If it were to happen that not a drop of water remained in the cooling system, two problems would arise. Firstly, the spent fuel assemblies, which are still generating heat, are stored in pools of water. If they dry out, the barriers between the fuel elements first corrode at 1200 degrees Celsius. At around 2400 degrees, the crystal structure of the fuel rods dissolves, releasing radioactive radiation. However, no new nuclear fission would start.
The second problem would be that when the power goes out, diesel generators kick in, running pumps and security systems. These must be cooled during operation. If this is not possible due to a lack of water, mobile diesel generators would step in. Should the emergency power fail completely, the main coolant pumps would also fail and cooling water would no longer be pumped through the fuel pools. So there could be a risk of melting this way too – but only after a few days, says Uwe Stoll. “The fact that the systems have been shut down for so long is the advantage in Zaporizhia: there is a lot of time before critical conditions arise.”
The fuel pools are shielded in the reactor building by steel and concrete. Melting down the fuel elements would therefore not necessarily mean that radioactivity would be released into the environment. If that were to happen, says Uwe Stoll: “In the worst case, if all the fuel elements in the storage pool should melt, an area comparable to that in Chernobyl around the reactor would probably have to be evacuated.” The exact area would depend, among other things, on the wind direction .