## 3 Results  
### Thermal Impact of HYNPP Operation on Intake and Outlet Water  
NPPs necessitate substantial circulating cooling water to regulate reactor temperature. Consequently, warm water containing waste heat is discharged into surrounding waters. A significant environmental impact of NPP operations is the alteration of discharge water temperature. This study examines long-term temperature variations in the intake and outlet areas of the HYNPP from March 2013 to November 2024. Key statistical parameters--maximum (Max), minimum (Min), Mean, and standard deviation (SD) of SST--were analyzed to assess the intensity of thermal discharge impacts. Max, Min, and Mean values primarily characterize the magnitude of temperature differences between intake and discharge waters before and after operations. SD reflects the influence of warm water discharge on the homogeneity of the temperature field in the surrounding sea area. Figure 2 presents the temporal variations in Max, Min, Mean and SD of SST values within the intake and outlet areas before and after the NPP commenced operations on 22 October 2018. Prior to the HYNPP's operation, as shown in Figure 2 and Table 3, both intake and outlet areas exhibited similar seasonal SST patterns, with Max reaching approximately 31.6 \({}^{\circ}\)C. Min fluctuations in Mean SST (<0.3 \({}^{\circ}\)C) and SD (<0.03) indicate a highly homogeneous temperature field. Following the NPP's operation, while the overall temperature trends remained consistent in both areas, a significant temperature increase was observed \begin{table}
\begin{tabular}{c c} \hline \hline
**Ranges of SST Increases** & **Levels** \\ \hline
[+1 \({}^{\circ}\)C, +2 \({}^{\circ}\)C) & \(R_{+1}\) \\
[+2 \({}^{\circ}\)C, +3 \({}^{\circ}\)C) & \(R_{+2}\) \\
[+3 \({}^{\circ}\)C, +4 \({}^{\circ}\)C) & \(R_{+3}\) \\ \(\geq\)4 \({}^{\circ}\)C & \(R_{+4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Temperature ranges for different levels of increases in SST (\({}^{\circ}\)C). Figure 2: Temporal changes in SST parameters at intake and outlet of HYNPP. in the outlet area. The Max in the outlet area rose from 33.49 \({}^{\circ}\)C to 35.79 \({}^{\circ}\)C, exceeding 2 \({}^{\circ}\)C. The Mean SST in the outlet area experienced a substantial increase from 12.95 \({}^{\circ}\)C to 18.02 \({}^{\circ}\)C, reflecting the influence of thermal discharge. The most pronounced change occurred in the SD of the discharge area, which increased from 0.2 to 1.12, indicating a significant disruption to the temperature field homogeneity caused by the continuous release of thermal discharge. These findings unequivocally demonstrate the substantial thermal impact of NPP operations on the surrounding water environment. The monthly mean SST difference between the outlet and intake waters exhibited distinct seasonal variations. As illustrated in Figure 3, prior to the NPP's operation, the temperature difference remained relatively low throughout the year, fluctuating within a range of approximately \(\pm\)0.5 \({}^{\circ}\)C.  
Following the plant's operation, a significant increase in the temperature difference was observed, reaching a maximum of 7.3 \({}^{\circ}\)C in January. This difference remained relatively stable from February to April before gradually decreasing from May to a minimum of 4.6 \({}^{\circ}\)C in September, after which it increased again. Seasonally, the average temperature difference was highest in winter (6.8 \({}^{\circ}\)C), followed by spring (5.9 \({}^{\circ}\)C), autumn (5.3 \({}^{\circ}\)C), and summer (4.9 \({}^{\circ}\)C). This \"high in winter, low in summer\" pattern is consistent with the plant's design, which prioritizes higher discharge water temperatures during colder months. This direct surface discharge leads to pronounced temperature increases, contributing to the observed seasonal variation, with greater increases in winter and smaller increases in summer.