*6.2. Nuclear Reactors*

Nuclear power plants are part of the energy network that has been adopted by many countries across the globe, such as France, USA, UK, Russia, Iran, and UAE, among others to support their growth in energy demands [308]. Unlike most energy sources, the power produced from the fission process of the fuel (i.e., enriched uranium or plutonium) within

the nuclear reactor can arguably be considered as one of the solutions for solving the problems associated with climate change and the increasing levels of CO<sup>2</sup> emissions in the atmosphere and its feasibility for none or low oil producing countries [309]. Nuclear technology has seen significant developments throughout the years to enhance the efficiency of these systems, reduce their construction size, and improve their safety standards [310,311]. Historically, the first generation of commercial nuclear reactors were inaugurated in the 1950s, whereas today, the newly introduced fourth generation of reactors are currently being either planned or under construction. In terms of the working fluid, these reactors can be classified into three main groups (i.e., water-cooled reactors (WCRs), gas-cooled reactors (GCRs), and molten solid cooled reactors (MSR)) [312]. The WCRs can be subdivided into further categories, namely, boiling water reactors (BWRs), pressurized water reactors (PWRs), and pressurized heavy water reactors (PHWRs). Furthermore, the thermal transport concept of the BWR and both PWR and PHWR is similar in the sense that the working fluid, in all cases, absorbs the thermal energy from the fuel when it undergoes an excited state. However, the main difference is that PWR and PHWR use pressurizing systems to maintain the working fluid in its liquid phase, and therefore must be separated from the electrical generating cycle for contamination safety concerns. On the other hand, the working fluid in the BWR is boiled to generate steam that is used directly to provide the needed mechanical power to rotate the steam turbine and generate electricity. In addition to being a thermal energy carrier for power generating purposes, the working fluid also takes the role of extracting heat from the nuclear fuel, which is primarily the main concern related to the safe and economic operation and lifespan of the reactor. In some cases where the cooling rate is insufficient or if the control rods fail to operate properly to stabilize or reduce the reaction process, the reactor can experience a loss-of-coolant accident (LOCA) [313]. In such scenarios, the nuclear fuel needs to be rapidly cooled down, using backup water tanks, to avoid a core meltdown crisis and possibly a hydrogen explosion in the chamber. From the aforementioned, one can generalize the modes of heat transfer inside the rector's core based on the driving force of the fluid motion into two main categories; the first is flow boiling, which is a forced convection phenomenon that occurs during normal operating conditions. The second is pool boiling, which is a natural convection heat mechanism that takes place following a reactor LOCA state. Enhancing the heat transfer coefficient (HTC) and critical heat flux (CHF), for flow boiling, or increasing the minimum film boiling temperature (Tmin) in pool boiling are essential for optimizing these thermal modes outcomes. Whether it comes to improving the energy efficiency or for safety reasons, the aforementioned shows how crucial the role of the working fluid in a nuclear reactor system. Therefore, utilizing working fluids of enhanced thermophysical properties, such as nanofluids, can help in further advancements in the field of nuclear power plants, especially in WCR systems, if properly handled and understood its role in both nuclear flow boiling and pool boiling [314]. This section demonstrates some of the available studies on nanofluids for both thermal modes (i.e., flow and pool boiling), but focuses more on the pool boiling mode due to its important role in designing an emergency core cooling system.
