Erkki Ilus

The radiological exposure of man from radioactivity in the Baltic Sea

A radiological assessment has been carried out considering discharges of radioactivity to the Baltic Sea marine environment since 1950. The sources of radioactivity that have been evaluated are atmospheric nuclear-weapons fallout, fallout from the Chernobyl accident in 1986, discharges of radionuclides from Sellafield and La Hague transported into the Baltic Sea, and discharges of radionuclides from nuclear installations located in the Baltic Sea area. Dose rates from man-made radioactivity to individual members of the public (critical groups) have been calculated based on annual intake of seafood and beach occupancy time. The dose rates to individuals from the regions of the Bothnian Sea and Gulf of Finland are predicted to be larger than from any other area in the Baltic Sea due to the pattern of Chernobyl fallout. The dose rates are predicted to have peaked in 1986 at a value of 0.2 mSv year-1. Collective committed doses to members of the public have been calculated based on fishery statistics and predicted concentrations of radionuclides in biota and coastal sediments. The total collective dose from man-made radioactivity in the Baltic Sea is estimated at 2600 manSv, of which approximately two-thirds originate from Chernobyl fallout, approximately one-quarter from atmospheric nuclear-weapons fallout, approximately 8% from European reprocessing facilities, and approximately 0.04% from nuclear installations bordering the Baltic Sea area. An assessment of small-scale dumping of low-level radioactive waste in the Baltic Sea in the 1960s by Sweden and the Soviet Union has showed that doses to man from these activities are negligible. Dose rates and doses from natural radioactivity dominate except for the year 1986 where dose rates to individuals from Chernobyl fallout in some regions of the Baltic Sea approached those from natural radioactivity.

Transfer and behaviour of 137Cs in two Finnish lakes and their catchments

The long-term behaviour of 137Cs was studied in two freshwater ecosystems in southern Finland in an area most loaded by the Chernobyl fallout in 1986. Samples were taken from water, sediments, aquatic plants and fish in the lakes and from soil, mushrooms and seed plants in the catchments. The activity concentrations of 137Cs in fish have remained at a relatively high level and decreased much more slowly in these two lakes than in other lakes studied by us. One reason for the continuously high concentrations in fish is evidently the prolonged stay of caesium at a relatively high level in the water of these lakes, which is associated with a slow sedimentation rate. The hydrographical properties of the lakes, i.e. the oligotrophic character associated with a deficiency of potassium in water and a low pH are other reasons for the effective uptake and long retention time of 137Cs in fish. The effect of humic substances on the uptake and delay of caesium in fish could not be proved clearly in this study. The swampy soil type of the catchment associated with a more oligotrophic status and lower pH of the water in Lake Siikajärvi explain at least partly the difference in activity concentrations and transfer of 137Cs between the two lakes studied. This refers to the higher transfer from the catchment to the lake and the higher uptake of 137Cs by fish and other biota in Lake Siikajärvi than in Lake Vehkajärvi. Perch and pike were more efficient accumulators of caesium than the best indicators among the aquatic plants. In the terrestrial environment, caesium was most effectively accumulated by mushrooms.

Uranium and plutonium containing particles in a sea sediment sample from Thule, Greenland

Particles composed of radioactive materials and probably originating from US nuclear weapons were identified in sea sediment samples collected from Thule, Greenland, in 1997. The weapons were destroyed close to the Thule Air Base in 1968 in an aeroplane crash, which dispersed radioactive materials in the environment. The presence of particulate radioactive materials in the sediment samples was revealed by combining gamma-spectrometry and autoradiography. Isolation and separation of a radioactive particle from a bulk sample were performed using autoradiography, phosphor plate imaging and scanning electron microscopy. Using X-ray microanalysis as well as alpha and beta activity analysis, U and weapons-grade Pu were detected in the granular, brittle particle.

Discharge of 137Cs and 90Sr by Finnish rivers to the Baltic Sea in 1986–1996

The total amounts of 137Cs and 90Sr transported from Finland by rivers into the Gulf of Finland, Gulf of Bothnia and Archipelago Sea since 1986 were estimated. The estimates were based on long-term monitoring of 137Cs and 90Sr in river and other surface waters and on the statistics of water discharges from Finnish rivers to the above sub-areas of the Baltic Sea. The total amounts of 137Cs and 90Sr removed from Finland into the Baltic Sea during 1986-1996 were estimated to be 65 and 10 TBq, respectively. The results show that, although the deposition of 137Cs was much higher than that of 90Sr after the Chernobyl accident, the amount of 137Cs removed from Finland is only six times as high as that of 90Sr. This emphasizes the importance of 90Sr while considering radiation doses from surface waters and 137Cs while estimating doses via pathways from catchment soil, lake sediments and biota after a fallout situation.

Joint Russian—Finnish study of radioactive contamination in the NW part of Lake Ladoga

In August 1992 a joint Russian-Finnish expedition was arranged to the NW part of Lake Ladoga to study radioactive contamination in the region. Special attention was paid to the area surrounding the Heinamaa Islands, where the wreck of the former mine carrier ship “Kit” had been lying about 30 years before it was moved to Novaya Zemlya in 1991. During this period the wreck had been used as a store for radioactive waste containing principally 90Sr, 137Cs and 239,240Pu. Lake water, bottom sediment and some biological samples were collected for strontium, plutonium and gammaspectrometric analyses. In all the samples the radioactivity concentrations were very low, indicating radioactive contamination of about the same level as caused by global fallout in the 1960’s and the Chernobyl fallout in the area. Only in two water samples taken close to the former site of the wreck slightly elevated 239,240Pu concentrations were detected. The great water volume of Lake Ladoga and effective water exchange at the wreck site may explain the very low levels of radioactive wastes detected in the aquatic environment.