Ukraine has accused Russia of planning to carry out a sabotage attack at the Zaporizhzhia nuclear plant that it has controlled since it seized it by force in March 2022. Although it reports this morning that this current threat is decreasing, the situation is fluid and the plant remains vulnerable to both accidents and attacks. While this ongoing crisis should not lead to panic, there is no cause for complacency either. Unfortunately, the American Nuclear Society (ANS) and other commenters have been busy attempting to dismiss the risks that either an accident or a deliberate attack could lead to a significant radiological release with far-reaching consequences. Simply put, the ANS is dead wrong here, and by minimizing the potential risk it is endangering Ukrainians and others who may be affected by lulling them into a false sense of security and undermining any motivation to prepare for the worst. Effective emergency preparedness requires a clear-eyed understanding of the actual threat.
As I have pointed out previously, the fact that the six reactors have been in shutdown mode for many months (with one in “hot”, as opposed to “cold,” shutdown) does reduce the risk somewhat compared to a situation where reactors are operating or have only recently shut down. The decay heat in the reactors’ cores decreases significantly over time, although the rate of decrease slows down quite a bit after a few months. However, this does not mean, as ANS misleadingly implies, that there is no risk of a major radiological release that could disperse over a wide area. What it does mean is that if cooling were disrupted to one or more of the reactors, then there would be a longer period of time—days instead of hours—for operators to fix the problem before the cooling water in the reactor cores would start to boil away and drop below the tops of the fuel assemblies, causing the fuel to overheat and degrade.
Timely operator actions are even more critical for reactors that are shut down than for reactors that are operating, since some automatic safety systems are not functional during shutdown. Indeed, in a 1997 report, the International Atomic Energy Agency (IAEA) points out that “acceptable results for most of events during shutdown modes cannot be achieved without operator intervention.” The IAEA report states that both “preventive and mitigatory capabilities are somewhat degraded” in shutdown conditions, and lists a number of shutdown accident initiators for VVER-1000s.
One class of events of particular concern are “boron dilution” accidents, in which the concentration of boron in cooling water necessary to maintain reactors in a subcritical state becomes reduced and nuclear fission inadvertently begins in the core. This would not only increase the reactor temperature and the amount of heat that would have to be removed, but would also generate new quantities of troublesome short-lived fission products, such as iodine isotopes, which have previously decayed away in the months since shutdown. (This is why it remains important that potassium iodide—a drug that can block uptake of radioactive iodine in the thyroid—continue to be available to communities who may be in the path of any plume.) It is also important to note that it is very unusual for reactors to be maintained for any length of time in either hot or cold shutdown modes with fuel remaining in the core, as is the case at Zaporizhzhia. Whenever nuclear reactors operate in unusual conditions that have not been thoroughly analyzed, risks increase.
Unfortunately, because of the incredible stress that the greatly reduced staff at Zaporizhzhia are under, and the unclear lines of command under Russian occupation, their ability to efficiently execute all the actions necessary to mitigate any accident or sabotage attack is in grave doubt. And if timely operator intervention does not occur, and the fuel assemblies are exposed, then a core melt accident similar to what was experienced in three of the reactors at Fukushima Daiichi is certainly possible. Once the water level has dropped below the tops of the fuel assemblies, the original decay heat in the reactor core is no longer a relevant factor because when the zirconium cladding surrounding the fuel rods overheats and reacts with steam or air, it produces additional heat through a so-called exothermic reaction. The heat released in this way would soon become far greater than the original decay heat load and would accelerate the heat-up and degradation of the reactor core. At that point, it would be much harder for operators to arrest the progression of the core melt. Eventually, the molten core would drop to the floor of the steel reactor vessel and melt through it onto the floor of the containment building, where it would react with concrete to generate hot gases. Then, there are multiple ways in which the radioactive gases and aerosols generated during the core melt could be released into the environment, including a containment melt-through mode that is possible in VVER-1000 reactors such as Zaporizhzhia.
There is no technical reason why any resulting radioactive releases could not disperse at least as far as occurred at Fukushima, depending on the meteorological conditions. The heat of the radioactive plumes, which determines how high they will rise in the atmosphere and hence how far they can travel, largely come from the heat released by zirconium oxidation. The magnitude and extent of the resulting environmental contamination would depend on the “source term,” or the inventory and characteristics of the radioactive materials released from the site. Since up to six reactors and six spent fuel pools could be involved—especially if the site is deliberately sabotaged—the source term could ultimately be larger than that of Fukushima, where only three reactors were involved and containments remained largely intact.
Thus it is imperative that the international community take Ukraine’s warnings seriously and provide all the assistance it needs for emergency preparedness. Unjustified complacency could lead to a lack of resolve for addressing the danger, only increasing the potential for a long-lasting disaster that will compound the misery of the Ukrainian people.