SENSITIVITY ANALYSIS OF ECOSYS-87 - AN EMPHASIS ON THE INGESTION PATHWAY AS A FUNCTION OF RADIONUCLIDE AND TYPE OF DEPOSITION

A sensitivity analysis of parameters associated with the ingestion pat hway was conducted for the computer model ECOSYS-87. The model is curr ently being used extensively throughout Europe for accident consequenc e analysis following a nuclear release. Individual parameter perturbat ion was used to develop sensitivity indices. The sensitivity indices i dentified parameters whose uncertainties had a large impact on model r esults. The relative ranking of the sensitive parameters depended on t he radionuclide (Cs-137, Sr-90, I-131, or Pu-239), whether dose 1 y or 50 y postaccident was being considered, and whether the deposition ev ent was dry or mixed. The most influential parameter for Pu-239 was re suspension. Parameters to which human dose was sensitive following exp osure to Cs-137, Sr-90, and I-131, Were as follows: yield (biomass) of vegetation, transfer of radionuclides from plants to animals, deposit ion velocity, changes in radionuclide concentrations due to food proce ssing, livestock feeding rates, and weathering of radionuclides from p lant surfaces. The ranking of I-131's parameters was governed by its 8 -d physical half-life. Parameters that affected the initial deposition , parameters that could rapidly affect the transfer of I-131 from the biota to humans, or parameters that allowed I-131 to decay prior to co nsumption were important. Important parameters specific to Sr-90 and C s-137 included transfer of radionuclides from soil to plant, leaching from the plant rooting zone, and resuspension. Parameters associated w ith the movement of radionuclides within the soil were not as importan t for Cs-137 as they were for Sr-90. With the exception of the deposit ion velocity, if a parameter proved to be sensitive for dry deposition , it was as sensitive, or even more so, for a mixed deposition event. Extending the model end point from 1 y to 50 y postaccident also cause d a shift in the relative ranking of sensitive parameters for Cs-137 a nd Sr-90. Parameters that were not important for any of the radionucli des considered under this scenario were those related to timing and le ngth of crop harvest, transfer of radionuclides from leaf surfaces to edible portions of plants, rate at which radionuclide concentrations i n plants decrease due to growth dilution, and time for animals to reac h marketable size.