Giant nuclear power plant in Ukraine is at risk of 'unlimited release' of nuclear materials unless a safety zone is established, UN warns

The United Nations' (UN) atomic watchdog has called for the creation of a demilitarized zone around Ukraine's Zaporizhzhia nuclear power plant, warning that shelling near the facility could cause the "unlimited release" of radioactive materials into the environment.

Russian forces took over the Zaporizhzhia plant, which is the largest nuclear power plant in Europe and produces nearly one-fifth of Ukraine's power, on March 4, the ninth day of their invasion of Ukraine, Live Science previously reported. Since then, Ukrainian nuclear plant workers have been operating the facility under Russian occupation. The site lies on the south bank of the Dnieper River, across from Ukrainian occupied areas, and has been attacked numerous times with both sides accusing the other of shelling the plant. 

Those attacks have produced damage across the plant, according to an International Atomic Energy Agency (IAEA) report produced after a site visit. Observers also experienced active shelling during the team's visit to the plant. Only one of the Zaporizhzhia plant's six reactors is operational, and with all four power lines from Ukraine's electrical grid to the plant disconnected, Zaporizhzhia has just one emergency backup line left: a nearby thermal plant that can pump vital cooling water around the reactor. If the active reactor does not constantly receive this coolant, a meltdown could occur. 

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"We are playing with fire, and something very, very catastrophic could take place," Rafael Mariano Grossi, the IAEA's director general, said Tuesday (Sept. 6) at an emergency session of the U.N. Security Council. "This is why in our report we are proposing the establishment of a nuclear safety and security protection zone limited to the perimeter and the plant itself."

The IAEA said it wants to consult with both Russia and Ukraine "immediately" about the steps needed to establish the security zone. During the Security Council briefing, U.N. Secretary General António Guterres said the first step would be for both sides to cease all military operations around the plant.

"As a second step, an agreement on a demilitarized perimeter should be secured," he said. "Specifically, that will include the commitment by Russian forces to withdraw military personnel and equipment from that perimeter and the commitment by Ukrainian forces not to move in."

The reaction at the heart of all operating nuclear power plants is nuclear fission, in which heavy isotopes of uranium and plutonium absorb incoming neutrons before splitting and releasing energy. This splitting also chucks out more neutrons, which other heavy isotopes absorb before splitting in turn, creating a chain reaction. The thermal energy produced by a chain reaction can be used to heat water, create steam and spin electricity-generating turbines. But if this process isn't carefully managed, a runaway reaction can occur, and a nuclear plant can go into meltdown. 

This is because nuclear reactors run extremely hot — with some parts reaching up to 3,272 degrees Fahrenheit (1,800 degrees Celsius) — and if coolant doesn't constantly circulate around them, the fuel inside can easily melt. This transforms nuclear fuel and its cladding into a radioactive magma-like lump, which sinks through the reactor and then the building, melting everything in its path. The melting of the cladding also generates hydrogen gas — meaning that when the molten goop finally breaks from the reactor and the hydrogen makes contact with the oxygen in the air, it explodes.

Of course, Ukraine has experienced a catastrophe of this kind before. On April 26, 1986, the disastrous meltdown at the Chernobyl nuclear power plant led to two enormous explosions that blew the 2,000-ton (1,800 metric tons) lid from one of the plant's reactors, blanketing the region with reactor debris and radioactive fuel. The explosion released 400 times more radiation into the atmosphere than was produced by the atomic bomb dropped on Hiroshima, and nuclear fallout rained down across Europe, according to a report by the European Parliament.

Despite the frightening surface similarities between the two events, Zaporizhzhia's reactors are safer than the ones at Chernobyl, nuclear scientists say. Unlike Chernobyl, which used RBMK-1000 reactors, Zaporizhzhia has more modern pressurized water reactors, which require significantly less uranium fuel in the reactor core, thus limiting the likelihood of a runaway chain reaction. Two layers of protection — a steel-reinforced concrete outer containment unit and an 8-inch-thick (20 centimeter) steel inner vessel — also surround the reactor. Both layers are designed to withstand earthquakes, explosions and collisions from incoming planes. Pressurized water reactors also shut down automatically in the event of an emergency. 

Nonetheless, a direct shell hit to the outside of a reactor could be dangerous, Robin Grimes, a professor of materials physics at Imperial College London, said in a statement. Be it from a shell outside or a meltdown inside, a puncture to the Zaporizhzhia reactors' twin shells wouldn't lead to an explosion like the one at Chernobyl, but it would still release a lot of dangerous material.

"It is not designed to withstand explosive ordinance, such as artillery shells," Grimes said. "While it seems to me unlikely that such an impact would result in a Chornobyl-like nuclear event, a breach of the pressure vessel would be followed by the release of coolant pressure, scattering nuclear fuel debris across the vicinity of the plant and a cloud of coolant with some entrained particles reaching further."

Zaporizhzhia's reactor cores still contain a decent amount of highly radioactive fuel for an explosion, however, and this is not the only source of concern. Environmentalists and nuclear experts have long warned that the plant's spent nuclear fuel rods, cooling in acres of open water pools and standing in open-air yards behind the site, could produce lethal airborne plumes of radiation if struck by a stray shell or missile.

Two weeks ago, after fire damage to one of the plant's transmission lines temporarily knocked the facility offline, Ukrainian officials began handing out iodine tablets to the residents of the nearby city of Zaporizhzhia. In response to the news, the European Union has donated 5.5 million of these tablets, which help to block the body's absorption of the deadly radioactive byproduct iodine, to Ukraine.

"While the ongoing shelling has not yet triggered a nuclear emergency, it continues to represent a constant threat to nuclear safety and security, with potential impact on critical safety functions that may lead to radiological consequences with great safety significance," the IAEA inspectors wrote in the report.

Originally published on Live Science.