Model-based analyses of the cesium dynamics in the small mesotrophic reservoir, Pond 4. I. Estimating the inventories of and the fluxes among the pond's major biotic components.

A model of cesium (Cs) dynamics among the principal biotic and abiotic components of an 11.4-ha impoundment is described. The model is derived from analyses of field measurements of Cs-133 concentrations in pond components for 500 days following the addition of 4 kg of stable Cs-133 to the system. This study differs from similar experiments in which radionuclides, or their stable analogs have been added to small ponds in that the biomasses of key pond components were also obtained. The Cs-133 concentrations and biomasses were used to compute the dynamics of Cs-133 inventories and fluxes among the pond components. The model permits interesting comparisons of Cs-133 transport and fate over time among the pond's abiotic components, primary producers, and two-orders of consumers. The importance of the submerged macrophyte and periphyton community in controlling the transport and fate of the added Cs-133 is quantified. Macrophytes intercepted much of the Cs-133 and slowed its ultimate sequestration by the sediments. The macrophytes' rapid absorption and slow release of Cs-133 prolonged the availability of the element to other pond biota. These data are being used within a subsequent paper to further develop the model into one in which the Cs-133 kinetics are described by transfer coefficients so that effects of changing environmental variables and remediation options can be explored.

Effect of the duration time of a nuclear accident on radiological health consequences.

This study aimed to quantify the effect of duration time of a nuclear accident on the radiation dose of a densely populated area and the resulting acute health effects. In the case of nuclear accidents, the total emissions of radioactive materials can be classified into several categories. Therefore, the release information is very important for the assessment of risk to the public. We confirmed that when the duration time of the emissions are prolonged to 7 hours, the concentrations of radioactive substances in the ambient air are reduced by 50% compared to that when the duration time of emission is one hour. This means that the risk evaluation using only the first wind direction of an accident is very conservative, so it has to be used as a screening level for the risk assessment. Furthermore, it is judged that the proper control of the emission time of a nuclear accident can minimize the health effects on residents.

Radiological risk assessment caused by RDD terrorism in an urban area.

This paper specifically discusses a radiological risk assessment due to RDDs (Radiological Dispersion Devices) containing Cs-137 in the metropolitan area of Seoul, South Korea. The comparison of an effective dose caused by airborne plume and deposited Cs-137 is performed with and without consideration of the wind direction. When the dose is computed conservatively, an effective dose is around twice that of a dose computed realistically. Monte Carlo simulations showed that the 95% confidence interval for morbidity was 2.40×10(-5) to 8.55×10(-5), and mortality was 3.53×10(-5) to 1.25×10(-4).

Analysis of atmospheric dispersion factors for building wakes at the Wolsung nuclear site in Korea.

The characteristics of atmospheric dispersion considering the building effects in the Wolsung nuclear site in Korea were studied using ISC-PRIME and ARCON96 models. The maximum 2-h average atmospheric dispersion factor (ADF) was six times larger when the building geometry was considered in ISC-PRIME and two times larger in the exclusion area boundary. Owing to different adjustments for wind speed by the stability class, the ADFs calculated using ARCON96 were smaller than those calculated using ISC-PRIME. Strategies for locating buildings need to be considered to maximise dispersion when planning for constructing several reactors and accessory buildings at the Wolsung nuclear site.

Development of a radioactivity mapping tool to support the decision making for radiological emergency events.

A radioactivity mapping program was developed to support the decision making in case of a radiological emergency event. Geostatistics and kriging methods were used in the program to make a more accurate radioactivity map for the polluted area. Two variogram models, i.e., linear and exponential models, were tested, and the exponential variogram model showed a better performance when compared with the linear interpolation for estimating unobserved data for radioactivity.

Statistical approaches to forecast gamma dose rates by using measurements from the atmosphere.

In this paper, the results obtained by inter-comparing several statistical techniques for estimating gamma dose rates, such as an exponential moving average model, a seasonal exponential smoothing model and an artificial neural networks model, are reported. Seven years of gamma dose rates data measured in Daejeon City, Korea, were divided into two parts to develop the models and validate the effectiveness of the generated predictions by the techniques mentioned above. Artificial neural networks model shows the best forecasting capability among the three statistical models. The reason why the artificial neural networks model provides a superior prediction to the other models would be its ability for a non-linear approximation. To replace the gamma dose rates when missing data for an environmental monitoring system occurs, the moving average model and the seasonal exponential smoothing model can be better because they are faster and easier for applicability than the artificial neural networks model. These kinds of statistical approaches will be helpful for a real-time control of radio emissions or for an environmental quality assessment.

Determination of the source rate released into the environment from a nuclear power plant.

A study was carried out to determine the source rate from a nuclear power plant using the tracer experimental data conducted at the Yeoung-Kwang nuclear site. The least squares method optimises the agreement between the released source rate and the calculated source rate by minimising the errors between the measured concentrations and the calculated ones using the Gaussian plume model. The least squares estimator generally estimates the source rate to be within a factor of 2. The forecasting ability of the source rate is improved by applying the modified dispersion coefficients that are calculated using the experimental data. Determination of the source rate in an early phase nuclear emergency will be helpful for the decision making when taking appropriate and prompt countermeasures in the case of a radiological emergency.