Radiation Shield Assembly
Shadow Shield and Neutron/Gamma Attenuation Assembly
The radiation shield assembly is a 400-kilogram, multi-layered disc positioned between a surface reactor core and payload bays to attenuate nuclear radiation along a 30-degree half-angle cone. Layering dense tungsten or depleted uranium with lithium hydride and boron carbide across a 40-centimeter diameter, it achieves a 100,000-fold neutron and 10,000-fold gamma attenuation. This shadow geometry reduces shield mass by 60 to 75 percent compared to omnidirectional designs while limiting 15-year electronic doses to 10 kilorads. Mass optimization remains critical on the lunar surface, where shielding must account for secondary radiation scattered off ambient regolith to maintain biological vault exposures below 1 milligray per year.
Shadow shield positioned between the reactor core and the Ark payload to attenuate neutron and gamma radiation to acceptable levels for electronics and biological specimens.
Purpose
Protect sensitive Ark payloads (biological specimens, electronics, data storage) from the reactor's neutron and gamma radiation field by providing sufficient attenuation in the direction of the payload, while allowing radiation to escape harmlessly in other directions to minimize shield mass.
Context
Positioned between the reactor core (L3-PWR-NUC-CORE) and the rest of the Ark. Uses a shadow shield concept where only the solid angle subtended by the payload is shielded, dramatically reducing mass compared to 4-pi shielding. May be supplemented by regolith burial or distance (L1-RAD).
Principles
- ▸Gamma rays attenuated by high-Z materials (tungsten, depleted uranium, lead) via photoelectric effect and Compton scattering
- ▸Fast neutrons moderated and absorbed by hydrogen-rich materials (LiH, polyethylene) and boron compounds
- ▸Shadow shield geometry protects only the half-space containing the payload, reducing mass by ~60-75% vs full shield
- ▸Shield effectiveness measured in dose reduction factor; must achieve 10^4-10^6 attenuation
- ▸Multi-layer shield design: gamma layer closest to core, neutron layer closest to payload
Typical implementations
- ▸KRUSTY/Kilopower: lithium hydride (LiH) neutron shield + depleted uranium gamma shield
- ▸SNAP-10A: shadow shield with LiH and stainless steel
- ▸SP-100: lithium hydride neutron shield with tungsten gamma shield
- ▸TOPAZ: beryllium and steel shadow shield
Lunar considerations
- ▸Regolith burial can supplement or partially replace manufactured shielding, saving mass
- ▸100-year mission: radiation scattered off regolith may require additional shielding analysis
- ▸Biological specimen preservation requires very low radiation dose (< 1 mGy/year at vault)
- ▸Electronics radiation tolerance varies; shield must reduce dose below weakest component limit
- ▸Shield temperature must be managed (LiH decomposes above ~700°C)
Specifications
Functional
| primary function | Attenuate reactor neutron and gamma radiation to safe levels in the payload direction |
| inputs | Neutron flux from reactor core (~10^12 n/cm2/s at core surface), Gamma flux from reactor core and activated structures |
| outputs | Attenuated radiation field: < 10 krad(Si) total dose at electronics over 15 years, Dose to biological vault: < 1 mGy/year |
| neutron attenuation factor | 100000 |
| gamma attenuation factor | 10000 |
| dose at electronics krad 15yr | 10 |
| dose at bio vault mgy yr | 1 |
| shield half angle degrees | 30 |
Physical
| mass kg | 400 |
| dimensions | Conical/disc shape, ~40 cm diameter x 30 cm thick (multi-layer) |
| materials | Depleted uranium or tungsten (gamma attenuation layer, ~10 cm), Lithium hydride (LiH) (neutron moderation and absorption layer, ~15 cm), Boron carbide (B4C) thermal neutron absorber liner, Stainless steel structural casing and containment |
| operating temperature c | 100-400°C (gradient from core-facing to payload-facing side) |
| radiation field | Extreme on core-facing side, attenuated on payload side |
| thermal management | Passive conduction; must keep LiH below decomposition temperature |
Operational
| power consumption w | 0 |
| thermal range c | 20, 400 |
| lifetime years | 15 |
Interfaces
Provides
- Primary radiation attenuation in the reactor-to-payload direction
- Reduced radiation environment for wiring harness routing through shield penetrations
Requires
- Source radiation field definition for shield sizing (neutron and gamma spectra)
- Structural support for ~400 kg shield mass, positioned between core and payload
Decomposes into
Cite this entry
Lunar Ark Codex. "Radiation Shield Assembly" (L3-PWR-NUC-SHIELD). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-PWR-NUC-SHIELD
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.