CROM is a generic environmental model code developed by the CIEMAT in collaboration with the Polytechnic University of Madrid based on Safety Report Series 19 with some variations from RP 72. It was designed to automate the calculation of radionuclide concentrations in different compartments of the environment and their impact in the food chain, as well as the effective dose calculation in humans, allowing the researcher to concentrate on the analysis of the model results. The models implemented in CROM and the associated user’s manual has been published by CIEMAT. A quality control analysis of the code was performed by CIEMAT and RPD-HPA (formally NRPB), for its worldwide distribution by the IAEA as the reference for those models. CROM 6 is available and freely distributed by contacting the IAEA or downloading it from: ftp://ftp.ciemat.es/pub/CROM.
CROM uses generic models for dispersion and dilution, and is flexible enough to be used for different degrees of realism. In order to estimate the radionuclide concentrations in environmental media, the quantities and types of radionuclides to be discharged (the source term), the mode and characteristics of the discharge and the receptor points (up to five), should be specified.
|Figure 1. Main screen of the CROM showing the main dispersion modules.|
The atmospheric dispersion model is a Gaussian plume model designed to assess annual averaged radionuclide concentrations in air (this model was validated for distances <20 km) used for the calculation of the rate of deposition at various points in the region of interest from long-term releases, provided that a 30 years continuous emission and a neutral atmospheric condition are deemed. The model accounts for the effects of any buildings in the vicinity of the release and the effect of the roughness of the ground in the winds profile. The basic meteorological variables required for each individual air concentration calculation are the wind direction and the geometric mean of the wind speed at the physical height of the release point. The code allows the use of other diffusion factors, for different stability categories than neutral (D), and also the introduction of effective heights but does not calculate them.
Surface water models account for dispersion in rivers, small and large lakes, estuaries and along the coast of seas and oceans. These models are based on analytical solutions to advection-diffusion equations describing radionuclide transport in surface water with steady state uniform flow conditions. The contamination of surface water from routine discharges to the atmosphere is considered for small lakes. All the models contain a great quantity of default values that can be used in absence of local specific information. The models can also be easily used for great lakes and sewerage systems with some considerations of the pathways involved.
The terrestrial food chain models accept inputs of radionuclides from both the atmosphere and the hydrosphere and they take account of the build-up of radionuclides on surface soil over a 30 year period. The process of radioactivity decay and build-up is taken into account in the estimation of the retention of radionuclides on the surfaces of vegetation and on soil, and in the assessment of the losses owing to decay that may occur during the time between collection and consumption. The food categories considered are milk, meat and vegetables. The uptake and retention of radionuclides by aquatic biota uses selected element specific bioaccumulation factors that describe an equilibrium state between the concentration of the radionuclide in biota and water. The types of aquatic food considered are freshwater fish, marine fish and marine shellfish.
The estimated radionuclide concentrations in air, soil, sediment, food and water (representative of 30 years of discharge) are combined with the annual rates of intake and the occupational factors and the appropriate dose conversion coefficients to obtain the maximum effective dose in one year for the combined external and internal exposure. The dose conversion coefficients for internal exposure are taken from Safety Series No 115 while for external exposure have been calculated in base of the coefficients and equations given in the Federal Guidance Report No 12. The model takes into account external gamma dose rates from radionuclides due to cloud immersion, contaminated soil and sediments and water submersion (for gamma and beta exposure). The effective doses from external exposure and radionuclide intakes are calculated for the six age categories recommended by IAEA4 and ICRP.
The software implements a default database with data for 152 radionuclides and also 8 examples of application based on examples included in SRS19.