Søren Thykier-Nielsen

Description of the riso puff diffusion model

An operational puff diffusion model has been developed at Riso National Laboratory to provide risk and safety assessments in connection with nuclear installations. The computer model releases a sequence of puffs with individual pollutant and heat contents, then calculates the time-dependent concentration field, which is provided by the collection of puffs. The puffs are advected through a three-dimensional grid on the basis of a sequence of either measured or simulated horizontal wind vectors. In one case study where the time duration of a pollutant release was varied, the puff model predicted a Gaussian dose distribution only when the source duration was relatively short. For use at distances up to about 1 km from the release point, experimental observations of nonstationary smoke plume diffusion seem to justify a puff advection scheme, where all the puffs in each time step are advected with the instantaneous velocity vector measured at the release point.

Testing the importance of accurate meteorological input fields and parameterizations in atmospheric transport modelling using dream - validation against ETEX-1

A tracer model, the DREAM, which is based on a combination of a near-range Lagrangian model and a long-range Eulerian model, has been developed. The meteorological meso-scale model, MM5V1, is implemented as a meteorological driver for the tracer model. The model system is used for studying transport and dispersion of air pollutants caused by a single but strong source as, e.g. an accidental release from a nuclear power plant. The model system including the coupling of the Lagrangian model with the Eulerian model are described. Various simple and comprehensive parameterizations of the mixing height, the vertical dispersion, and different meterological input data have been implemented in the combined tracer model, and the model results have been validated against measurements from the ETEX-1 release. Several different statistical parameters have been used to estimate the differences between the parameterizations and meterological input data in order to find the best performing solution.

Atmospheric dispersion of argon-41 from a nuclear research reactor: measurement and modelling of plume geometry and gamma radiation field

An atmospheric dispersion experiment was conducted using a visible tracer along with the routine releases of 41Ar from the BR1 research reactor in Mol, Belgium. Simultaneous measurements of plume geometry and radiation field from 41Ar decay were performed as well as measurements of the 41Ar source term and the meteorological parameters. Good overall agreement is found between measurement data and model results using the mesoscale atmospheric dispersion and dose rate model RIMPUFF.

An Operational Real-Time Model Chain for Now- and Forecasting of Radioactive Atmospheric Releases on the Local Scale

A comprehensive atmospheric dispersion modelling system, designed for real-time assessment of nuclear accidental releases from local to European scale, has been established by integrating a number of existing preprocessors, wind, turbulence, and dispersion models together with on-line available meteorology. The resulting dispersion system serves the realtime on-line decision support system for nuclear emergencies RODOS (Ehrhardt 1996; Kelly et al., 1996; Ehrhardt et al., 1997) with a system-integrated atmospheric dispersion module. This module is called met-rodos, Mikkelsen et al. (1997).

Validation of a Combination of Two Models for Long-Range Tracer Simulations

The accident at the Chernobyl nuclear power plant on April 25, 1986 initiated great interest in development of warning systems that could be used as a part of the emergency planning in connection with radioactive and toxic releases due to major industrial accidents. Forecasts of the transport and dispersion of the releases by use of long-range transport models are used as an important tool in the emergency planning and it is therefore crucial to test the quality of the models, for example, by comparison of model simulations with full scale atmospheric tracer experiments.

Experimental Evaluation of a PC-Based Real-Time Dispersion Modeling System for Accidental Releases in Complex Terrain

The local-scale real-time dispersion modelling system LINCOM/RIMPUFF is evaluated using data from the complex terrain Guardo experiments carried out in Northern Spain November 1990.