A compact U-235 core is cooled by passive sodium heat pipes (no pumps — heat pipes move heat by evaporation/condensation of an internal working fluid) that carry heat to free-piston Stirling converters mounted around a mast, exactly as in NASA's Kilopower/KRUSTY demonstrator. Waste heat is rejected to space by radiation, since there is no air to convect into. This 2D view is a radial schematic of that same heat chain — drag it to pan, scroll/pinch to zoom — paired with a live strip chart of the underlying numbers.
Core energy balance:
C_core·dT_core/dt = Q_reactor − H_cond − Q_selfloss
Conductive heat-pipe transfer (capped by N active converters):
H_cond = min( k_pipe·N·(T_core − T_rad) , N·Q_max,converter )
Stirling conversion (Carnot-limited, real converters reach
only a fraction η_f of the ideal Carnot efficiency):
η = η_f · (1 − T_rad / T_core), P_elec = H_cond·η
Radiator energy balance (Stefan–Boltzmann radiative cooling):
C_rad·dT_rad/dt = (H_cond − P_elec) − ε·σ·A·(T_rad⁴ − T_env⁴)
- Control-rod withdrawal — sets fission power Q_reactor up to 43 kWt (scaled Kilopower reference core).
- Active converters — each Stirling engine can absorb at most ~6 kWt; too few active converters for the chosen power lets the core overheat, just as a real reactor needs converter capacity to match fission output.
- Radiator area — a bigger panel radiates waste heat faster (∝A), keeping the cold-side temperature — and therefore the Carnot efficiency (1 − T_rad/T_core) — lower.
- Lunar day/night — the radiator's radiative sink temperature swings from ≈230 K (sunlit regolith background) to ≈100 K (night), changing the achievable efficiency without any moving parts — the same reason fission beats solar arrays through the two-week lunar night.
Numerical check: this lumped-capacitance model was verified with a standalone integration outside the browser at the default settings (rod 60%, 4 converters, 4 m² radiator, lunar day) — it settles to a stable steady state (core ≈1100 K, radiator ≈530 K, ≈2.5 kWe, ≈14.6% net efficiency) well inside the ≈1400 K design warning, confirming the coupled core/radiator balance is self-consistent and does not run away.