## 2 Overview of Microreactors
### Microreactor Designs
The main thermodynamic process is configured as a gas Brayton cycle, utilizing helium or carbon dioxide as a coolant [113; 114; 115; 116; 117]. Incorporating a system for waste heat recovery and conversion, its thermodynamic efficiency can be boosted from 45% to 60% [111; 112; 113; 114; 115; 116; 117]. The advantages of the Holos Generators microreactor include its simple and highly efficient turbo-jet engine-like design. The ORC-Brayton combination allows for thermodynamic efficiency of up to 60%. The reactor offers scalable power options from 1 MWe to 100 MWe. However, this innovative design has not yet been widely applied commercially, and potential safety and operational challenges are not fully known. Figure 1: Overview of the eVinci(tm) microreactor developed by Westinghouse Electric Company [94]. The Xe-Mobile (Xe-100) reactor concept developed by X-energy (Rockville, MD, USA) (Figure 4) is suitable for small power generation applications on land, sea, and air [122; 123; 124; 125; 126; 127]. It can fit into a standard container and operate for three years without refueling. It uses TRISO fuel and generates at least 1 MWe of electricity. It can operate autonomously without on-site operators [122; 123; 124; 125; 126; 127]. The Xe-Mobile reactor stands out with its portability and autonomous operation capabilities. It can fit into a standard container and operate for three years without refueling. It uses TRISO fuel and generates at least 1 MWe of electricity. However, the portability and autonomous operation features require sophisticated control systems to ensure safe operation without on-site operators, potentially increasing costs [122; 123; 124; 125; 126; 127]. Figure 3: Overview of the Holos-Quad microreactor designed by Holos Generators [122]. Figure 2: Overview of the Aurora microreactor designed by Oklo Inc. [110]. The SEALER microreactor developed by LeadCold (Stockholm, Sweden) (Figure 6) is a lead-cooled microreactor [25; 144; 145; 146; 147]. It generates 3 MWe of electricity and can operate for 30 years without refueling. The core can be enclosed to boost safety and mitigate proliferation concerns [25; 144; 145; 146; 147; 148]. Passive removal of decay heat is achieved through natural convection and thermal radiation. The U-Battery microreactor, which uses helium as a coolant and graphite as a moderator, produces 4 MWe and has a core lifespan of 5 Effective Full Power Years (EFPY). Its design features a compact, underground reactor cavity. However, using an indirect Brayton cycle with helium coolant can introduce operational complexities due to the extensive development needs of the helium turbine [144; 145; 146; 147; 148; 149; 150]. The SEALER reactor's benefits include its lead-cooled design and the capability to operate for 30 years without refueling. Sealing the core enhances safety and decreases proliferation risks. Additionally, decay heat is removed passively. However, there may be technical challenges related to managing and operating the lead coolant. The U-Battery developed by Urenco (Stoke Poges, UK) (Figure 7) is an advanced high-temperature gas reactor. This reactor is designed using modern nuclear technology for reliable and efficient energy production [151; 152; 153; 154; 155; 156]. The U-Battery is cooled by helium gas and uses a graphite moderator to slow down neutrons and control nuclear reactions [154; 155; 156; 157; 158]. This design aims to increase energy efficiency by operating at high temperatures. The reactor is capable of producing 4 megawatts of electrical power (MWe) and uses an indirect Brayton cycle to convert energy into electrical energy [153; 154; 155; 156; 157; 158]. The Brayton cycle uses nitrogen gas as the working fluid to convert energy into electrical energy. This method ensures high efficiency and safety in energy production [151; 152; 153; 154; 155; 156; 157; 158]. The U-Battery's core lifetime is planned to be 5 Effective Full Power Years (EFPY), meaning the reactor can operate continuously at full electrical power [159; 160; 161; 162].