Deposition

Radioactive materials originating from nuclear tests performed decades ago are still detected in Finland. A nuclear power plant accident happened in 1986 in Chernobyl, Ukraine. Radioactive materials spread into many countries over many days. Even today, the Chernobyl fallout increases the radiation dose received by Finns. However, this represents only one hundredth of our average annual dose of 3.7 millisieverts.

Deposition refers to radioactive materials that have fallen to the ground or water from air. Deposition may come with rain (wet deposition) or as dry deposition.

Nuclear tests in the atmosphere

Fallout from nuclear tests performed decades ago is still detected in Finnish nature. Altogether 512 nuclear tests have been performed in the atmosphere at a number of sites. Most of these tests took place between 1945 and 1963. The last nuclear tests were performed in 1980. The blasts generated plenty of different radioactive substances, their half-lives ranging from a few seconds to many thousand years.

The blast sent part of the materials propelling to the upper layers of the atmosphere, and over the years they gradually fell down to earth. The heaviest fallout was received after tests performed in the atmosphere in the early 1960s. With regard to radiation doses, the most important radioactive materials in the fallout are cesium-137 and strontium-90. Their presence in the ground, food and people has been monitored since the early 1960s.

Changes in the cesium-137 activity in Finns has also been followed through direct measurement of people. The highest cesium concentrations were detected in the mid-1960s, but before the Chernobyl accident, small amounts of cesium were still detectable in people. The average radiation doses were a few tenths of a millisievert.

In Finland, there is one group of people who received a clearly higher radiation dose from the cesium-137 in the fallout compared with others: reindeer breeders in Lapland. They received cesium in food. The elevated cesium-137 activity in reindeer breeders is due to the fact that the food chain lichen–reindeer–human efficiently enriched cesium. From 1955 to 1985, cesium-137 exposed reindeer breeders to a dose of approximately 13 millisieverts (mSv), that is, approximately 0.4 mSv per year. Their dose of other radioactive materials originating from nuclear testing totalled 0.3 mSv, that is, 0.01 mSv per year.  Even today, reindeer breeders receive an annual dose of approximately 0.03 mSv from the cesium-137 originating from the nuclear testing fallout.

Changes in cesium-137 activity in people in the Helsinki Capital Region from 1965 to 2014

Changes in cesium-137 activity in people in the Helsinki Capital Region from 1965 to 2014. Until 1986, the cesium came from the fallout caused by nuclear weapon testing in the atmosphere. Since 1986, almost all of it has come from the fallout caused by the Chernobyl accident.

The Chernobyl fallout

The Chernobyl fallout will expose Finns to a total radiation dose of two millisieverts during 50 years. We receive a similar dose each year from radon. Half of the total dose from Chernobyl came during the first ten years after the accident. The situation is illustrated in the figure through a calculation of radiation doses received from different sources over 50 years. The average dose is 184 millisieverts for each Finn.

Total radiation doses received by Finns from 1986 to 2036  
Medical use of radiation  27 mSv
Chernobyl fallout 2 mSv
Natural background radiation 55 mSv
Radon 100 mSv

 

Radioactive fallout fell down with rain

In the accident, high amounts of radioactive materials were released and propelled more than 1,000 metres up in the air by the force of the explosion and fire. At this altitude, winds blew towards Finland, Sweden and Norway at first. Later, as the direction of winds changed, radioactive substances were also carried to other areas, mainly Central Europe. In Finland, the concentrations of radioactive materials were at their highest in the evening of 28 April. More than 30 radioactive substances were identified in air. After two or three days, the air was almost completely clean again. A couple of weeks after the accident, Finland received another plume of radioactive materials from Chernobyl. However, the concentrations were lower compared with the first plume.

Some of the radioactive substances were drawn to the ground by gravity, but a considerably larger amount rained down from the plume.  The amount and intensity of the rain varied greatly in different parts of Finland. For this reason, there was great variation also in the amount of radioactive materials ending up in soil.

The fallout carried a lot of short-lived radioactive materials that disappeared from nature in just a few days or months. In the long run, with regard to radiation doses, the most important substances are the radioactive cesium isotopes, cesium-137 and cesium-134. Their half-lives are 30 and approximately two years, respectively.

Cesium-137 deposition

Cesium in soil causes exposure to external radiation

The external radiation dose results from depositions, that is, radioactive materials fallen to the ground. At first, the highest measured dose rate resulting from the Chernobyl fallout was 5 microsieverts per hour. It is 30 to 50 times higher than the dose rate of normal natural background radiation. Protective measures are necessary if the radiation level exceeds 100 microsieverts per hour. Short-lived radioactive materials decayed rapidly. At the end of May, the dose rate was less than one tenth of the value measured immediately after the fallout.

In 1986, the Chernobyl accident exposed every Finn to an average external radiation dose of 0.15 millisieverts. By 1996, the dose had decreased to 0.02 millisieverts per year. Currently, most of our external radiation dose is caused by cesium-137.

Average external and internal doses received by Finns after the Chernobyl accident.

Average external (blue) and internal (green) doses received by Finns after the Chernobyl accident.

Internal radiation dose is mainly caused by cesium in natural produce

Our internal radiation dose is almost completely caused by radioactive substances received in food. Milk, meat and other farm produce were the main source of cesium in the first years after the accident. Since 1988, the main sources of cesium have been freshwater fish, game, wild mushrooms and wild berries. Cesium concentrations in farm products fell quickly, while concentrations in natural produce have been falling slowly. STUK monitors radioactivity in food.

Cesium concentrations in people were at their highest in summer 1987, when the average cesium-137 level in a Finn was 2,000 becquerels.  In the region with the heaviest fallout, the average level of cesium was 4,000 becquerels. In heavy users of natural produce the doses were 5- to 10-fold compared with others. Today, the average cesium level in a Finn is approximately 200 becquerels. For comparison, it should be noted that everyone has a few thousand becquerels of naturally-occurring potassium-40 in their system.

In 1987, Finns received a radiation dose of approximately 0.13 millisieverts from radioactive cesium in food. By 1996, the internal radiation dose had decreased to a fifth, that is, 0.02 millisieverts per year. Today, cesium-137 accounts for almost all of the dose.

 

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