L2-RAD-POLY CRITICAL ENVIRONMENTAL Level 2 · hardware

Hydrogen-Rich Shielding

Hydrogen-Rich Material Radiation Shielding Subsystem

The hydrogen-rich shielding subsystem is an interior secondary radiation barrier comprising polyethylene panels, water-filled containment tanks, and borated sheets positioned beneath the regolith overburden. It captures secondary neutrons produced in the regolith layer and attenuates galactic cosmic ray heavy ions, yielding at least a fivefold reduction in neutron dose. Hydrogen provides maximum mass efficiency for nuclear collision moderation, with a 5 to 10 percent boron loading terminating thermal neutrons. Because all hydrogenous materials must be launched from Earth, mass optimization is critical. In the lunar vacuum, water walls require rigorous leak containment, while polyethylene faces outgassing and radiation embrittlement over a 100-year operational lifetime.

Secondary radiation shielding layer using polyethylene, water walls, and borated materials to provide optimized attenuation of GCR heavy ions and secondary neutrons within the Ark interior.

Purpose

Supplement regolith bulk shielding with materials specifically optimized for hydrogen content, which is the most mass-efficient element for GCR attenuation and neutron moderation, particularly for capturing secondary neutrons produced in the regolith layer.

Context

Positioned between the regolith overburden and the interior volumes, hydrogen-rich shielding provides the second defense layer. Water walls can serve dual-purpose as radiation shielding and thermal mass. Borated materials capture thermal neutrons that would otherwise contribute to dose.

Principles

▸Hydrogen is the most effective element per unit mass for GCR shielding due to efficient energy transfer in nuclear collisions
▸Polyethylene (CH2)n provides ~10% hydrogen by mass with excellent structural properties
▸Water provides 11% hydrogen by mass and can serve as thermal mass and potential emergency resource
▸Boron-10 has a very high thermal neutron capture cross-section (3840 barns), making borated polyethylene effective for neutron termination
▸Secondary neutrons from GCR interactions in regolith must be moderated and captured to prevent dose contribution

Typical implementations

▸HDPE or UHMWPE panels lining interior walls
▸Water-filled bladders or tanks integrated into wall cavities
▸Borated polyethylene (5-10% boron loading) sheets for neutron capture
▸Layered configurations: polyethylene for moderation, borated layer for capture
▸Structural polyethylene composite panels with integrated shielding function

Lunar considerations

▸All hydrogen-rich materials must be launched from Earth, making mass optimization critical
▸Water walls offer dual-use potential but require leak containment in vacuum environment
▸Long-term hydrogen outgassing from polyethylene in vacuum must be characterized
▸Radiation damage to polyethylene over 100 years may cause embrittlement and hydrogen loss
▸Temperature cycling effects on polyethylene mechanical properties in lunar environment

Specifications

Functional

primary function	Provide hydrogen-rich radiation shielding layer optimized for GCR attenuation and secondary neutron moderation/capture
inputs	Partially attenuated radiation field from regolith layer, Structural mounting from L1-STR
outputs	Further attenuated radiation environment for interior volumes, Thermal mass contribution (water walls)
polyethylene areal density g per cm2	>=10
boron loading weight percent	5-10
neutron dose reduction factor	>=5x
gcr dose reduction factor	>=1.5x additional
water wall thickness cm	>=10
coverage of critical volumes percent	100

Physical

materials	High-density polyethylene (HDPE), Ultra-high molecular weight polyethylene (UHMWPE), Borated polyethylene (5-10% B4C or B loading), Water (for water walls), Bladder/containment materials for water walls, Mounting hardware and structural brackets
temperature range c	-40, 60
vacuum adjacent	True
radiation exposure	True

Operational

power consumption w	0
thermal range c	-40, 60
lifetime years	100

Interfaces

Provides

→Radiation Shieldingenvironmentalcritical
Optimized hydrogen-rich radiation attenuation layer, particularly effective against neutrons and GCR heavy ions
→Active Thermal Control Systemthermaldesirable
Thermal mass contribution from water walls, aiding temperature stabilization of interior volumes

Requires

←Primary Structuremechanicalcritical
Structural mounting points and wall cavity space for polyethylene panels and water bladders
←Active Thermal Control Systemthermalessential
Temperature management to keep water walls above freezing and polyethylene within operational range