Dyatlov broke every rule we have, and pushed a reactor to the brink of destruction. He did these things believing there was a fail-safe. A-Z-5. A simple button to shut it all down. But in the circumstance he created, there wasn't. The shut-down system had a fatal flaw. At 1:23 and 40 seconds, Akimov engages AZ-5. The fully-withdrawn control rods begin moving back into the reactor. These rods are made of boron, which reduces reactivity. But not their tips. The tips are made of graphite, which accelerates reactivity.
Why?
Why? For the same reason our reactors do not have containment buildings around them like those in the West. The same reason we don't use properly enriched fuel in our cores. The same reason we are the only nation that builds water-cooled graphite-moderated reactors with a positive void coefficient... It's cheaper.
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