Control Rod Displacer Effect
The direct mechanical cause of the Chernobyl explosion. Each RBMK control rod has a graphite displacer tip that adds reactivity before the boron absorber can reduce it.
Rod Fully Withdrawn
The control rod assembly sits above the core. The channel is filled with water (neutron absorber). Press "Insert Rod" to begin.
The Design Flaw
Each RBMK control rod is 10.75 meters long, consisting of three sections: a 5.0m boron carbide absorber at the top, a 1.25m water-filled telescope gap in the middle, and a 4.5m graphite displacer at the bottom. When fully withdrawn, the channel is filled with water, a neutron absorber.
When the rod is inserted, the graphite displacer enters the lower core before the boron absorber reaches the active zone. The graphite pushes out neutron-absorbing water and replaces it with a moderator. For the first ~3 seconds of insertion, the rod actually increases reactivity instead of decreasing it.
Why This Matters
When operators pressed AZ-5 (emergency scram) at 01:23:40, all 187 top-entry control rods began inserting simultaneously. Each rod's graphite displacer added a small positive reactivity pulse. The combined effect across all rods created a system-wide positive reactivity spike of approximately +0.5$, enough to push the already-unstable reactor into a prompt supercritical excursion.
Toggle the "No displacer" checkbox above to see how a properly designed control rod would behave, immediately negative, with no positive spike. This is how all modern reactor designs work.