## 2 Overview of Microreactors
### Technological Features and Advantages  
The advanced technological innovations in microreactor designs offer various advantages to users. Microreactors continue to attract attention due to the intensive technological infrastructure required in their production processes and the benefits provided during their use. One critical technological feature is reactivity control, essential for the efficient and safe operation of microreactors. Reactivity control can be achieved through three main options to ensure high energy production within a small core [20]. Firstly, higher neutron leakage occurs in small cores compared to larger plants, making it crucial to manage these leakages. The first method to achieve this is by increasing the neutron flux level. This can be achieved by choosing core and coolant materials that have low absorption cross-sections within the neutron spectrum and incorporating neutron multipliers, such as beryllium compounds, into the system [176; 177]. However, increasing the flux level also increases irradiation damage within the core and incurs additional costs due to the use of neutron multipliers [178; 179]. The second option is to increase the concentration of fissile material. This can be achieved by selecting fuel compounds with a higher proportion of uranium and high U-235 enrichment levels [180; 181; 182; 183; 76; 184; 185; 186; 187; 188; 189; 35; 35; 36; 37; 38; 35; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83]. For instance, HALEU fuel can be used. However, higher enrichment levels lead to increased costs and the risk of forming a plutonium-rich core [180; 181; 76; 182; 183; 180; 76; 181; 182; 183]. The third approach involves leveraging the thermal portion of the neutron energy spectrum to increase the likelihood of fission. This can be accomplished by employing efficient neutron moderators like water, heavy water, or graphite. Given the compact nature of the core, it is essential to optimize all three methods. Balancing these methods with considerations of reactor safety, proliferation resistance, and cost-effectiveness is crucial. Consequently, reactivity control plays a critical role in microreactor design [180; 181; 182; 183; 76; 180; 181; 76; 182; 183]. Table 4 provides a comparative view of the spectrum and fuel enrichment levels across different microreactor designs. The table illustrates the diversity in spectrum types--thermal and fast--and the range of enrichment levels, predominantly between 5% and 19.75%. For example, the eVinci(tm) and Holos Generators microreactors utilize thermal spectrums with fuel enrichment levels of 5-19.75% and 8-15%, respectively. In contrast, the Aurora microreactor operates on a fast spectrum with fuel enrichment levels below 20%. This variation in spectrum and enrichment levels reflects the tailored approaches to achieving optimal reactivity and efficiency in different operating environments. The diversity in fuel types, such as HALEU and TRISO, further underscores the adaptability of microreactors to specific needs, enhancing their performance and safety profiles. By examining these tables, it becomes evident that microreactors offer versatile and innovative solutions for energy production. However, the successful deployment of these reactors hinges on overcoming challenges related to initial costs, operational data, and safety measures, emphasizing the need for continuous research and development [25]. Another crucial technological feature is the choice of coolant. This system is fundamental for heat control and removal. The ideal coolant for microreactors should possess