By Guy Faulconbridge and Francois Murphy

MOSCOW/VIENNA (Reuters) - U.S. President Donald Trump said he would speak to Russia's Vladimir Putin on Tuesday about ending the Ukraine war, with the question of control of the Zaporizhzhia nuclear power plant likely to be an important part of the talks.

Russia and Ukraine blame each other for drone attacks and other military activity at or around Zaporizhzhia, Europe's largest nuclear plant, and the International Atomic Energy Agency has long warned of the risks of a disaster there.

The plant is 500 km (300 miles) from the site of the world's worst nuclear accident, the 1986 Chornobyl disaster. In February, a drone caused significant damage to the structure built to stop any radioactive release from the reactor damaged at Chornobyl.

The IAEA in February blamed military activity for the cancellation of a rotation of a monitoring mission at the Zaporizhzhia plant, without apportioning responsibility. In December an IAEA car was hit by a drone on the road to the plant.

What nuclear material is at the Zaporizhzhia plant, what are the risks and why are Russia and Ukraine fighting over it? 

WHAT IS IT AND WHAT WAS ITS CAPACITY?

The Zaporizhzhia nuclear power plant has six Soviet-designed VVER-1000 V-320 water-cooled and water-moderated reactors containing Uranium 235. They were all built in the 1980s, though the sixth only came online in the mid-1990s after the collapse of the Soviet Union. 

All of the reactors are in cold shutdown.

IAEA Director General Rafael Grossi says that fighting a war around a nuclear plant has put nuclear safety and security in "constant jeopardy". 

WHAT ARE THE RISKS?

Russian forces took control of the plant in early March 2022, weeks after invading Ukraine. Special Russian military units guard the facility and a unit of Russia's state nuclear company, Rosatom, runs the plant.

Nuclear reactors' containment structures like Zaporizhzhia's are made of steel-lined reinforced concrete designed to withstand the impact of a small plane crash so there is little immediate risk from a minor attack on those structures.

A 1989 study by the U.S. Department of Energy found that the model of containment structure used in Zaporizhzia "exhibits vulnerabilities to the effects of an aircraft crash" and a fighter jet crashing downwards into the dome, where the structure is thinner, could penetrate it, causing concrete chunks and aircraft engine parts to fall inside.

External power lines essential to cooling nuclear fuel in the reactors are a softer potential target. Cooling fuel, even in reactors in cold shutdown, is necessary to prevent a nuclear meltdown.

Since the war began the plant has lost all external power several times, forcing it to rely on emergency diesel generators for power. Water is also needed to cool fuel.

Pressurised water is used to transfer heat away from the reactors even when they are shut down, and pumped water is also used to cool down removed spent nuclear fuel from the reactors.

Without enough water, or power to pump the water, the fuel could melt down and the zirconium cladding could release hydrogen, which can explode. 

WHAT ABOUT THE SPENT FUEL?

Besides the reactors, there is also a dry spent fuel storage facility at the site for used nuclear fuel assemblies, and spent fuel pools at each reactor site that are used to cool down the used nuclear fuel.

Without water supply to the pools, the water evaporates and the temperatures increase, risking a fire that could release a number of radioactive isotopes.

An emission of hydrogen from a spent fuel pool caused an explosion at reactor 4 in Japan's Fukushima nuclear disaster in 2011.

WHAT HAPPENS IN A MELTDOWN?

A meltdown of the fuel could trigger a fire or explosion that could release a plume of radionuclides into the air, which could then spread over a large area. 

The Chornobyl accident spread Iodine-131, Caesium-134, Strontium-90 and Caesium-137 across parts of northern Ukraine, Belarus, Russia, northern and central Europe.

Nearly 8.4 million people in Belarus, Russia and Ukraine were exposed to radiation, according to the United Nations. Around 50 deaths are directly attributed to the disaster itself. 

But 600,000 "liquidators", involved in firefighting and clean-up operations, were exposed to high doses of radiation. Hundreds of thousands were resettled. 

There is mounting evidence that the health impact of the Chornobyl disaster was much more serious than initially presented at the time and in the years following the accident.

Incidence of thyroid cancer in children across swathes of Belarus, Russia and Ukraine increased after the accident. There was a much higher incidence of endocrine disorders, anaemia and respiratory diseases among children in contaminated areas.

(Reporting by Reuters bureaus; Editing by Alex Richardson)