Kilopower: Space Fission Reactor with Stirling Conversion

Reactor Controls

Control-rod withdrawal 60%

Active Stirling converters 4 / 8

Radiator panel area 4.0 m²

Environment

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⚠ Core exceeding design temperature — add converters or reduce power

Reactor thermal power

0 kWt

Electrical output

0 We

Core temperature

300 K

Radiator temperature

200 K

Thermal → electric efficiency (% of Carnot limit)

0% (0% of Carnot)

How it works

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.

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.

Fission core Heat pipes / converters Radiator panels

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