My nuke E prof (who worked on the Manhattan Project) in college took one look at a picture of Chernobyl and said - "that's not a power plant, that's a bomb factory".
Something about being able to access the reactor while it was running.
1. No containment structure.
2. Positive power feedback coefficient of reactivity in certain operating regimes (which is what they were in when they had their accident).
LWR technology as deployed in the West has neither of these shortcomings.