Mr. Dokter, what do you know about the current situation at the Zaporizhia NPP?
After Thursday’s power outages, all six reactor units at Zaporizhia were down by Friday afternoon. Blocks 5 and 6 were still operating beforehand, but were subsequently closed. Unit 6 operated for a short time yesterday at reduced capacity in order to generate the electricity needed by the entire system for cooling. This block was also shut down and the plant was fed via a so-called backup line from a nearby conventional power plant. The latest information is that the high voltage line that was damaged yesterday has also been repaired and that the operator is working again as of today at noon on one of the two blocks.
What are the risks in this confusing situation?
There are three issues that can be mentioned. One is the understandable concern about what happens when it is struck by fire. We still believe that no one has an interest in attacking the facilities with the aim of destroying them. With the kind of bombing we’ve seen in recent weeks, an accidental hit on the facility itself would not automatically lead to a serious accident. It is designed to be relatively robust and protected from crashing by smaller aircraft, for example. Another issue is that during the Russian occupation, the factory workers had to work under psychological stress that we can hardly understand. It is an important part of safety to have a handling team that is rested and able to operate without stress. The third set of issues is what we have been experiencing since yesterday: that it is not the facilities themselves that are being destroyed, but, for example, the power connection. The systems are also designed for this case. But it is true that you have a level of security less. For a catastrophic accident to occur, there must eventually be a chain of many such factors.
How long would a power outage have to be for a truly serious situation to occur?
If the state grid goes down and electricity is no longer supplied through the backup line, we have what we call an emergency blackout. The system would then need to be supplied with emergency diesel power so that the reactor cores could be further cooled. In Zaporizhia there are three such diesel units for each unit, one of which would be sufficient for cooling. They can also be switched laterally, so that an adjacent block can also be supplied with diesel from one block. According to the design of the plant in Zaporizhia, diesel should be available for seven days per unit. This could be used to bridge a week or two in order to restore network connectivity in the meantime.
What would happen in the worst case from your point of view?
If one assumes a scenario in which the plant can no longer be supplied with electricity for a certain period of time, even through diesel, then there will eventually be a core meltdown. This would roughly correspond to what happened at Fukushima, when large amounts of radioactive substances were released into the environment after a long blackout. The time it takes cannot be said in general, because it largely depends on how long the affected block has been idle. In the worst case, when the plant has just been shut down, we are talking about a few hours before a reactor core failure occurs. It is important to the classification that both the reserve line and almost all diesels do not need to be available for this to happen. In this regard, the plant in Zaporizhzhya is also faring much better than the one in Fukushima before the disaster in March 2011.
If released, where will the radioactive plume migrate?
This depends on several factors. Weather conditions are very important. Investigations by the Federal Radiation Protection Agency have shown that there are only about 60 days a year of weather conditions on which such air currents from Ukraine could reach Germany. And even if that happened after a disaster, we wouldn’t need short-term emergency measures like taking iodine tablets or staying indoors because of the relatively long distance. However, measures may need to be taken in agriculture, for example. This could be, for example, measurements of certain products or exclusion from transactions.
How do you rate warnings from political figures such as Ukrainian President Zelensky, who spoke of a possible “global radiological catastrophe” on Thursday?
For us as a scientific organization, what ultimately matters is what you can say about it in a technically correct way. You should be guided by calculations such as those of the Federal Radiation Protection Agency.