“When you release toxins, which is what you might see this as, it’s good to know where they go. For example, if something unexpected happens, you have an idea of the local impact. Since it may be relevant to expand Swedish nuclear power, it’s especially important to have as much knowledge as possible about how the radioactive substances spread in brackish water, such as the Baltic Sea,” says Per Törnquist, who recently defended his thesis at the Department of Health, Medicine and Caring Sciences at Linköping University and is a teacher at Katedralskolan in Linköping.
Nuclear research has been carried out at the Studsvik site, ten kilometres south of Stockholm, since the late 1950s. This research and development has been very important for Swedish nuclear power. The last research reactor was decommissioned in 2005, but the plant still handles radioactive material.
Research into nuclear power causes some radioactive waste in liquid form, which may be released into the sea. Waste radiation levels are strictly regulated and controlled by the Swedish Radiation Safety Authority. But it is not clear how different types of radioactive substances behave in the marine environment and how they spread in nature. In his research, Per Törnquist has investigated where these substances end up in the marine environment.
A natural calender
Similar to how the annual growth rings in the trunk of a tree can reveal its age and growth, layers of sediment on the seabed can function as a natural calendar.
The researchers have taken long test cores from the seabed outside Studsvik and cut them into thin slices, corresponding to time periods. This way, they can track when in time different radioactive substances got there. Based on this data, they have created maps of how various radioactive substances are spread in the seawater in the vicinity of Studsvik.
“Now we have good maps that show the probability of where emitted particles will end up. They can be used in modelling the spread and making risk assessments of how radioactive emissions spread in the event of an accident,” says Per Törnquist
At Studsvik, the most radioactive liquids have been encapsulated for final disposal in a repository. Less radioactive liquids have been purified in Studsvik's own wastewater treatment plant and, after control measurement, released into the Baltic Sea. The researchers have examined Studsvik's emission reports from 1959 onwards and compared these with the radioactive substances they measured in the samples from the bottom of the sea. They can conclude that the research facility has remained within the permitted limits for the release of radioactive waste.
Releases reflect politics
Until 2002, the radioactive element plutonium was reported as a part of the total amount of substances that emit a certain type of radiation, alpha radiation. Therefore, it was not clear how much plutonium and which variants of plutonium, known as isotopes, had been released before 2002.
The researchers have been able to recreate the composition of the releases of plutonium that came from Studsvik before 2002. They used an accelerator mass spectrometer in Spain which allowed them to distinguish the different isotopes of plutonium. The ratios between the different isotopes differ depending on the type of plutonium producer, which makes it possible to trace the origin of the plutonium. In their measurements, the researchers found that Studsvik had released plutonium, mainly in the 1960s.
The 1956 report “The Swedish line” aimed at Sweden becoming self-sufficient in uranium and even becoming a nuclear weapons nation, with the production of plutonium-239 (239Pu) that is optimal for the construction of nuclear weapons. In the late 1960s, “the Swedish line” was abandoned. This policy is reflected in the researchers' measurements, which clearly show that the composition of plutonium isotopes differs completely between the 1960s and the 1970s.
Traces of accidents and tests
Global events have also left their mark on the seabed.
“The most interesting thing is that we can see with the help of 244Pu that in the early 1950s plutonium seems to have come here from tests of the earliest hydrogen bombs. It’s really cool that it can be seen here, because those bombs were detonated in the Bikini Atoll in the southern hemisphere. Traces of nuclear explosions are clearly visible until 1963, when the United States and the then Soviet Union agreed to stop atmospheric nuclear weapon explosions,” says Per Törnquist.
Nuclear tests and nuclear accidents contribute to the global radioactive fallout. The researchers note that at Studsvik, the largest source of radiation in the area is natural background radiation. The second largest source of radiation is the Chernobyl accident in 1986, which released cesium-137 (137Cs), which can be found throughout the Baltic Sea. Radioactivity in 137Cs halves in 30 years and has thus fallen significantly, but it will continue to be a significant source of radioactivity in the marine environment.
The research is now continuing, to contribute to a better understanding of how the radioactive substances spread in the marine ecosystem.
The dissertation: Tracing radioactive contamination: a case study of a research facility and its surrounding marine environment, Per Törnquist, Linköping University Medical Dissertations No. 1938
