Molten Salt Reactor 2D: Thermal Feedback & Drain-Tank Safety

Reactivity & Flow

Control-drum reactivity ρ_ext 0.00 $

Fuel-salt flow rate 60%

Live Readouts

Power (n / n₀)

1.00

Core temp T

650 °C

Total reactivity ρ

0.00 $

β_eff (drift)

0.0065

Salt level (core)

100%

Elapsed time

0.0 s

How it works

Unlike a solid-fuel reactor, the fissile material here is dissolved in a circulating molten fluoride salt. Two effects that don't exist in a solid core drive this model:

Delayed-neutron drift. As fuel salt leaves the core through the external loop, some fraction of its delayed-neutron precursors decay outside the core, where they can't contribute neutrons back to the chain reaction. Faster flow strips out more precursors, so the effective delayed-neutron fraction falls: β_eff = β₀·(1 − 0.4·flow).
Strong negative temperature feedback. Molten-salt reactors are famous for being self-regulating: as the core heats up, thermal expansion of the salt pushes fuel out of the neutron flux, adding negative reactivity automatically (α ≈ −4.8 pcm/°C here, expressed in dollars via β₀).

Each frame integrates the coupled system with RK4 (not an analytic shortcut — the temperature coupling makes the system nonlinear):

dn/dt = (ρ_tot − β_eff)/Λ · n + λ·C
dC/dt = β_eff/Λ · n − λ·C − k_loss·C
dT/dt = (k_gen·n − k_cool·flow·(T − T_in)) / C_th
ρ_tot = ρ_ext + α·(T − T₀)

where k_loss removes precursors that drain out with the salt while it is flowing but not decaying inside the core — the same mechanism as β_eff drift, applied continuously to the precursor inventory.

Freeze-valve drain. Real circulating-fuel reactors (like the 1965 Molten Salt Reactor Experiment) rely on a plug of solid, actively-cooled salt at the bottom of the core. If cooling is lost — including simply losing electrical power — the plug melts on its own and the entire fuel inventory drains by gravity into a geometrically subcritical drain tank, ending the chain reaction with no valve, pump, or operator action required. The "Open Freeze Valve" button reproduces exactly that: the core's salt level falls, ρ_tot collapses toward −3 $ as fuel geometry is lost, and power decays to zero — a passive, walk-away-safe shutdown rather than a control-rod SCRAM.

PWR n/n₀

1.00×