Verification of the NACAC atmospheric dispersion calculation using a hypothetical accident in a neighboring nuclear power plant

Abstract

Calculating atmospheric dispersion in a hypothetical nuclear accident plays an essential role as a predictive tool in nuclear emergency planning. The Nuclear Accident Consequence Analysis Code (NACAC) has been developed as an in-house code by the Thailand Institute of Nuclear Technology to develop its capability for understanding the transboundary radiological effect of a hypothetical nuclear accident. To verify the performance of the NACAC, a comparison between the calculated results using the NACAC with those from the well-developed Java-based Real-time On-line Decision Support (JRODOS) code is performed with representative cases of all seasons in a year. Two hypothetical severe accidents at the Fangchenggang pressurized water reactor, namely, (i) a loss of off-site power and (ii) a large-break loss of coolant, are utilized as the initial boundary conditions. The air concentration characteristic maps obtained from both codes are analyzed, revealing that the dominant radionuclide dispersion pathways are similar in all study cases. The air concentration, ground concentration, and total effective dose rate at several locations up to 500 km away from the release point are directly compared. Typically, the calculated results from the two codes are in reasonable agreement. However, for specific locations and directions, the NACAC predictions of the concentrations and total effective dose rate are lower than those from the JRODOS. These differences may be influenced by the different trajectory calculations between the two codes, which resulted in varying plume densities in certain areas. However, it does not significantly affect the radiation effect assessment. The NACAC still provides a total effective dose rate corresponding to the JRODOS, lower than one mSv throughout the main dispersion direction.