People around the world watched via livestreamed security camera as Russian forces attacked and took over Ukraine’s Zaporizhzhia nuclear power plant—the largest in Europe—on Friday morning local time. Amid the shelling and gunfire, a fire broke out at a training facility in the complex and was later extinguished, according to news reports. The incident raised alarm among world leaders and nuclear experts about the potential for purposeful or accidental reactor damage that could cause radiation leaks or, in a worst-case scenario, reactor core meltdowns.
Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), told the United Nations Security Council that the plant’s operations were normal after the attack and has said that no radioactive material was released. But he and other nuclear experts have warned that there is a danger of accidents there and at other nuclear plants in Ukraine as the conflict continues.
Scientific American spoke with Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, to explain the concerns about such facilities during wartime and to talk about some of the safety measures that are in place.[An edited transcript of the interview follows.]
What type of reactors does the Zaporizhzhia complex have, and how might they differ from those at Chernobyl or U.S. nuclear power plants?
The six reactors at Zaporizhzhia are called VVER-1000s, and they are cooled and moderated by light [ordinary] water. So in that respect, they’re similar to U.S. pressurized water reactors. They are somewhat more advanced models than the earlier versions of [this type of reactor], so they do have some features that are more in line with modern safety philosophy—but not entirely. And they’re different from the Chernobyl-type reactors, called RBMKs, which used a different moderator material—graphite—and had a lot of technical flaws, which contributed to the occurrence and the severity of the [Chernobyl] accident in 1986. So the type of accident that occurred at Chernobyl, which is essentially a power excursion, is very unlikely in a light water reactor.
In any nuclear reactor, the purpose is to maintain a nuclear fission chain reaction in the fuel that generates heat and also additional neutrons, which are used to propagate the chain reaction. When uranium atoms fission, they release heat, so the fuel gets hot. Water in this type of reactor is pumped through the core and is heated and then is transferred to another loop that’s under high pressure. And then it’s transferred to another system of steam generators, where water is turned into steam, and that turns a turbine.
These [Zaporizhzhia reactors] were designed in the Soviet Union, and they date as far back as the early 1980s. So they are past their expiration date, but the Ukrainians extended their licenses.
Does the age of the reactors factor into the safety concerns here?
Well, it always has to be taken into account, because they got license extensions. They have been refurbished to some extent, but there are always systems that can’t be replaced. For instance, the vessel that holds the reactor fuel—and which becomes embrittled over time—that poses a risk in certain types of accidents, so you do have to factor it in.
What are the kinds of safety systems that this type of reactor would have against accidents?
The big danger in any nuclear reactor is that somehow cooling of the fuel is disrupted, because without enough cooling, the fuel will heat up to the point where it can destroy itself. This is what happened, to a lesser extent, at Three Mile Island [in Pennsylvania in 1979] and, to a greater extent, at Fukushima [in Japan] in 2011. In addition, these plants store their spent nuclear fuel on-site—and some of that fuel is stored in cooling water, which also has to be replenished with pumps.
The modern reactors of this type have emergency core cooling systems so that, if there’s a breach in a cooling pipe, they have systems that can inject emergency coolant directly. And these [VVER-1000s] do have those kinds of systems, unlike some of the earlier versions of these Soviet reactors.
In addition to a pipe break, you can have a loss of power, which is what affected Fukushima. These plants normally draw electricity from the grid to operate their systems, and if that’s interrupted, they have to rely on backup power with emergency diesel generators. Each reactor has three, and then there are a couple extra, so there are a lot a lot of backup diesels at the plant. But there’s always the possibility that something happens that can disable multiple units at once—like at Fukushima, where the site was flooded, and even though they had plenty of backup diesels, they stopped operating. After Fukushima, as in many other countries, Ukraine developed additional measures to cope with that kind of Fukushima-like accident, where there’s a long-duration loss of electrical power. And that included acquiring additional mobile pumps that do not require electricity and run on diesel fuel.
What are the main concerns with fighting at or near these nuclear facilities?
Clearly there’s the potential for many different kinds of damage: either direct, destructive damage to the plant systems, safety systems, infrastructure or indirect damage to support systems such as the off-site power. And [there is] potential for fire, which can propagate and disable the instrumentation control system. Very few of those are addressed that would need to be, if you’re going to worry that there is a real possibility of having this plant in the middle of a war zone.
I guess the moral is that if you want to potentially seriously damage the plant, you don’t have to go after the containment building, which is the hardest part. There are other systems that are not as well protected. But even those containment buildings are not necessarily able to withstand certain types of military attack. Even if they are not breached, they can spall, and you can have concrete falling down onto the reactor vessel. Or just strong vibrations might also cause damage.
Before this attack, was there growing concern in the nuclear community that something like this could and potentially cause accidents?
It’s sort of like an unspoken fear. There have been certain individuals who have raised this for a long time, but these concerns have basically been dismissed. There are a lot of concerns you can have about nuclear power, and some of them seem very improbable—and until they happen, people tend to dismiss them. And this is one example.
The cost of hardening commercial nuclear power plants so that they might survive a military onslaught is probably prohibitive. At the beginning of the nuclear era, people such as Edward Teller [a theoretical physicist and member of the Manhattan Project] thought nuclear power plants needed to be underground.
If there were damage at the Zaporizhzhia plant, what kind of explosion or radiation leakage might happen?
It depends on the progression of the accident, how severe it is and whether these emergency measures can be brought to bear or not. In the worst case, if you have an unmitigated loss of cooling capability, the nuclear fuel can overheat and melt and burn through the steel reactor vessel that holds it and drop to the floor of the container. And in that case, the containment is the only remaining barrier between the radioactive material in the reactor and the environment. It’s designed to withstand certain types of events but not others.
At Three Mile Island, the core partially melted, but operators were able to stop in time—before it reached the point where it melted through the reactor vessel. But even then, from the fuel that was damaged, there were a lot of radioactive gases that were generated and had to be vented, though they had relatively low radiotoxicity. If the containment is not breached, they do leak—no building can be perfectly tight. But that leakage is designed and tested to be relatively low. [In] the U.S., there’s a regulatory limit for how bad an accident can get in terms of exposure to the public, and the safety systems and the containment have been designed to meet that. But, again, that assumes certain things about what the nature of the accident is that can prove to be false or can be exceeded, such as at Fukushima.
I think the particular type of release from Chernobyl—which was pretty large and was injected high into the atmosphere and was widely distributed—that’s probably less likely with a reactor like this. If you look at Fukushima, the releases were smaller, and they didn’t disperse over as wide an area. It’s certainly a regional concern. But if multiple reactors are affected at the same time, if the spent fuel is damaged, if the containment is mechanically breached, then all bets are off.
Reports indicate that after the Russian takeover of the Chernobyl site in Ukraine, its staff are still working and have been unable to switch out. What are the concerns there and at Zaporizhzhia if shifts are unable to relieve one another?
Well, having well-rested operators is critical because the tasks they have to perform are complex, and they need to be alert. You have to ensure that fatigue is being monitored. If there’s a plant staff, and they’re not getting any relief, and they can’t go home, and they’re working under duress, it’s a dangerous combination. There will have to be measures for that.
What do combatants and the IAEA need to do?
The IAEA doesn’t have very much authority in this area, and I see the director general, Grossi, is struggling with this. He made this offer to go to Chernobyl and negotiate some sort of deal. But it’s not clear exactly what, and it’s going to depend on the good graces of the parties. That means addressing this in the context of a military conflict.
Ukraine and Russia have agreed on these temporary cease-fires in particular zones for safe passage of the population, and they could model that until they’ve reached some sort of an agreement on how nuclear power plants are going to be operated.
This raises these difficult issues of how far Ukraine would go to prevent a military takeover of a nuclear power plant, and the potential for damage.