The operation of nuclear power stations is inevitably associated with routine releases of radioactive materials in gaseous effluents. In order to assess the maximum radiological dose to the public, an assessment of atmospheric dispersion and of ground deposition around the nuclear site of concern must be conducted using meteorological data. The U.S. Nuclear Regulatory Commission (NRC) provides a model for the assessment of annual averaged atmospheric dispersion and of annual averaged ground deposition in their Regulatory Guide 1.111. The NRC also provides a computer program (XOQDOQ) that allows analyses to be carried out according to their guidelines [1, 2]. The program uses a joint frequency distribution of atmospheric stability, derived from meteorological data, to calculate projected atmospheric dispersion and ground deposition.

The NRC Regulatory Guide 1.23 presents several methods of classifying atmospheric stability. The vertical temperature difference (ΔT) method and the standard deviation of horizontal wind direction fluctuations (σ_θ) method are the general methods for determining atmospheric stability [3]. The Korean Nuclear Safety and Security Commission Notification No. 2014-25 concerning the assessment standards of the meteorological conditions of nuclear facilities follows and endorses the NRC guidelines for classifying atmospheric stability [4]. Methods of classifying and evaluating atmospheric stability differ among countries; nuclear facilities in Korea principally use the ΔT method to estimate atmospheric stability and the σ_θ method as an ancillary method. In cases where the meteorological data required for assessing ΔT cannot be obtained due to the limited height of meteorological tower, the σ_θ method must be used as an alternative.

Since the calculation of the atmospheric dispersion factor and the ground deposition factor are significantly influenced by atmospheric stability, the choice of classification system for atmospheric stability warrants special attention. In this study, we compared meteorological measurements at the Daedeok and the Wolsong nuclear sites. We compared the annual averaged atmospheric dispersion factors and the ground deposition factors between these two sites, using values derived from estimates of atmospheric stability based on either the ΔT or the σ_θ method.

We used the joint frequency distribution according to atmospheric stability class to calculate annual averaged dispersion factor and annual averaged deposition. After estimating atmospheric stability on the basis of ΔT or σ_θ, we performed a comparative analysis of the meteorological characteristics derived from these two methods of classifying stability. Assuming a ground level release, we used meteorological data obtained 10 meters above ground level. For the assessment of ΔT, measurements were taken at 10 and 67 meters above ground level at Daedeok and at 10 and 58 meters above ground level at Wolsong. Other meteorological characteristics, such as the building wake effect and the recirculation and stagnation of building turbulence that are specifications of each nuclear facility were not considered. Rather, in this study, we chose only to investigate the effects of meteorological characteristics on atmospheric stability. In accordance with the NRC Regulatory Guide 1.23, we subdivided wind speed into 11 categories.

Figures 2 and 3 show the occurrence probability according to the classification method (ΔT or σ_θ) at the Daedeok site and at the Wolsong site. For the Daedeok site, the distribution of atmospheric stability results based on ΔT was generally homogeneous, showing a slight peak at category E (slightly stable atmospheric conditions). In contrast, the distribution of atmospheric stability results based on σ_θ showed a highly skewed distribution; the occurrence of category A (extremely unstable) atmospheric conditions had a probability over 90%. For the Wolsong site, ΔT-based atmospheric stability was most strongly associated with category E (slightly stable) atmospheric conditions and σ_θ-based atmospheric stability was most strongly associated with category A (extremely unstable) atmospheric conditions. However, these associations were not as strong as those observed for the Daedeok site.

Joint frequency distribution based on these two classification methods was input into the XOQDOQ program to calculate the atmospheric dispersion factor and the ground deposition factor. Radioactive decay and plume depletion were not taken into account. A total of 7 distances from a release point were chosen for comparison and discussion of the results: 400 m, 800 m, 1,200 m, 1,600 m, 2,400 m, 3,200 m, and 4,000 m. Figures 4 and 5 illustrate the atmospheric dispersion factors at each calculation distance according to the classification method for the Daedeok site. The maximum value of atmospheric dispersion factor based on ΔT was 1.46×10⁻⁴ sec·m⁻³ in the east direction, and that based on σ_θ was 2.25×10¹ sec·m⁻³ in the east-northeast direction. Likewise, Figures 6 and 7 illustrate the atmospheric dispersion factors according to the classification method at various calculation distances of the Wolsong site. The maximum atmospheric dispersion factor based on ΔT was 6.62×10⁻⁵ sec·m⁻³ in the east-southeast direction and the maximum atmospheric dispersion factor based on σ_θ was 1.50×10⁻⁵ sec·m⁻³ in the east-southeast direction. The maximum values of the ground deposition factor, regardless of the classification method used, were found in the east-northeast direction at the Daedeok site (Figures 8, 9) and in the east-southeast direction at the Wolsong site (Figures 10, 11). The maximum values of the ground deposition factor were as follows: at the Daedeok site, 6.24×10⁻⁸ m⁻³ (ΔT method) and 6.28×10⁻⁸ m⁻³ (σ_θ method); and at the Wolsong site, 7.92×10⁻⁸ m⁻³ (ΔT method) and 7.76×10⁻⁸ m⁻³ (σ_θ method). Our results suggest that atmospheric stability based on ΔT is the more conservative in calculating the annual averaged atmospheric dispersion factor and the ground deposition factor, with the exception of the ground deposition factor at the Daedeok site.

To protect radiation-induced health detriment of the public, the atmospheric dispersion factor and the ground deposition factor should be evaluated based on their maximum values when determining acceptability within the limits of legal standards. Therefore, we compared the maximum values of these factors according to methods of classifying atmospheric stability. Table 2 shows the highest atmospheric dispersion factor evaluated by each classification method at varying distances from the nuclear facilities in Daedeok and in Wolsong sites. At both sites, the atmospheric dispersion factor based on ΔT was higher than that based on σ_θ. The difference between the two atmospheric dispersion factors by different classification systems appears to be correlated to the distance from the nuclear power station: the greater the distance from the nuclear facilities, the larger the difference between the factors. The atmospheric dispersion factor based on ΔT was 6.5 to 14.5 times higher than the atmospheric dispersion factor based on σ_θ at the Daedeok site and 4.4 to 6.8 times higher at the Wolsong site.

The maximum values of the annual averaged ground deposition factor according to the atmospheric stability classification at varying distances from the nuclear facilities are presented in Table 3. For both sites, we found that the ratio of the ground deposition factors determined using the different methodologies (ground deposition factor based on ΔT to ground deposition factor based on σ_θ) was not influenced by distance. The corresponding ratios were almost same for the Daedeok and Wolsong sites. The reason that the impact of the classification method on the atmospheric dispersion factor was found to be sensitive to the distance from the nuclear site, whereas no distance dependency was observed for the ground deposition factor, may be because the ground deposition factor is evaluated through the relative deposition rate, which takes distance into account, and because ground-level release is calculated independently of atmospheric stability.

Using atmospheric stability classified on the basis of either ΔT or σ_θ, we analyzed the atmospheric dispersion factor and the ground deposition factor at the Daedeok site and the Wolsong site. For both sites, we found that the atmospheric dispersion factor derived from the atmospheric stability class obtained using ΔT was higher than that derived from the atmospheric stability class obtained using σ_θ. For the ground deposition factor, since the ground-level release, which is estimated through empirical graphs presented in the NRC Regulatory Guide 1.111, is assessed independently of atmospheric stability, the results of the two methods were the same. Since our study was an investigation for selecting sites in a specific year, it is difficult to say that the findings of our study have strong implications for different cases. However, our findings suggest that in general, ΔT is the more conservative method for offsite dose calculations for nuclear facilities. If at the time of preliminary study for the determination of site distance, no local meteorological tower is tall enough to measure ΔT, σ_θ may have to be measured. In this case, the margin for site distance should be considered to meet the national standards to protect radiation-induced health detriment to the public.