RBMK Positive Scram Effect: Graphite-Tip Reactivity Spike

AZ-5 Emergency Scram

Rods withdrawn — reactor at steady power

Rod & Channel Parameters

Rod drive speed 0.40 m/s

Graphite displacer length 4.5 m

Rods included in this scram 30 / 30

Live Readouts

Time since AZ-5

0.0 s

Rod insertion

0 %

Core reactivity Δk/k

0 pcm

Peak positive spike

0 pcm

How it works

RBMK control rods carry boron-carbide absorber above a graphite displacer, with a short water-filled gap below the displacer when the rod is fully withdrawn. Pressing the AZ-5 scram button drives every rod down at speed v; the graphite tip enters the water gap first, and — because graphite moderates neutrons far better than the water it replaces — that swap briefly increases reactivity in the lower core before the trailing absorber column reaches the same region and drives it sharply negative.

Each rod's contribution is modelled as two overlapping fronts of an insertion fraction s = v·t / H (H = 7 m core height):

g(s) = smoothstep(s / w_g)      graphite replacing water (fast, narrow)
a(s) = smoothstep(s / w_a)      absorber replacing graphite (slow, wide)
ρ_rod(s) = +A_g·g(s) − A_a·a(s)
ρ_core(t) = Σ over active rods of ρ_rod(s(t))

Rod drive speed — AZ-5 moved at only ≈0.4 m/s historically, so full insertion took ~18–20 s; a faster drive shortens the positive-reactivity window.
Graphite displacer length — a longer displacer covers the water gap faster (narrower w_g) but banks more positive reactivity per rod before the absorber arrives.
Rods included in this scram — on 26 April 1986 the operating reactivity margin had been eroded and only ~6–8 of the normal complement of rods were available to insert. Fewer rods means less negative shutdown worth is ever reached, so the same per-rod positive spike dominates for longer relative to the (weaker) net effect — the "1986 scenario" preset reproduces that imbalance.

This is a simplified, illustrative model of a real, documented reactor-physics effect (INSAG-7's account of the "positive scram effect"), not a re-run of the actual accident — it isolates the graphite-tip mechanism from the reactor's separate positive void coefficient and xenon-135 poisoning, which are different mechanisms covered by their own simulations on this site.

Graphite displacer Boron absorber Coolant water gap

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RBMK Positive Scram Effect: The Graphite-Tip Reactivity Spike