**3.1 ISRU technologies**

ISRU technologies on the Moon will cover a vast number of activities ranging from collection, storage, manipulation, recycling, treatment, and post-processing. Regarding habitat construction, it should be noted that first, the construction area needs to be cleared, leveled and compacted to control the spread of lunar dust. Then, such an area can be used for building a habitat.

Currently, the global space community investigates sintering, molding, brickmaking, and 3D-printing with regolith. The techniques require different types of expertise, machinery, level of automation/human presence, power, and supplementary materials. The readiness levels of the technologies varies drastically as some techniques have been investigated for a number of decades while other started to gain a significant level of industrial and engineering interest in more recent years. As such, the idea of piling up loose regolith dates back to the Apollo era, cement and concrete production has been investigated since the 1980s [32], and additive manufacturing has been attracting a lot of attention in the last tens of years.

#### **3.2 Regolith simulants**

Typically simulants are made by crushing down terrestrial rocks of basaltic origins that largely resemble the chemical composition of the rock component of the lunar soil. The mixture can be improved by adding any particular minerals or glasses, as was done for the very first lunar regolith simulant JSC [13] when knowledge about lunar soils advanced thanks to sample return.

Including both mare and highlands types, there are a few tens of simulants that are being produced and used across academia and industry. These simulants respond to different engineering and scientific needs, and are used in technological demonstrations and experiments. In a user guide [11], NASA suggests that particle composition, size distribution, shape distribution and bulk density are the most important properties in a regolith simulant. Indeed, these factors will largely define the thermo-mechanical and chemical properties of the raw material and also outline how it will interact with its environment (e.g. static charge) and other materials (e.g. abrasive nature of the material). For radiation protection purposes, chemical composition and areal density are key factors that will define the effectiveness of regolith shielding. Radiation cross-sections are calculated from molecular formulas and are used to predict the interactions between the incoming radiation and matter. Areal density in g/cm2 , measures how much passive shielding is present in the way of the incident particles. Simply put, in dense materials where molecules sit closely together, there is a higher chance for an incoming primary particle to interact either with the nucleus or the electrons of the molecules. Bulk density in g/cm3 , defines whether and how much the simulant needs to be compressed in order to reach the areal density required for radiation protection.

#### **3.3 Radiation protection**

Deviation, distance, time, counter measures, and materials are the only units to put forward at the front line against radiation. In a lunar habitat however, large-scale particle deviation is not a feasible option. Increasing the distance to radiation source in space is impossible as the primary particles are omnipresent in interstellar space, and reducing time exposure may be in conflict with the scientific and exploratory missions' objectives. Although biological counter measures are currently being explored, this research is in its early stages and it is further challenged by individual responses to repeated exposures and hyper sensitivity to low doses. This leaves it to the strategic choice of passing shielding to protect astronauts from radiation. The best choice consists in the material that will absorb the maximum amount of primary radiation while producing the least amount of secondary emissions. The complexity and diversity of the space radiation environment makes this choice all the more difficult. However due to the large shipment costs to the Moon and the abundance of loose regolith on the surface, it becomes the main shielding material in a habitat.

As most units do, the radiation protection community has a guiding motto—a principle proposed by NASA—As Low As Reasonably Achievable (ALARA). It pushes the community to engineer ways to bring down the organ and whole-body doses, ultimately aiming at lower health risks associated with exposure.

As outlined in previous sections, one possible way to maximize radiation protection on the Moon is to build a habitat underground. Studies [3, 39] suggest that several meters under the surface, GCR exposure levels become comparable to those on Earth—a few mSv/year. However due to the major drawbacks of using and living in lava tubes expressed in Section 2.1, this option is not considered for an early settlement here. However, lava tubes should be investigated further for the potential use as shelters from SPEs.

#### **Figure 2.**

*Effective dose equivalent from GCR in CAF/180 days behind highlands regolith (HR) and multilayer shielding (HR—3 mm aluminum, Al—5 cm polyethylene, PE) as a function of regolith areal density. Based on results in [39].*

The best way to optimize passive shielding is to utilize the most effective molecules in terms of radiation protection. Extensive studies [23, 40, 41] show that low atomic mass materials act best as shielding against heavy ions and high-energy protons as they present more nuclei in the path of the incoming particles, thus maximizing the stopping power for the same shield thickness in mass per unit area, if compared to heavier atomic mass counterparts. The top sergeant in this respect is protium or hydrogen 1 H because on top of its low atomic mass, it contains no neutrons and thus its utilization enables to bring down the secondary neutron production.

When the choice of chemistry of the main shielding is done or limited, the two cards left to play are areal density (of regolith in the lunar case) and the combination of supplementary materials which can be brought from Earth in moderate amounts, or possibly fabricated in-situ in the future. The term *combination* here includes both the types of added materials (their chemistry, density, H-richness, etc.) and the order or composition of the different layers together. Dense materials present a higher probability for primary radiation to interact with the target molecules since they sit tightly together. Therefore compression and sintering techniques are investigated with regolith and regolith simulants. If compressed regolith is complemented by low-atomic mass or hydrogen-rich materials in a multilayer structure then ALARA principle may be approached. This effect is illustrated in **Figure 2** which summaries the results of a deterministic study [39] with highlands regolith and a multilayer of regolith, aluminum and polyethylene, the latter being rich in hydrogen. The results demonstrate how the addition of aluminum and especially polyethylene leads to a significant reduction in the effective (whole-body) dose equivalent in a Computerized Anatomical Female (CAF) model in lunar GCR environment, simulated in OLTARIS [42] using the BON2014 model.

## **4. On the lookout for new solutions**

To follow the ALARA principle implies to be on the lookout for material enhancements, new materials, and the evolution of ISRU technology. Starting with an evaluation of commonly used materials, it is wise to look into possible combinations of those with regolith. As such, a study of 59 space materials [40] concluded that polymers should be used instead of metals in space where possible. In parallel, polymer 3D printing and sintering techniques with regolith are being developed (e.g. [43, 44]).

Besides the development of new materials, the utilization of multilayered structures is being investigated. The use of multiple layers of different complementary materials is not a new concept is space, as it has been used since the very first days of exploration, in particular in Extravehicular Mobility Units (EMUs). However, the radioprotective properties of such commonplace materials as Kevlar in EMUs has only been investigated recently [45]. The ROSSINI study [45] performed accelerator-based tests of several multilayers with He, F, and C beams of 1000, 962–972, and 430 MeV/u respectively. Among other, it concluded that the addition of LiH to a Moon regolith simulant enhanced protection from radiation by up to 20%. However, any such study is limited to the particular energy and type of primary particles. Overall recommendations require further tests and consideration of secondary emissions—especially neutrons [45].
