As you know, the Russkies built DUAL-PURPOSE nukes; one of the purposes was to create weapons-grade uranium.
Therefore, the Chernobyl plant's exterior was built of corrugated metal and there were plenty of entry-exit points for bomb-plant trucks to haul stuff.
Further, the plant was a "go-critical-by-itself" design using ancient carbon technology.
But the Russkies, just like the RedChinese, don't really give a damn about a few million citizens...