*8.3. Limitations in Nuclear Reactor Systems*

As with most heat transfer systems, nuclear reactors were shown to have potential performance benefits from replacing their working fluids with nanofluids. Although on some occasions, certain types of nanofluids cannot compile with such application requirements (e.g., gold and platinum), due to the high temperature nature of such systems and the presence of emitted radiations that effects the dispersed particles [496]. Nevertheless, carbon-based materials have shown the capability of being an acceptable candidate for these systems. Despite that, the common challenge with almost every application that uses this category of suspensions remains rounded on the feasibility of such fluids and their particles clustering issue within the hosting system. When focusing on nuclear reactors, these systems design, sizes, and mechanisms can be seen changing rapidly throughout the past 20 years [308,312,497]. While this shows how this area of science is advancing, it also constrains the exploration capability of researchers working in the field of nanofluids. Thus, scientists specialized in nanofluids cannot investigate the performance enhancement caused by their suspensions on pre-existing reactors. Still, at the same time, they need to take into account the operation lifetime of the facility and understand how the isotopes build up and decay within these systems. This is because such changes in these isotopes could cause different behaviors when exposed to the dispersed particles. In addition, the fact that some of the dispersed particles may deposit on the nuclear fuel surface needs to be also considered and evaluated with respect to the possible corrosion development on the outer surface of the fuel. Moreover, studies on the long-term physical stability of nanofluids, when employed in nuclear reactors, remain unknown and need to be investigated. In addition, further work is needed to determine the effect of surfactants, when used as stabilizers, on the heat transfer rate in such application. This is because most (if not all) of these chemicals cannot withstand high temperature operating conditions. In terms of LOCA scenarios, nanofluids can help stop (or reduce) the level of damages that the fuel of elevated temperature may cause to the facility, but the method in which the newly introduced waste can be dealt with remains questionable and needs to be solved. This is because, unlike conventional liquids, the dispersed particles conserve more radiations, and hence remain radioactive for a very long time before they decay and stabilize. When it comes to pool boiling during quenching, to the best of our knowledge, there is still no existing literature that covers the effect of nanofluids on the minimum film boiling temperature (Tmin) such as what was presented in Section 6.2.2. In general, there is a lack of studies about the impact of nanofluids on Tmin during quenching. Owing to the importance of Tmin, various types, concentrations, and sizes of nanofluids should have experimented with to investigate their effects on this parameter in specific. Furthermore, most of the investigations that are concern the effect of nanofluids on the CHF uses block plates, flat plates, or wires. However, research work on other geometries is crucial because it is evidence that the CHF will strongly be influenced by it. In addition, the currently employed models (e.g., Zuber's correlation) fails to accurately predict CHF when using thin wires [498], and therefore scholars need to focus more into developing a universal model that can withstand such limitation.
