Aerosol dynamics and dispersion of radioactive particles

In an event of a nuclear power plant failure with release of radioactive material into the atmosphere, dispersion modelling is used to understand, how the released radioactivity is spread. For the dispersion of particles, Lagrangian Particle Dispersion Models, LPDMs are commonly used in which model particles, representing the released material, are transported through the atmosphere. These model particles are usually inert and undergo only first order processes such as dry deposition and simplified wet deposition along the path through the atmosphere. Aerosol dynamic processes including coagulation, condensational growth, chemical interactions, formation of new particles and interaction with new aerosol sources are usually neglected in such models. The objective for this study is to analyse the importance of including more advanced aerosol dynamic processes in LPDM simulations for the use in radioactive preparedness. In this investigation, a fictitious NPP failure, commencing with hourly separation for a full year, is studied for three geographically and atmospherically different sites. We conclude that: a) modelling of wet deposition by incorporating an advanced cloud parameterisation is advisable since, it significantly influence simulated levels of airborne activity as well as the formation of hotspots, and b) with advanced cloud parametrisation in the model, the inclusion of full aerosol dynamics can make a difference in single events, especially for formation of hot spots e.g. in 5 % of the simulated cases the decrease of airborne radioactivity concentration differed with more than 60 %-points compared to a simplified version of the model.

Design of the Important Area Airborne Radioactivity Monitoring System and Calculation of the Alarm Threshold of HTR-PM

Unlike the conventional pressurized water reactor (PWR), helium is adopted as the coolant in the high-temperature gas-cooled reactor pebble-bed module (HTR-PM). Due to the leakage of the primary coolant, in certain process rooms and some important areas, the airborne radioactivity should be monitored for the purpose of personnel radiation protection and emergency control. The system executing above function in HTR-PM is called important area airborne radioactivity monitoring system. This system consists of five separate monitoring channels, in which online and/or sampling monitoring devices will be included. For radiation monitoring system of a reactor, the calculation and setting of alarm thresholds for online devices is an important issue. The set of alarm thresholds requires not only accurate calculation but also feedback from actual operational experience. The design of the high-temperature reactor is different from the PWR, so the set of the alarm thresholds need special considerations. Based on the calculation of source terms and production-removal model, the theoretical radioactive levels of the sampled gases of all the online monitors were calculated and the alarm thresholds adopted in the current design are presented in this paper. The set of the alarm thresholds takes the influences of the background and other factors into account. This paper provides a method for calculating and determining the alarm thresholds of the radiation monitoring system, which can provide references for other nuclear power plants.