A Screening Level Ecological Risk Assessment and ranking method for liquid radioactive and chemical mixtures released by nuclear facilities under normal operating conditions

Titre du congrès :International Conference on Radioecology and Environmental Radioactivity
Ville du congrès :Bergen
Date du congrès :15/06/2008

Ecological Risk Assessment is used to evaluate the potential hazards to the environment that are attributable to emissions of pollutants from industries. There is guidance available regarding the general ecological risk assessment process including problem formulation, analysis, and risk characterization (in Europe, the technical guidance document for chemicals and recently the ERICA integrated approach for radionuclides). In a first step, the Screening-Level Ecological Risk Assessment (SLERA) is used to evaluate whether the emissions can put the receptor ecosystems at risk or not. Concerning releases from nuclear facilities under authorization, any SLERA is a challenging task because of (1) the large number of substances, (2) the various quantities that may be emitted to the aquatic ecosystems and (3) the various environmental situations to be considered. This task must be performed for two categories of pollutants, radionuclides and chemicals, each exhibiting specificities in terms of dose-effect relationships. Since these relationships for radioactive substances are based on the tissue-absorbed dose in Gray, the first step is to express critical exposure values to radionuclides in a consistent way with the critical concentration used for chemicals. We describe here the screening and ranking method and an application to the electronuclear sites along the Rhône River. The method is based on the calculation of the ecotoxicological effect factor (EF) using similar underlying principles and assumptions than those adopted in Life Cycle Assessment methodologies. Assuming linearity of the Species-Sensitivity Distribution fitting a log-logistic distribution for substance i between the origin and a benchmarck concentration (HC50 – Hazardous Concentration affecting 50% of species), the gradient (PAFi/Ci ; PAF for Potentially Affected Fraction of species) expresses the change in effect per unit change of exposure: . In presence of multiple substances ms, and for both concentration addition or response addition, the change in PAF becomes: . For each chemical, the HC50 is the geometric mean of a set of chronic EC50 at least for 3 phyla. The AMI (Assessment of the Mean Impact) database was used to extract the HC50 and IC95% for the substances of interest (copper, zinc, iron, boric acid, ammonium, morpholine, lithine, hydrazine). For radionuclides, radiological chronic effect data from FRED–EDR50 or Dose Rate giving 50% effects of the exposed group compared to the control – are expressed in Gy per unit of time. The HDR50 is calculated as the geometric mean of the chronic EDR50 data. It is then converted into HC50 expressed in mol/L for each radionuclide while using Dose Conversion Coefficients associated to a conceptual model for fate taking into account external irradiation from the habitat and internal irradiation due to the radionuclide bioaccumulation. The results and discussion will be focused on the following comparison: (1) the relative importance of released chemicals and radioactive substances in terms of ecotoxicological impact (chemicals>>>radionuclides); (2) the major contributor in terms of impact for each category (chemicals: Cu and radionuclides: C-14) and (3) the relative contribution of different nuclear sites at the watershed scale (for chemical impact: St Alban>Cruas>Tricastin>Bugey).