Analysis of the influence of meteorological data collection duration on prospective atmospheric dispersion evaluation during normal operation of NPP

To assess the impact of gaseous radioactive materials released from nucleaer facilities on the environment around the facilities, the behavior of radioactive materials should be estimated and evaluated using site-specific meteorological data. Agencies for nuclear regulatory provide guidelines on the minimum meteorological data collection period and method of processing meteorological data because the assessment results may vary depending on which meteorological data is used and how it is processed. This thesis evaluated the effect of the collection period of the meteorological data on the site-specific representativeness the atmospheric dispersion assessment and compared the results according to the meteorological data processing method in the process. The compared length of collection period are 2 years, the minimum collection period in the regulatory guide, and 5 years considered as more proper length of collection period. By the comparison, whether there is any difference in the site-specific representative of the meteorological data was examined. The sitespecific representativeness of data was judged as the number of annual data that could be considered similar by evaluating the similarity with the annual meteorological data during the entire observed period of the meteorological data used in the study. Comparisons between meteorological data are based on the distribution of wind direction, wind speed, and atmospheric stability or the atmospheric dispersion factors for 16 directions on exclusion area boundary (EAB). The atmospheric dispersion factor is defined as the ratio of the concentration of pollutants in the atmosphere to the rate of pollutant released from the stack and is calculated using the downwind distance at the point, height of release, the meteorological characteristics, such as wind direction, wind speed and atmospheric stability, and released radionuclides. In this study, the released radionuclide was set to I-131, the downwind distance was set to the distance up to EAB, and the height of release was divided by 0 m corresponding to ground-level release and 58 m corresponding to elevated release. The similarity between the two distributions of wind speed, wind direction, and atmospheric stability were judged by phi coefficient calculated by chi square test. The atmospheric dispersion factors were matched to each other in the same direction and Wilcoxon sign ranked test was performed to determine whether there was a significant difference between the two data. As the result of the study, there is no significant difference in the number of annual data that can be represented using 2 and 5 year meteorological data. However, 5-year averaged data may be more advantageous to reduce the variability of the representativeness depending on the period to be averaged and to reduce the error when the meteorological pattern is significantly different from common pattern. The comparison of atmospheric dispersion factors shows more pairs of meteorological data considered to be similar than the comparison of meteorological characteristics such as wind direction, wind speed and atmospheric stability. Especially, assuming the ground-level release, every pair of meteorological data was judged to be similar. Looking at the distribution of atmospheric dispersion factors according to the height of release, the case of ground-level release shows much larger atmospheric factor values at EAB than the case of elevated release, but they decrease sharply as the downwind distance increases and the atmospheric dispersion factors of elevated release are larger from the point of about 2 km. From the results of the study, it was confirmed that the collection period of the meteorological data does not have a significant effect on the result of evaluating the expected exposure by the gaseous effluent at the EAB of the Kori site. Also, assuming ground-level release, the maximum individual exposure dose at EAB can be evaluated more conservatively rather than the elevated release, but it may be smaller than the value of elevated release when evaluating the collective dose in the emergency planning zone