Characterization and modelling of naturally occurring radioactive material releases from a phosphogypsum disposal area in Greece

摘要

At the north part of Greece a phosphate fertilizer industry is in operation since 1975. The produced 145 m3/h phosphogypsum (wet production 30% b.w) is deposited continuously near the industry, by the sea side, in a 6 basins (height: 2 -12m) open land disposal area of 563240 m~3. The underground clay layer is not uniform. Moreover, it has a slope and in some areas it disappears, so the vertical leakage is favored. The underground water pH is significantly low (min= 1.93), specially at the north part of the disposal. The low pH values are correlated with increased SO_4 and P_2O_5 concentrations. The underground water pollution is mainly due to underground phosphogypsum drainage water runoffs (10-20 m~3/h), mainly at north, east and south where water permeable layers at the underground water level exist. At the west side, the existing draining channel, the very low permeable layer (clay), the absence of a leakage point (based on piezoelectric measurements) and the direction (west to east) of the underground water flow, prevent the surface and underground runoffs. In order to evaluate the releases of the natural radionuclides from the disposal area to the nearby environment, phosphogypsum measurements with gamma spectrometry and determination of uranium isotopes & ~(226)Ra with alpha-spectrometry in underground and surface water samples collected from different sampling points, have been performed. The ~(238)U concentration in underground water varies from 120 up to 1060 mBq/l, while the ~(226) Ra from 8 up to 5880 mBq/l. High ~(238)U and ~(226) Ra concentrations have been observed at the north part of the disposal area. A model was applied for assessing the behavior of the natural radionuclides U and Ra in the surrounding environment of disposal phosphogypsum site. The model includes the performance assessment, obtaining the water flow and radiological parameters based on experimental data. From field and laboratory data, it has been possible to obtain parameters for the driving processes considered in the model, water fluxes, source term definition, and distribution coefficients. The model consider for both waters flows and mass balance of the radionuclides in a natural environment, provides assessments of radionuclides behavior in groundwater, surface water and including processes of radionuclide migration through land and water. The model has been implemented in a flexible software package RESRAD permits either deterministic or probabilistic calculations to be undertaken for the specific data of the site. In the presented work deterministic calculations will be presented.