evacuations, farm stoppages and many billions of dollars in cleanup costs.

William J. Broad and Anna Lukinova contributed reporting.