RBMK Positive Scram Effect: 2D Core Reactivity Map

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Rods withdrawn — reactor at steady power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

This 2D variant renders the same RBMK "positive scram effect" as three linked panels instead of a 3D scene: a top-down channel map (colour = local front position), a reactivity-vs-time strip chart, and a reactivity-vs-insertion phase portrait that traces the positive-then-negative loop in state space.

Each rod's contribution still uses two overlapping fronts of 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))

Bug fix vs. the 3D original: that version scaled the per-rod graphite worth A_g down as displacer length grew (A_g ∝ (H−tipLength)), which contradicts its own documentation that a longer displacer "banks more positive reactivity per rod." This 2D build instead scales A_g ∝ tipLength (still normalised to the reference A_g0 at the 4.5 m reference length), so a longer displacer now correctly produces a larger positive spike.

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 without changing its peak height.
Graphite displacer length — a longer displacer narrows the water-gap crossing time and banks a larger positive spike per rod.
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. Fewer rods means the (weaker) net negative shutdown worth is reached later, so the same per-rod positive spike dominates for longer — the "1986 scenario" preset reproduces that imbalance.

Simplified, illustrative model of a real, documented reactor-physics effect (INSAG-7's account of the "positive scram effect"), isolating the graphite-tip mechanism from the reactor's separate positive void coefficient and xenon-135 poisoning.

Water gap Graphite front Absorber front Withdrawn (idle)

Reactivity vs. time

Phase portrait (ρ vs insertion)