## 7 Conclusions  
An overarching conclusion of this work supports current state-of-the-art on regolith utilisation for radiation protection [6; 16; 56]. It consists of three parts:  
1. Regolith has a real potential for radiation protection of a surface habitat on the Moon. 2. Minimisation of secondary emissions should be targeted by specific materials in the habitat walls. 3. More diversified in situ measurements are necessary for a definite design of any habitat and closing the gap between simulations and real doses. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Non-Compressed Regolith (1.6 g/cm\({}^{3}\))** & **Compressed Regolith (4.0 g/cm\({}^{3}\))** \\ \hline Solar flux (W/m\({}^{2}\)) & 600 & 1300 & 600 & 1300 \\ Temp gradient (\({}^{\circ}\)C/m) & \(-\)143 & \(-\)45 & \(-\)136.6 & \(-\)35 \\ Escape flux (W/m\({}^{2}\)) & 2.145 & 0.675 & 2.05 & 0.525 \\
**Heating reg. (W)** & **252.7** & **79.5** & **241.4** & **61.9** \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Heating required to keep 20 \({}^{\circ}\)C inside a lunar habitat with a highlands regolith wall. Within the context of given model limitations, more specific conclusions can be drawn about the radiation protection of a habitat on the surface of the Moon, using OLTARIS surface function:  
* To maintain the effective dose equivalent under 150 mSv for 180 days, the habitat needs at least 160 g/cm\({}^{2}\) of highlands regolith shielding. That corresponds to 100 cm at the nominal volumetric density of 1.6 g/cm\({}^{3}\), or 40 cm of regolith compressed to 4.0 g/cm\({}^{3}\). * The habitat should rather have a multilayer shield with for instance 8.5 cm of highlands regolith compressed to 2.7 g/cm\({}^{3}\), followed by 3 mm of aluminium and 5 cm of polyethylene. * For the shelter, the shield must comprise a multilayer structure with at least 405 g/cm\({}^{2}\) of regolith. That corresponds to 150 cm of 2.7 g/cm\({}^{3}\) regolith and 5 cm of polyethylene. * Due to the long-term unpredictability of SPE occurrences in future solar cycles, the shelter area has to be designed with large margins. The option of adding more layers inside the habitat as materials become available during a mission should be considered. In particular, most of the tools, items, food portions are typically packed in plastic bags for the travel. Plastic compression techniques should be evaluated for production of supplementary shielding from used materials and plastic bags in particular. Native highlands regolith has good thermal insulation properties which could save a lot of heating energy inside a habitat, whatever the extreme external temperatures are on the surface of the Moon during days or nights. A considerable part of the simulated habitat wall remains largely unaffected by the thermal cycling and temperature constraints. Compressed regolith provides an additional benefit of \(\frac{1}{4}\) of the wall's thickness that remains in isothermal state. This concludes that both for radiation protection and thermal insulation of habitats, regolith compression is desirable. Finally, having taken the CAF model as a reference in this study, the results show that female astronauts may not suffer excessively of the radiation exposure if the proposed solutions are followed. Mixed crews can stay safely on the surface of the Moon for extended periods of time, if appropriate protection guidelines are followed. Future work will use the newer Female Adult voXel 2005 (FAX) anatomical model, possibly cross-comparing results with CAF, as well as male phantoms. Further, we aim to compare the results to those from probabilistic models and experimental data with biological materials irradiated behind prototype walls. Conceptualization, Y.A. and Y.G. ; methodology, Y.A. ; simulations, Y.A. ; validation, Y.A. and Y.G. ; formal analysis, Y.A. ; investigation, Y.A. ; resources, Y.A. ; writing--original draft preparation, Y.A. ; writing--review and editing, Y.A. ; visualization, Y.A. ; supervision, Y.G. ; project administration, Y.G. ; funding acquisition, Y.G.