## 2 Overview of Microreactors
### Technological Features and Advantages
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Design** & **Spectrum** & **Fuel Enrichment** \\ \hline eVinciTM & Thermal & 5-19.75\% \\ Aurora & Fast & \textless{}20\% \\ Holos generators & Thermal & 8–15\% \\ Xe-Mobile & Thermal & \textless{}20\% \\ NuScale & Thermal & \textless{}20\% \\ SEALER & Fast & 19.75\% \\ U-Battery & Thermal & \textless{}20\% \\ MMR & Thermal & 19.75\% \\ Kaleidos & Thermal & 20\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Spectrum and enrichment levels of microreactors [25]. characteristics such as high volumetric heat capacity, stability without phase change under normal and accident conditions, low neutron absorption, operation at low pressures, limited activation in neutron-rich environments, chemical compatibility with core and structural materials, and excellent thermal conductivity. Analyzing the coolant choices for microreactors, molten salts, sodium, and lead-based coolants emerge as the most effective due to their high power density, substantial heat capacity, phase stability, and capability for low-pressure passive heat removal [20]. Molten salts provide advantages like natural convection cooling, large temperature differentials within the core, and high volumetric heat capacities, though they also present issues such as corrosion and high melting points [184; 185; 186; 187; 188; 189; 190; 191; 192; 193]. Sodium provides advantages such as a low melting point, established technical knowledge, the potential to achieve high-temperature differentials, and natural circulation cooling. However, sodium has drawbacks, including restricted natural circulation control and chemical reactivity with water and air. Lead-based coolants offer benefits such as natural convection cooling, significant thermal inertia, and high volumetric heat capacity, but also present challenges like high melting points, corrosiveness, and the production of volatile polonium compounds [20; 184; 185; 186; 187; 188; 189; 190; 191; 192; 193]. Most microreactor designs convert power using a Brayton cycle with an intermediate heat exchanger [20; 194]. Notably, in some microreactor designs, heat transfer from the core to the intermediate heat exchanger is facilitated by heat pipes, which is a significant deviation from traditional and advanced large nuclear plants [195]. Microreactors and small modular reactors (SMRs) encompass a broad spectrum of technologies, such as light water reactors (LWRs); high-temperature gas reactors (HTGRs); and advanced reactor concepts like liquid metal fast reactors (FRs), molten salt reactors (MSRs), and heat pipe (HP) reactors [196; 197; 198; 199; 200; 201]. These designs can differ greatly in fuels, materials, coolants, reflectors, manufacturing techniques (e.g., additive manufacturing), and heat exchangers [203]. In terms of radioactive material containment, microreactors require simpler and more economical barriers compared to larger reactors. This advantage stems from factors such as lower source terms, lower system pressures under normal conditions, and a reduced likelihood of chemical reactions. In configurations using molten salt as a coolant, the fuel can dissolve into the molten salt. This configuration offers various advantages [20]. For instance, it provides features such as a strong negative temperature coefficient, high burnup rates, continuous fuel cleanup, and the ability to facilitate backup shutdown mechanisms by transferring fuel to subcritical tanks. These attributes are essential for improving the safety and performance of microreactors [203; 204; 205; 206; 207; 208]. However, this technology is still developing and encountering specific obstacles. For example, the depth of defense capacity is reduced, meaning coolant loss equates to active fuel loss. Moreover, the upfront investment and construction expenses for a system integrated with chemical salt purification are significantly high. Establishing such a system requires a significant amount of capital, which is a factor to consider in terms of cost-effectiveness [203; 204; 205; 206; 207; 208].