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RBMK Positive Scram Effect: The Graphite-Tip Reactivity Spike

Interactive 3D model of the RBMK control-rod 'positive scram effect' — the graphite-displacer design flaw that briefly raised reactor reactivity when the AZ-5 emergency shutdown was pressed at Chernobyl, before the boron absorber caught up and drove it negative.

Nuclear Physics3DAdvanced60 FPS📱 Mobile-adapted⇄ 2D version

This simulation isolates one specific, documented reactor-physics mechanism inside the Chernobyl accident chain: the RBMK control rod's "positive scram effect." Each rod's graphite displacer briefly replaces a water-filled channel gap as the rod drives downward, transiently raising local reactivity before the trailing boron absorber column catches up and drives it negative. A 3D grid of fuel channels shows graphite tips (amber) leading boron absorbers (dark) down into the core, while a live reactivity chart and readouts track core Δk/k, the peak positive spike reached, and rod insertion depth. Adjust rod drive speed, graphite displacer length, and how many rods are actually available to the scram — including an "1986 scenario" preset — to see why a weakened shutdown margin let this brief positive transient matter.

⚙ Under the hood

A 3D model of the RBMK control-rod 'positive scram effect' — the graphite displacer design flaw that briefly raised reactor reactivity when the AZ-5 emergency shutdown was pressed, before the trailing boron absorber drove it negative.

nuclear reactorRBMKcontrol rodsreactivityChernobyl physicsreactor safety

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install