What are the risks facing Zaporizhzhya, the nuclear power plant in a war zone?

Nuclear power plants (NPPs) are complex and sophisticated facilities with several layers of safety measures, but no NPP in the world is built to withstand military conflict – yet this is just the risk that has befallen the Zaporizhzhya NPP in Ukraine. It was taken over by Russian forces in May 2022 and has since often had to operate in conditions that threaten safety at the facility.

In June, a Lithuania-based NGO named the Bellona Foundation published a report, authored by its nuclear advisor, Dmitry Gorchakov, analysing the risks associated with the hostilities around, near, or at the Zaporizhzhya NPP based on the facility’s design, safety measures, and the local geography.

Is Zaporizhzhya comparable to Chernobyl?

Whenever the Zaporizhzhya NPP has been threatened, various media reports have repeatedly drawn comparisons to the Chernobyl NPP and the infamous accident there in 1986. The Bellona report’s headline finding, however, is that any damage to the Zaporizhzhya NPP is unlikely to play out in the same fashion or at the same scale.

The principal difference between Chernobyl and Zaporizhzhya is that the former had RBMK reactors and the latter has VVER-1000 reactors. (This is the same reactor design installed at the Kudankulam NPP in India.) As the Bellona report also noted, Zaporizhzhya also takes advantage of safety measures installed in the aftermath of the Chernobyl and Fukushima disasters.

What is the reactor design at Zaporizhzhya?

Zaporizhzhya NPP is located southwest of Zaporizhzhia city, along the Dnieper river. It has six VVER-1000 reactors for a total power generation capacity of 6 GW.

The reactor complex consists of the reactor vessel, in which fuel rods are surrounded in water. Control rods are inserted at the top. The water acts as both coolant and moderator. A pressuriser holds the water at a high but constant pressure – around 150 atm – to prevent it from boiling. This is the primary cooling circuit.

As the water heats up, the heat is moved to a secondary cooling circuit, where it converts a separate resource of water into steam. This steam is fed to turbines that generate electricity.

In this design, the primary coolant and the moderator are the same substance (water), and it doesn’t leave the reactor vessel at any time.

In RBMK reactors like at Chernobyl, on the other hand, the coolant and the moderator are different (light water and nuclear graphite, respectively) and the coolant – which is radioactive for having been exposed to the nuclear fuel – flows out of the reactor vessel. One reason Chernobyl was so bad was that when the reactor was breached, the superhot graphite caught fire when it came in contact with air.

Unlike Chernobyl, the VVER-1000 reactor and its power-generation units at Zaporizhzhya are also placed inside a large airtight chamber called a containment. Its walls are 120 cm thick and made of prestressed concrete.

What is the risk at Zaporizhzhya today?

The Bellona report evaluated the risk of different types of accidents at the facility based on different types of damage sustained. In the worst case scenario, the containment is completely damaged and a projectile strikes a reactor while it is generating power.

The principal danger here is that the primary circuit water could depressurise as steam and escape into the air, along with radioactive material and other volatile substances. This mixture will contain the isotope iodine-131, which is easily dispersed by winds and accumulates in and damages the thyroid gland in humans. It has a half-life of around eight days and so, per the report, “would only pose a threat for several weeks”.

A breach and depressurisation would also release caesium-137, which has a half-life of 30 years and was responsible for contaminating much of Chernobyl’s surroundings after the accident.

How either isotope is dispersed depends on the immediate weather, especially the strength and direction of the wind. This said, due to design differences, what was released in sustained fashion at Chernobyl for around a week is likely to be released in a single, short burst at Zaporizhzhya. This in turn could keep the fallout to within around a few hundred kilometres, according to one estimate cited in the report.

What are other threats to Zaporizhzhya?

The worse scenarios are separated from the better ones depending on the reactors’ operational statuses. If the reactors have been shut for a few months, the iodine-131 will have almost completely decayed, removing an important threat. If a reactor has been in cold-shutdown – i.e. shutdown and the primary circuit is almost at atmospheric pressure – then the chances of an explosive leak also drop.

Since September 10, 2022, the six reactors at Zaporizhzhya have been shut. In late 2022, two were placed in a state of semi-hot shutdown, meaning the primary circuit was held at 200 degrees Celsius with heat from the decaying nuclear fuel. This was required to provide heat at the facility and for the nearby town of Energodar.

As of May 2023, all reactors but the sixth were in cold-shutdown.

The Bellona report discussed several possibilities based on combinations of conditions. One was the ‘Fukushima scenario’ – when the NPP becomes disconnected from the external power grid. This is dangerous because, when nuclear reactions aren’t happening in the reactor, the nuclear fuel has to be cooled, which means the coolant pumps need to operate. If they don’t, the fuel could become hot enough to melt through the reactor’s bottom, where it will contaminate soil, air, and water.

The report’s ultimate concern? The working conditions of the 3,000 or so people at the plant, most of whom have refused to sign new employment contracts with their new Russian employers, amidst — among other things — uncertainties over the management, violation of protocol, and “suspicions of disloyalty”.

What does the report recommend?

On June 6, 2023, the Kakhovka dam, which is downstream of the Zaporizhzhya NPP and in whose reservoir the plant is located, was breached. While the water level in the reservoir subsequently dropped, the Zaporizhzhya NPP wasn’t affected because the cooling pond from which it draws water is isolated from the water in the reservoir. The only way water can enter or exit the pond is through controlled sluice gates. The Bellona report suggested that the walls of the pond were built to withstand a water-level differential of 6 metres.

After considering the possibility of this breach as well, the report made the following recommendations (reworded):

* All reactors should be in shutdown or cold-shutdown states

* There should be no effort to move fuel at the same time as hostilities around the plant

* Hostilities should be kept out of the territory of the plant itself

* If/when Russian troops withdraw from the plant, plant staff should be rehabilitated

• Zaporizhzhya NPP is located southwest of Zaporizhzhia city, along the Dnieper river. It has six VVER-1000 reactors for a total power generation capacity of 6 GW.
• The Bellona report evaluated the risk of different types of accidents at the facility based on different types of damage sustained. In the worst case scenario, the containment is completely damaged and a projectile strikes a reactor while it is generating power.
• If the reactors have been shut for a few months, the iodine-131 will have almost completely decayed, removing an important threat. If a reactor has been in cold-shutdown – i.e. shutdown and the primary circuit is almost at atmospheric pressure – then the chances of an explosive leak also drop.