Uranium concentrations in Finnish bedrock and waterways

Finnish bedrock contains dozens of uranium and thorium deposits and mineralizations that have been investigated through diamond drilling and/or by excavating exploratory trenches.

Of the known uranium deposits, among some of the most important are the stratiform Kesänkitunturi deposit in the Kolarin-Kittilä area, a number of small uranium deposits found in the Kuusamo area, the Koli-Kaltimo quartzite belt in North Karelia, the  Rompas gold-uranium deposit in Ylitornio and, in the south of Finland, Uusimaa, a province that stands out both in terms of radiometry and geochemistry as an area that exhibits, across large areas, higher uranium concentrations in the bedrock and groundwater than most of Finland.

Exploration for uranium began in Finland in the mid-1950s, continuing in a systematic manner, although with limited resources, until the late 1980s. After a long pause, a new phase in exploration began in 2004, when the first claim reservations were submitted. Between 2004 and 2006, claim reservations or claims, in accordance with the Mining Act, were submitted for practically all of the known uranium and thorium occurrences. The applicants include a number of international companies.

Uranium in the bedrock

In Earth’s crust, uranium is primarily enriched in granitic rocks as well as in migmatites consisting of granites and shists. Uranium is found in large concentrations in Southern Finland, in areas with rocks of younger granites. Naturally occurring uranium is found in three isotopes: U-238 (99.9%); U-235 (0.71%) and in U-234 (0.005%).

Isotope U-235 is naturally capable of spontaneous fission when its nucleus is bombarded by neutrons, with the energy then released being used in various applications, including nuclear power plants.
Isotope U-238 has a 14-step radioactive decay series where uranium is transformed – in the final phase – into lead via a number of progeny (Pb-206).

Uranium has a half-life of 4.5 billion years. Of the progeny, the most interesting, in terms of geology, are radium-226, with a half-life of 1.620 years, and its progeny radon-222 (half-life 3.82 days). Being a gas, radon has a marked tendency to move through the soil, both in the air in the voids as well as dissolved in water.

Uranium concentrations are often indicated in parts-per-million (ppm). It is generally estimated that the average uranium concentration in Finnish bedrock is 4ppm. The Nuclear Energy Decree specifies uranium as a mineral in which the average concentration of uranium is at least 1,000ppm.

The half-life of uranium-238 is significantly longer than that of uranium-234. This means that when the uranium concentration is expressed as mass-per-volume-unit or per-weight-unit (mg/l or mg/kg = ppm), the mass of uranium is composed almost exclusively of uranium-238. Regarding the activity of uranium, it should be borne in mind that the total activity of uranium is the combination of the activity of all the three isotopes of uranium, of which the activities of U-238 and U-234 in a solid rock are equally high (1mg of naturally occurring uranium contains 12.4Bq of both U-238 and U-234, and 0.58Bq of U-235.)

Uranium concentrations in bedrock vary greatly. It is generally estimated that the average uranium concentration in Finnish bedrock is 4ppm. The map shows that granitic rock types commonly have higher-than-average concentrations (4–50ppm), particularly in the rapakivi areas of South-East Finland. For example, in the black schist of Talvivaara, the uranium concentration is 15–20ppm; in other words, the concentration is not exceptionally high with regard to the variations in concentration in the Finnish bedrock. Typical uranium concentrations in foliated rock and gneiss zones are not higher than in granites, even though practically all of Finland’s uranium deposits are located in them. In the deposits surveyed so far, the average uranium concentration has been 300–1,200ppm and the total amount of uranium approximately 1,000 tonnes.

Uranium in natural water

In bore well water in Finland, the average uranium activity concentration (U-238 and U-234) is approximately 0.3Bq in a liter (Bq/l) (21 microgrammes in a liter), but there is a wide variation with the maximum concentrations ranging from 150 to 300Bq/l. The activity content in the water of wells dug in soil is only one tenth on the average of the activity level measured in the water of bore wells, with the maximum values of uranium concentration remaining at a far lower level, at around one microgramme per litre.

In natural waters in Finland, the uranium content typically ranges from 0.0001 to 0.001Bq/l (less than 0.01–0.1 microgrammes per litre.) However, in eastern Uusimaa and in mid-Uusimaa for example, such concentrations are clearly above the average: 0.006–0.03Bq/l (0.5–2.4 microgrammes per litre.)

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