Radioactivity of the deposition
Deposition refers to radioactive substances that have fallen to the ground or water from air. Deposition may come with rain as a wet deposition or as a dry deposition. Analyzing deposition samples provides information on the quantities of radioactive materials in different parts of the country and forms the basis for reports and studies on the migration of radioactive substances in the soil and bodies of water.
Collection of samples
Deposition samples are taken on a continuous basis in eight localities. The collection period for deposition samples is typically one month. If analyses of air samples or external radiation indicate signs of fresh fallout, deposition samples are collected at shorter intervals. This ensures that information on the composition of the fallout and the amount of radioactive substances in different localities is obtained promptly.
Deposition samples are collected with a stainless-steel instrument with a collection area of 0.07 m². The instrument is not able to differentiate between wet depositions caused by rainfall and dry depositions, and measures them as a total. To facilitate samples during winter months, the instruments are fitted with heat resistance to keep the samples unfrozen and to ensure that all snow collected in the collection container is included in the sample.
Analysis of samples
Usually, the volume of deposition samples is reduced by evaporation prior to analysis. This helps to detect even the lowest concentrations of radioactive materials. The evaporation residue is placed in a small container and analysed for gamma radiation-emitting radioactive substances, such as cesium-137, with a gamma spectrometer. In addition, samples can be analysed for beta or alpha emitters, such as strontium-90 and plutonium, that require radiochemical detection. Furthermore, rainwater samples taken from two localities are analysed for tritium concentrations.
Measurement data
STUK publishes the measurement data once the analysis has been completed.
Deposition of radioactive substances in eight localities in 2023
Table 1: The tables shows quarterly totals of cesium-137 depositions in the different localities. For strontium-90, the samples are combined annually according to location. The data is presented in becquerels per square meter (Bq/m2).
| City | Cs-137 (Bq/m2) | Sr-90 (Bq/m2) | |||
|---|---|---|---|---|---|
| January-March 2023 | April-June 2023 |
July-September 2023 | October-December 2023 | January-December 2023 | |
| Imatra | 0,23 | 0,44 | 0,18 | 0,40 | 0,65 |
| Ivalo | <0,08 | 0,09 | 0,11 | 0,08 | <0,05 |
| Kajaani | 0,06 | 0,17 | 0,19 | <0,11 | 0,11 |
| Kotka | 0,27 | 1,5 | 0,55 | 0,27 | 0,09 |
| Kuopio | <0,10 | 0,18 | 0,10 | 0,08 | <0,05 |
| Rovaniemi | 0,09 | 0,09 | 0,11 | 0,07 | <0,05 |
| Sodankylä | <0,06 | 0,13 | 0,11 | 0,10 | <0,04 |
| Vantaa | 0,10 | 0,11 | 0,11 | 0,10 | <0,05 |
Tritium concentrations in rainwater samples in 2023
Tritium concentrations of rainfall in 2023 were low. Tritium activity concentrations ranged between 1–2 Bq/l or below the detection limit.
Table 2: Tritium concentrations in rainwater samples in 2023
| Reference date | Rovaniemi H-3 (Bq/l) |
Vantaa H-3 (Bq/l) |
|---|---|---|
| 19.1.2023 | <1,1 | <1,1 |
| 20.2.2023 | <1,1 | <1,1 |
| 19.3.2023 | <1,1 | <1,1 |
| 17.4.2023 | 1,5 | <1,2 |
| 19.5.2023 | 1,3 | 1,5 |
| 19.6.2023 | 1,9 | <1,2 |
| 20.7.2023 | 1,3 | 1,8 |
| 21.8.2023 | <1,2 | 1,6 |
| 18.9.2023 | 1,1 | 1,3 |
| 19.10.2023 | <1,2 | <1,2 |
| 20.11.2023 | <1,2 | <1,2 |
| 18.12.2023 | <1,2 | 1,2 |
Cesium-137 and strontium-90 depositions
Radioactive materials identified in depositions preceding 1986 originate from the fallout resulting from nuclear weapon testing performed in the atmosphere. Measurements from this type of fallout typically vary between the different seasons of the years. The Cs-137 and Sr-90 detected from post-1986 depositions mainly originate from the explosion at the Chernobyl power plant. The radioactive substances carried by fallout migrate locally, with a small portion of them re-entering the atmosphere.
