## 5 Conclusions  
In summary, the objective of this study was to analyse the cycle coolant inlet temperature after it exits from the precooler and to understand the effects on the seawater heat sink and the cycle thermal efficiency. The results provide a good basis to support preliminary design, testing, validation, and verification activities of Gas Cooled Fast Reactors (GFRs) and Very High Temperature Reactors (VHTRs) for Generation IV Nuclear Power Plants (NPPs). The main conclusions are:  
* The Intercooled Cycle without recuperation (IC) has the highest hot gas temperature at the inlet to the precooler for a compressor inlet temperature \(\leq\)50 \({}^{\circ}\)C. At a compressor inlet temperature \(>\)50 \({}^{\circ}\)C, the Simple Cycle Recuperated (SCR) has the highest hot gas temperature at the precooler inlet. This indicates that the type of precooler design is critical to both cycles in order to ensure that modest heat sink exit temperatures are achieved. Modest heat sink temperatures will have a favourable effect on the helium temperature at the compressor inlet. This is necessary to maintain the design point thermal efficiencies of both cycles. The Intercooled Cycle Recuperated (ICR) has the lowest hot gas temperature observed, hence the precooler basis within the ICR configuration is design compactness. * The effect of the hot gas temperature at the inlet to the co-current precooler is negligible. The heat sink exit temperatures are modest. This indicates that recirculation for the purpose of cooling  
Figure 13: Size and Technology vs. Heat Exchange Comparison. Diagrams show that for the same size and technology level, the heat exchange is less than 50% for the co-current precooler. the helium will not lead to adverse temperatures at the inlet of the compressor, and thus no unfavourable effect on the cycle thermal efficiencies. With regards to the counter-current design, rises in the heat sink outlet temperatures are pronounced at cycle inlet temperatures that are <35 \({}^{\circ}\)C, but generally show a significant downward trend as cycle inlet temperatures exceed 35 \({}^{\circ}\)C. This indicates that the recirculation effect using this precooler design in the temperature range of concern can result in unfavourable helium coolant temperatures at the compressor inlet, which will reduce the cycle thermal efficiency. * When the baseline cycle inlet helium temperature of 25 \({}^{\circ}\)C is considered, the IC has the highest heat sink outlet temperature for the counter-current design, the SCR has a temperature above the International Standard Atmosphere (ISA), and the ICR is below the ISA temperature. This is evident in the significant differences of the temperature ratios between 25 and 35 \({}^{\circ}\)C and is a stark contrast to the co-current design. At high baseline temperatures, the heat sink outlet temperatures become very unfavourable and not ideal for maintaining cycle thermal efficiencies. * The co-current precooler design is ideal for all cycle inlet temperatures as the heat sink exit temperature is not significantly affected. However, for a moderate heat exchange rate per square meter, the benefits of a compact design are compromised because a larger heat exchanger is required. This impacts the size of the plant and the maintenance costs. * The compactness offered by the counter-current design due to the high exchange rate per square meter results in excess heat sink outlet temperatures. At these temperatures, a phase change (vapourisation) of the seawater will be observed, prompting the need for a condenser in the return medium, which will add to the cost of the precooler assembly. Hot sea water corrosion is also likely to occur at the heat sink passage, which will reduce the time between the maintenance and replacement of parts. Furthermore, the expelled seawater could potentially increase the mean temperature of the incoming seawater, if the expelled water at the outlet is not recirculated effectively.