Let's also remember a couple of other facts:
Chernobyl was in an office building, with no additional containment built around the reactor core.
Chernobyl was an outdated, graphite-modulated reactor. Several years prior to the disaster, German nuclear engineers had found that this reactor design would go uncontrollably critical if certain conditions were met. The Chernobyl engineers didn't know about this, thanks to the Soviet government.