## 2 Overview of Microreactors
### Technological Features and Advantages
The absence of containment structures typically mandated for large nuclear power plants (NPPs) can result in several issues. This gap means fewer barriers separating radioactive materials from the environment and a higher risk of incidents such as aircraft impacts, which are usually considered in the design of large NPPs. The initial generation of small modular reactors (SMRs) will likely utilize low enriched uranium (LEU) fuel with uranium-235 enrichment levels between 3% and 5% [209; 210; 211]. Conversely, microreactor designs typically employ highly enriched low enriched uranium (HALEU) fuels with enrichment levels above 5% but below 19.75% [212; 213; 214; 215; 216; 217; 218]. This higher enrichment level improves reactor performance and lengthens the refueling intervals from several years to the reactor's full operational life [198; 216; 217; 218; 219]. According to economic studies by the Massachusetts Institute of Technology (MIT), the fuel costs for microreactors can be substantial. Therefore, the advantages of these more costly fuels need to be assessed in light of performance requirements and current production constraints [26]. In the United States, commercial light water reactors (LWRs) produce electricity using low enriched uranium (LEU) fuel, which contains between 0.7% and 20% uranium-235. The existing LWR fleet typically utilizes LEU with uranium-235 levels below 5%. However, some advanced reactors and LWR designs are planned to use LEU with uranium-235 concentrations ranging from 5% to 20%. These higher enrichment fuels, known as HALEU, can enhance fuel efficiency and positively impact the economic performance of the facility [20; 220; 221; 222; 223]. HALEU fuel intended for microreactor use is commonly available in metallic, ceramic, or TRISO forms [194; 198]. While conventional light water reactors use fuel with less than 5% uranium-235, microreactors and some advanced reactor designs use HALEU fuels with uranium-235 enrichment levels between 5% and 19.75% [20; 220; 221; 222; 223]. The higher enrichment level of HALEU enhances reactor performance and extends refueling intervals, thus improving operational efficiency. The various forms of HALEU fuel provide flexibility for different reactor types and optimize fuel management [20; 220; 221; 222; 223]. Economic assessments and studies in the United States suggest that using HALEU fuel can increase cost efficiency in microreactors. However, the production and processing of these fuels involve complexities due to current technical and logistical limitations. Therefore, the advantages of HALEU fuel should be carefully evaluated in terms of performance requirements and cost-effectiveness [220; 221; 222; 223]. As advanced reactor technology progresses, both new and existing power reactors will need HALEU fuel [29; 279; 217; 224]. The US nuclear fuel cycle infrastructure is currently not equipped to provide new HALEU sources or the necessary qualified packaging for HALEU transportation [29; 279; 217; 224]. It is likely that commercial supply will not be realized until the microreactor market becomes established. Microreactors stand out with their technological innovations and the advantages they offer. These reactors, equipped with autonomous and remote operation systems, can operate safely without continuous human intervention, making them suitable for use in remote and inaccessible areas [225; 226; 227; 205; 225; 226; 227]. The installation of microreactors is relatively quick compared to large nuclear power plants; they can be set up and start generating power within a few days. These features provide a significant advantage by reducing installation time and costs [225; 226; 227; 225; 226; 227]. Additionally, these reactors can be easily removed from one site and transported to another, enhancing flexibility and reducing costs. Microreactors are also an ideal solution for providing electricity during natural disasters or system outages.