No unusual radiation or radioactivity is measured at Risø DTU or in its surrounding area. And that’s the way it has been since the inauguration of Risø in 1958. At that time the purpose was peaceful use of nuclear energy. Twice a year Risø DTU’s Radiation Research Division prepares a data report to Danish Decommissioning (DD), who works on decommissioning the nuclear facilities at Risø. The report is a commissioned work required by the authorities. It contains measurement results for radioactivity and radiation at and around Risø’s area.
The data report, published on Risø’s website recently, contains a lot of tables and graphs illustrating the data measured either in the previous year or data covering all the years since the foundation of Risø. Among other things, the report deals with air samples of cesium-137, a manmade radioactive isotope.
“The present cesium-137 levels are very low, but previously we have seen fluctuations and higher ’normal’ levels than now. When the United States and the Soviet Union were still carrying out atmospheric nuclear weapons tests, we were able to measure it here in Roskilde. We could also measure significant fluctuations when the Chernobyl accident occurred in 1986 – the whole division, in fact everyone at Risø familiar with nuclear techniques, was involved in measurements of the radioactive fallout,” explains Sven P. Nielsen.
Radiation above – radiation below
Another thing that is measured is beryllium-7. Beryllium-7 comes from interaction of cosmic radiation from space with the upper atmosphere and is mixed with the atmosphere below. Also lead-210 is measured. It is a naturally occurring radioactive substance that comes from uranium decay in soil. The occurrence varies with the season, because for instance the air pressure helps to release lead-210 and radon from soil. Furthermore there is a substantial quantity of uranium ore at the area of Risø, and therefore it is important that not more than expected is released (see fig. 1.2 in the report)
Two rain collectors are used in order to measure the tritium content of rainfall, showing Risø’s pollution from nuclear facilities, but also here cosmic radiation may influence the figures (see fig. 2.3.1 + 2.3.2 in the report).
“Tritium is also called super-heavy hydrogen or H-3. Deuterium is heavy hydrogen (H-2). The occurrence of tritium is a natural consequence of Denmark’s reactor 3 (DR 3) which was placed at Risø and was heavy water cooled. And as heavy water was used for cooling, tritium appeared causing the levels to rise in the cooling water,” Sven says and continues:
“The graph shows that we have only had very small amounts of tritium in rainwater since 2000, when DR 3 was shut down permanently. The ’peaks’ seen in the past reflect the maintenance work that has been made on DR 3.”
Samples from Roskilde Fjord
The last figures in the report show, among other things, bottom and water samples from Roskilde Fjord. The most interesting in these samples is probably the wide fluctuations in 1986 of cesium-137, because of Chernobyl. But also the peak of tritium in 2000, is interesting. The peak reflects technical problems related to DR 3, where some of the heavy water was released to the fjord. However, the spill was so small that the maximum concentration of tritium in Roskilde Fjord corresponded to the limit value set for tritium in drinking water.
The waste management plant at Risø is the only place in Denmark to receive radioactive waste. The waste is handled by the company Danish Decommissioning (DD), and it is mainly hospitals and the industry that deliver the waste. DD is also responsible for storing the waste afterwards. Part of the work is to distil radioactive waste water.
“Tritium is found in water (HTO), meaning that tritium cannot be separated from the distilled waste water at the waste management plant, but has to be discharged into the fjord. Only particulate substances and salts can be separated.
It is also worth noting that the limit value for tritium in drinking water has been set considering that a person drinks two liters of water a day, which will even then cause very low radiation dose. And Roskilde Fjord does not provide drinking water – though it is neither pure seawater, but rather brackish water. Besides, tritium exists naturally in nature,” Sven reassures.
In addition measurements are done of beta radioactivity from Risø’s settling basins. These measurements serve as screening and do not identify which isotopes contribute to the radioactivity.
“If there are fluctuations, even if they, like this year, by no means are critical, we always do additional gamma measurements. In many cases it appears to be organically bound tritium, that cannot be distilled,” Sven says, adding that they are now doing additional measurements on the latest fluctuations and that everything is as it should be.
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