Heat Pipe Assembly
Sodium Heat Pipe Thermal Transport Array
The sodium heat pipe thermal transport array is a passive heat-transfer system that moves 43 kWt of thermal energy from a lunar reactor core at 800°C to Stirling engine converters at 780°C. Comprising eight 316L stainless steel pipes with sintered internal wicks, the assembly circulates sodium through continuous phase change and capillary action, eliminating mechanical pumps. In the lunar environment, capillary forces must reliably maintain liquid return across 1/6 g, while the assembly must tolerate sodium freeze-thaw cycles during reactor startup and resist high-temperature liquid-metal corrosion over a 15-year operating life.
Array of sodium-filled heat pipes that passively transfer 43 kWt thermal energy from the reactor core to the Stirling engine hot-end heat exchangers.
Purpose
Provide highly reliable, passive thermal transport from the reactor core to the Stirling converters using two-phase sodium capillary action, eliminating the need for pumps or active circulation equipment.
Context
Bridges the reactor core (L3-PWR-NUC-CORE) and Stirling converters (L3-PWR-NUC-STIRL). The Kilopower architecture uses multiple independent heat pipes so that failure of one pipe causes graceful degradation rather than total loss. Each pipe operates as a self-contained two-phase thermal circuit.
Principles
- ▸Sodium evaporates in the core-embedded evaporator section absorbing latent heat of vaporization
- ▸Sodium vapor flows to the condenser section at the Stirling hot-end heat exchanger
- ▸Capillary action in the sintered stainless steel wick returns liquid sodium to the evaporator
- ▸No moving parts: heat transport is entirely passive via surface tension and phase change
- ▸Each heat pipe operates independently, providing inherent redundancy
Typical implementations
- ▸KRUSTY test used 8 sodium heat pipes, each ~2.5 cm diameter, stainless steel envelope
- ▸Kilopower 10 kWe design uses 8 heat pipes feeding 8 Stirling converters
- ▸Los Alamos National Laboratory sodium heat pipe testing at 800°C for thousands of hours
- ▸High-temperature alkali metal heat pipes used in various NASA thermal management concepts
Lunar considerations
- ▸Vacuum environment is ideal for heat pipe operation (no gas loading interference)
- ▸Lunar gravity (1/6 g) affects wick return flow but capillary-driven design is gravity-independent
- ▸Freeze-thaw cycling of sodium during reactor startup requires careful thermal management
- ▸Long-term corrosion of stainless steel by sodium at 800°C is a life-limiting factor
- ▸Micrometeorite protection required for any exposed heat pipe sections
Specifications
Functional
| primary function | Passively transfer 43 kWt from reactor core to Stirling converter hot ends |
| inputs | Thermal energy from reactor core at ~800°C (evaporator section) |
| outputs | Thermal energy to Stirling hot-end heat exchangers at ~780°C (condenser section) |
| thermal capacity kwt | 43 |
| evaporator temperature c | 800 |
| condenser temperature c | 780 |
| number of pipes | 8 |
| redundancy | N-1 (system operates with one pipe failed) |
| startup from frozen | Must survive sodium freeze-thaw |
Physical
| mass kg | 25 |
| dimensions | 8 pipes, each ~2.5 cm OD x 50-80 cm length |
| materials | 316L stainless steel envelope, Sintered stainless steel wick structure, Sodium working fluid (high-purity NaK-free), Molybdenum interface coating at core junction |
| operating temperature c | 780-800°C |
| internal pressure kpa | ~100 kPa (sodium vapor pressure at 800°C) |
| external | Vacuum |
Operational
| power consumption w | 0 |
| thermal range c | 20, 800 |
| lifetime years | 15 |
| mtbf hours | 200000 |
Interfaces
Provides
- 43 kWt delivered to Stirling hot-end heat exchangers via condenser sections at ~780°C
Requires
- 43 kWt from reactor core at 800°C via evaporator sections embedded in core
- Structural support for heat pipe routing between core and converters
Decomposes into
Cite this entry
Lunar Ark Codex. "Heat Pipe Assembly" (L3-PWR-NUC-HPIPE). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-PWR-NUC-HPIPE
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.