I don't have the HBO scene at hand, but I found this documentary on youtube which the scene was probably based on. Physicist Nesterenko says the experts conclud

I don't have the HBO scene at hand, but I found this documentary on youtube which the scene was probably based on. Physicist Nesterenko says the experts concluded there was a possibility that the corium explosion due to contact with water tank could have magnitude of 3-5 megaton and that Minsk would be erased [1]. Now obviously the nuclear explosion of the corium is not entirely impossible, there are known mechanisms which could set it up. But even if it happened, it would not erase Minsk, that is not believable, because Minsk is too far away. So that puts the main claim into question. How probable such corium explosion is is not clear, some things about nuclear bombs which those experts knew are probably still classified, but internet pundits say not very likely. [1] https://youtu.be/ZKV1piLjoa0?t=58 https://youtu.be/ZKV1piLjoa0?t=58