Radionuclides in sedimenting material from the Biotest basin

In the biotest basin, outside the nuclear power plant at Forsmark, two sedimentation traps have been stationated during 1984. Samples have been collected every fortnight. The total amount of the sedimented material, organic content and the concentration of radionuclides have been determined for the samples. A subsample will be examined by microscope, and the results reported elsewhere. The sedimentation rate in the biotest basin is low. A maximum sedimentation rate, about 2.4 mg/cm2 day, was found in the summer. During the rest of the year it declines to 1 mg/cm2 day. Usually the nuclides Co-60, Zn-65, Mn-54, Ag-110m, Co-58 and K-40 are determined and occasionally also Nb-95 and Cr-51. The concentration is relatively high, about 400-8000 Bq/kg d.w. for Co-60 and Zn-65. In some samples even much higher values were found. One explanation could be that some fragments of ion exchanger, which might be discharged with the cooling water, was recovered in the traps. There was no correlation between the radionuclide concentration in the samples and the amount of radionuclides in the discharge at the southern sampling station. The station is located off the main current and the influence of the discharge varies with the wind. At the other location, about 800 m to the north, in the middle of the stream, there is good correlation between the nuclide concentration and the discharge. Particularly for Co-60 and Zn-65 and to some extent even for Mn-54. For Ag-110m no correlation have been determined. (author)