## 4 Discussion  
### Impact of Thermal Discharge on SST in EFZs  
Following the commencement of nuclear power operations, the discharge of warm water leads to an increase in the temperature of the receiving water body. The magnitude of this thermal impact can be assessed by comparing the changes in SST between the affected and unaffected areas. To assess the extent and intensity of this thermal impact, the EFZs surrounding the HYNPP were divided into three areas: thermal mixing zones A and B, and the functional zone (Zone C). Zone A, located west of the plant, has an operational drainage outlet, while  
Figure 4: Temporal variation in the spatial morphology of the thermal plume from HYNPP under typical tidal conditions. Panels (**a–c**) illustrate the plume’s evolution during flood tide, while panels (**d–f**) depict the ebb tide. Zone B, situated to the east, is designated for a future outlet but is currently inactive. Zone C encompasses the entire EFZ, excluding the thermal mixing zones. As illustrated in Figure 1b, this spatial distribution of the EFZs provides a framework for analyzing the thermal impact. By comparing the temperature variations among these zones, we can gain insights into the spatial extent and intensity of the thermal influence. To quantify the thermal impact, we compared the multi-year average change in SST (\(\Delta T\)) within each effluent field zone to a reference zone unaffected by the discharge. This reference zone (\(T_{\text{Ref}}\)) is situated far from the outlet and, consequently, is not influenced by the thermal plume. Prior to the plant's operation, the \(\Delta T\) values for all three zones ranged from 0.06 to 0.13 \({}^{\circ}\)C, which can be seen in Figure 5a, reflecting small inherent geographical variations in SST. Post-operation, \(\Delta T\) values for Zone B and Zone C increased to 0.51 \({}^{\circ}\)C and 0.67 \({}^{\circ}\)C, respectively, indicating the long-term influence of thermal discharge. However, these increases were relatively modest, less than 0.6 \({}^{\circ}\)C in both cases. In contrast, Zone A experienced a dramatic increase in \(\Delta T\) from 0.13 \({}^{\circ}\)C to 2.51 \({}^{\circ}\)C, highlighting the significant impact of warm water discharge in this region. The preceding analysis demonstrates the significant warming impact of discharged warm water on the receiving water body. To further investigate the rate of warming and any potential seasonal variations, we analyzed the annual averaged \(\Delta T\) values for each zone. Figure 5b illustrates the results of scatter plot and linear regression analyses, indicating that Zone B and Zone C experienced an average annual temperature increase of approximately 0.05 \({}^{\circ}\)C. In contrast, Zone A exhibited a substantially higher warming rate of approximately 0.3 \({}^{\circ}\)C per year, with a strong correlation (\(R^{2}\) = 0.76, \(p\) = 0.00024). These findings clearly demonstrate the concentrated impact of thermal discharge, as evidenced by the significantly elevated temperature increase in Zone A compared to the other two zones. Analysis of seasonal variations in warming rates (Figure 6) reveals a consistent pattern across all three zones: the highest rates occur in winter, followed by autumn and spring, with the lowest rates observed in summer. This seasonal trend aligns with the NPP's design, which prioritizes higher discharge temperatures during winter months. As a result, more heat is injected into the receiving water body in winter, leading to a more pronounced warming effect. Figure 5: Spatial Variability of SST within EFZs attributed to thermal discharge. (**a**) Change in \(\Delta\)T within EFZs pre- and post-operation; (**b**) Change in EFZ Warming Rate. Among the three zones, Zone A consistently exhibited the highest warming rates, reflecting its proximity to the outfall and direct exposure to the thermal plume. The winter warming rate in Zone A reached 0.41 \({}^{\circ}\)C/a, approximately three times higher than that of the other zones.