## 4 Discussion
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Multiplication Factor** & **Year for Site Release (Years)** \\ \hline \multirow{3}{*}{External gamma shielding factor} & upper & \(\times\)2 & 7.14 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.5 & - a \\ \hline \multirow{3}{*}{Density of contaminated zone} & upper & \(\times\)2 & 11.19 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.5 & - a \\ \hline \multirow{3}{*}{Runoff coefficient} & upper & \(\times\)1.6 & 3.44 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.63 & 3.64 \\ \hline \multirow{3}{*}{Kd of \({}^{60}\)Co in contaminated zone} & upper & \(\times\)2 & 4.56 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.5 & 1.96 \\ \hline \multirow{3}{*}{Evapotranspiration coefficient} & upper & \(\times\)1.45 & 7.99 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.69 & 3.44 \\ \hline \multirow{3}{*}{Kd of \({}^{108m}\)Ag in contaminated zone} & upper & \(\times\)2 & 4.56 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.5 & 3.44 \\ \hline \multirow{3}{*}{Kd of \({}^{134}\)Cs in contaminated zone} & upper & \(\times\)2 & 4.07 \\  & mid & \(\times\)1 & 4.07 \\  & lower & \(\times\)0.5 & 4.07 \\ \hline \multirow{3}{*}{Kd of \({}^{90}\)Sr in contaminated zone} & upper & \(\times\)2 & 4.31 \\  & mid & \(\times\)1 & 4.07 \\ \cline{1-1}  & lower & \(\times\)0.5 & 3.85 \\ \hline \hline \end{tabular} a: Total exposure dose is less than 0.1 mSv-year\({}^{-1}\) at t = 0 year.  
\end{table} 
Table 7: Year for site release for the unrestricted reuse of Wolsong NPP site calculated based on the sensitivity analysis for various input parameters. According to Equation (2), the leach rate (L\({}_{\text{i}}\)) has an inverse relationship with the density of the contaminated zone (\(\rho\)). In other words, with an increase in the density of the contaminated zone, the leaching of radionuclides to groundwater decreases relatively. Thus, the radionuclides stay longer in the contaminated zone, which increases the direct external exposure to receptors, and consequently, the total exposure dose increases as well. Further, the relationship between the evapotranspiration coefficient and the infiltration rate is expressed in the RESRAD-ONSITE code as follows [10]:  
\[\text{I = (1 - C}_{\text{e}})[(1 - C_{\text{r}})\text{Pr + Irr} \tag{3}\]  
where  
I = infiltration rate (m-year\({}^{-1}\))  
C\({}_{\text{e}}\) = evapotranspiration coefficient (dimensionless)  
C\({}_{\text{r}}\) = runoff coefficient (dimensionless)  
Pr = precipitation rate (m-year\({}^{-1}\))  
Irr = irrigation rate (m-year\({}^{-1}\))  
Here, if the evapotranspiration coefficient increases, the infiltration rate of the infiltration water decreases relatively. Thus, similar to the case of the density of contaminated zone, the transport of radionuclides is relatively delayed. Consequently, there is an increase in the external exposure to receptors which increases the total exposure dose. The results of this study showed that when the evapotranspiration coefficient was increased by 1.45 times, the year for site release for unrestricted reuse increased to approximately 8 years from the reference case of 4.07 years because of a significant increase in the total exposure dose. The external gamma shielding factor is also considered to be a major factor that significantly affects the evaluation of the receptors' radiation exposure to radionuclides remaining in the contaminated zone. Among the parameters related to external radiation, which is a major exposure path, the external gamma shielding factor is used to derive the environmental transport factor (ETF), which is expressed as follows:  
\[\text{ETF(t) = FO \times FS(t) \times FA(t) \times FCD(t)} \tag{4}\]  
Figure 3: (**a**) Sensitivity analysis result for the density of contaminated zone and (**b**) relevant exposure pathways for the reference case. Minor pathways are not shown.