HYDROS: a model of radionuclide migration in surface waters

The submodel HYDROS has been developed for the safety assessment code COSMOS-S/D to represent radionuclide migration in surface waters. Model assumptions are generally conservative (i.e. pessimistic) in keeping with the safety assessment philosophy. Dilution is the dominant mechanism for reducing contaminant concentration. Actual removal of contaminant occurs through delay and decay, evaporation (if volatile), or sorption in lake sediment, the latter modelled as an irreversible process. The same system of equations applies equally to both river reaches and lakes, with the difference apparent only in the relative effects of dispersion and delay mechanisms for a given radionuclide. Experience since the original model was developed has led to simplification of input so that the submodel is easier to use