Modelling the atmospheric dispersion of radiotracers in small-scale, controlled detonations: validation of dispersion models using field test data.

A series of modelling exercises, based on field tests conducted in the Czech Republic, were carried out by the 'Urban' Working Groups as part of the International Atomic Energy Agency's Environmental Modelling for Radiation Safety II, Modelling and Data for Radiological Impact Assessment (MODARIA) I and MODARIA II international data compilation and model validation programmes. In the first two of these programmes, data from a series of field tests involving dispersion of a radiotracer,99mTc, from small-scale, controlled detonations were used in a comparison of model predictions with field measurements of deposition. In the third programme, data from a similar field test, involving dispersion of140La instead of99mTc, were used. Use of longer-lived140La as a radiotracer allowed a greater number of measurements to be made over a greater distance from the dispersion point and in more directions than was possible for the earlier tests involving shorter-lived99mTc. The modelling exercises included both intercomparison of model predictions from several participants and comparison of model predictions with the measured data. Several models (HotSpot, LASAIR, ADDAM/CSA-ERM, plus some research models) were used in the comparisons, which demonstrated the challenges of modelling dispersion of radionuclides from detonations and the need for appropriate meteorological measurements.

Urban working groups in the IAEA's model testing programmes: overview from the MODARIA I and MODARIA II programmes.

The IAEA's model testing programmes have included a series of Working Groups concerned with modelling radioactive contamination in urban environments. These have included the Urban Working Group of Validation of Environmental Model Predictions (1988-1994), the Urban Remediation Working Group of Environmental Modelling for Radiation Safety (EMRAS) (2003-2007), the Urban Areas Working Group of EMRAS II (2009-2011), the Urban Environments Working Group of (Modelling and Data for Radiological Impact Assessments) MODARIA I (2013-2015), and most recently, the Urban Exposures Working Group of MODARIA II (2016-2019). The overarching objective of these Working Groups has been to test and improve the capabilities of computer models used to assess radioactive contamination in urban environments, including dispersion and deposition processes, short-term and long-term redistribution of contaminants following deposition events, and the effectiveness of various countermeasures and other protective actions, including remedial actions, in reducing contamination levels, human exposures, and doses to humans. This paper describes the exercises conducted during the MODARIA I and MODARIA II programmes. These exercises have included short-range and mid-range atmospheric dispersion exercises based on data from field tests or tracer studies, hypothetical urban dispersion exercises, and an exercise based on data collected after the Fukushima Daiichi accident. Improvement of model capabilities will lead to improvements in assessing various contamination scenarios (real or hypothetical), and in turn, to improved decision-making and communication with the public following a nuclear or radiological emergency.

Mid-range atmospheric dispersion modelling. Intercomparison of simple models in EMRAS-2 project.

An intercomparison of atmospheric dispersion models has been carried out for a hypothetical accident occurring in a nuclear power plant in the center of Spain. The accident consisted of a steam generator tube rupture, and two radionuclides have been considered for the exercise: 137-Cs and 131-I. Meteorological conditions and radionuclide release rates were supplied. Models provided deposition maps, timeintegrated concentrations in air and arrival times of the plumes to specific locations. The effect of the meteorological conditions used in the modelling was clear, with different behavior of the plume with neutral stability vs. stable conditions. The predicted arrival times of the plume at specific locations showed much less variability than deposition and air concentrations. This variability in part reflects the uncertainties inherent in atmospheric dispersion modelling and in the selection of parameter values, such as deposition velocities or diffusivities.