General Purpose Heat Source Module
Plutonium-238 GPHS Fuel Clad Assembly
The General Purpose Heat Source module is a radioisotope thermal unit containing plutonium-238 dioxide fuel pellets encapsulated in iridium DOP-26 alloy cladding and nested graphite containment shells. It supplies continuous, non-throttlable thermal power through alpha decay at 0.39 W/g, operating independently of sunlight or reactor systems. In the lunar vacuum, the non-refuelable assembly must reject heat solely through radiative cooling without convective assistance. Across long-duration missions, thermal output decreases predictably along an 87.7-year half-life, leaving roughly 55% power remaining after 100 years, while spontaneous fission within the fuel generates a constant secondary neutron flux requiring dedicated accommodation.
Radioisotope heat source using Pu-238 alpha decay providing continuous thermal power
Purpose
Generate reliable thermal energy independent of sunlight or reactor operation for emergency power
Context
Child of L2-PWR-RTG within the Lunar Ark power generation system
Principles
- ▸Pu-238 alpha decay produces 0.39 W/g with 87.7-year half-life
- ▸GPHS module design provides containment through multiple barriers (clad, graphite impact shell, aeroshell)
- ▸Iridium cladding (DOP-26 alloy) encapsulates PuO2 fuel pellets
- ▸Decay heat is continuous and cannot be throttled - always producing thermal power
Typical implementations
- ▸GPHS-RTG on Cassini, New Horizons, Galileo, Ulysses missions
- ▸MMRTG (Multi-Mission RTG) on Curiosity and Perseverance Mars rovers
- ▸Next-Gen RTG (NGRTG) under development by NASA/DOE
- ▸Pu-238 production restarted at Oak Ridge National Laboratory (~1.5 kg/year)
Lunar considerations
- ▸87.7-year half-life means ~55% thermal output remaining after 100 years
- ▸Radiation from Pu-238 (primarily alpha) requires minimal shielding but neutrons from spontaneous fission need consideration
- ▸Heat source operates in lunar vacuum with radiation-only cooling
- ▸Non-refuelable - power output decreases predictably over mission life
Specifications
Functional
| primary function | Radioisotope heat source using Pu-238 alpha decay providing continuous thermal power |
Physical
| materials | PuO2 fuel pellets, Iridium DOP-26 cladding, FWPF graphite impact shell, CBCF graphite aeroshell |
Interfaces
Provides
- Continuous thermal power ~250Wt per GPHS module
Requires
- Radiation containment and shielding integration
Cite this entry
Lunar Ark Codex. "General Purpose Heat Source Module" (L3-PWR-RTG-GPHS). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-PWR-RTG-GPHS
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.