So, after the apocalypse ravaged the world in WW3, people in the western parts of Russia were in a serious predicament. The radiation in the Western part of Russia (Moscow, Stalingrad, Kaluga) made the place uninhabitable, and the population there were forced to make an exodus to the eastern parts of Russia like Siberia, or went south to places like Kazakhstan or Mongolia.
The two halves of Russia are separated by a zone called the “Velikiy Pylat” or "Great Glow", for us English speakers, which goes from Kazan to the Russia Belarus border, and the center of the Zone is Moscow.
The radiation exposure for an unprotected human ranges from 300 rads/second at the edge of the zone to 7000 rads/second at the very center. This Radioactive Zone has to last for at least 750 years without the radiation dissipating too much. What conditions could cause this Radiation Zone to exist?