Reactor Core Assembly
U-Mo Monolithic Fast Spectrum Fission Core
The reactor core assembly is a monolithic cast uranium-molybdenum cylinder, measuring 11 cm in diameter by 25 cm in height and massing 80 kg, that produces 43 kWt of thermal energy at 800 °C via a fast-spectrum fission chain reaction. Designed to provide continuous base heat independent of solar illumination, the core operates in a lunar vacuum that prevents convective heat loss, necessitating conductive heat transfer directly to sodium heat pipes. Mounting systems must mechanically endure deep moonquakes reaching magnitude ~5, while surrounding lunar regolith is utilized as shielding to compress the operational radiation keep-out zone from 100 meters to approximately 10 meters.
Monolithic cast uranium-molybdenum alloy fission core producing ~43 kWt thermal energy via controlled neutron chain reaction.
Purpose
Serve as the primary thermal energy source for the Lunar Ark by sustaining a controlled fission chain reaction in enriched uranium fuel, providing continuous heat output independent of solar illumination.
Context
Central heat-producing element of the nuclear power subsystem. Heat generated here is transferred via sodium heat pipes (L3-PWR-NUC-HPIPE) to Stirling converters (L3-PWR-NUC-STIRL). Reactivity is managed by the beryllium oxide reflector assembly (L3-PWR-NUC-CTRL). Based on the NASA KRUSTY/Kilopower demonstrated architecture.
Principles
- ▸Fission of U-235 nuclei releases ~200 MeV per event as kinetic energy of fission products, converted to heat
- ▸Fast neutron spectrum eliminates need for moderator, enabling compact core geometry
- ▸Monolithic cast U-Mo alloy provides structural integrity and high thermal conductivity (~20 W/m-K)
- ▸Negative temperature coefficient of reactivity provides inherent passive safety feedback
- ▸Core criticality maintained by surrounding beryllium oxide neutron reflector controlling neutron leakage
Typical implementations
- ▸NASA KRUSTY test reactor: cast U-Mo core, ~28 kg HEU, 4 kWt demonstrated at Nevada NNSS in 2018
- ▸Kilopower 10 kWe design: scaled-up KRUSTY with ~43 kWt core
- ▸SNAP-10A (1965): NaK-cooled U-ZrH reactor, only US reactor flown in space
- ▸Soviet BUK/TOPAZ: fast-spectrum space reactors with thermionic conversion
Lunar considerations
- ▸Vacuum environment prevents convective heat loss, simplifying thermal design but requiring radiative rejection
- ▸Regolith shielding can reduce radiation keep-out zone from ~100m to potentially ~10m
- ▸100-year mission may require fuel burnup management or staged reactor replacement
- ▸Seismic loads from deep moonquakes (magnitude ~5) must be accommodated in core mounting
- ▸Autonomous startup sequence without human operators requires robust instrumentation
Specifications
Functional
| primary function | Generate 43 kWt of thermal energy through sustained uranium fission chain reaction |
| inputs | Enriched uranium fuel (U-7Mo alloy, pre-loaded), Neutron reflector positioning (from L3-PWR-NUC-CTRL) |
| outputs | Thermal energy: 43 kWt conducted to heat pipe interfaces, Neutron/gamma radiation field, Telemetry: core temperature, neutron flux |
| thermal output kwt | 43 |
| core temperature c | 800 |
| fuel burnup limit pct | 1.5 |
| negative temp coefficient | True |
| passive safety | Inherent shutdown on overtemperature |
Physical
| mass kg | 80 |
| dimensions | Approximately 11 cm diameter x 25 cm height (monolithic cylinder) |
| materials | U-7Mo alloy (93% enriched U-235 baseline, HALEU 19.75% alternate), Mo barrier coating at heat pipe interfaces, Stainless steel core containment vessel |
| operating temperature c | 800 |
| radiation field | Intense neutron and gamma at core surface |
| thermal gradient | Radial gradient from centerline to heat pipe wells |
Operational
| power consumption w | 0 |
| thermal range c | 20, 800 |
| lifetime years | 15 |
| mtbf hours | 131400 |
Interfaces
Provides
- 43 kWt conducted into sodium heat pipe evaporator sections embedded in core periphery
- Neutron flux and core temperature measurements for reactivity feedback
Requires
- Beryllium oxide reflector positioning to control neutron economy and reactivity
- Radiation shielding to protect downstream electronics and biological payloads
- Structural support and vibration isolation for core assembly
Decomposes into
Cite this entry
Lunar Ark Codex. "Reactor Core Assembly" (L3-PWR-NUC-CORE). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-PWR-NUC-CORE
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.