Risks of ground deposits

The two modes of deposition: dry and wet deposition

Editorial review 2026

In everyday life, away from the immediate vicinity of the sites of the most serious accidents, contamination residues and their impact are very low. Their effects diminish over time. The most serious of these accidents, Chernobyl, is now a quarter of a century old. Not enough time has yet passed for this process to have taken effect at Fukushima, where ground deposits are less significant. For the record, we should also mention the explosions that occurred in 1957 at Kyshtym in the former Soviet Union and at Windscale in the United Kingdom. These two earlier events, which were smaller in scale, were linked to military programmes.

Radioactive deposits released during a major accident cause long-lasting soil contamination. If the contamination exceeds permitted limits, the affected areas must either be decontaminated, where possible, or the population must be evacuated. This was the case at Chernobyl, where a circular area with a radius of 30 km was evacuated. In the case of Fukushima, the population was evacuated within a 20 km radius around the accident site. Today, some villages, homes, roads and public squares have been decontaminated. Some residents have returned to their homes. To give the French example in terms of regulations, following an accident of this type, the following provisions apply: The reference level for a person’s exposure to radioactive substances resulting from a radiological emergency situation is set at an effective dose of 20 mSv during the year following the end of the radiological emergency situation. This reference level is reassessed each year in order to eventually reach an effective dose of 1 mSv over one year, in addition to the level of radioactivity that existed before the radiological emergency situation.

Radioelements present in the atmosphere following a release are deposited on the ground in two ways, known as dry and wet deposition.

Dry deposition occurs during the dispersion of a radioactive plume. It forms on surfaces in contact with radioactive particles in the air. It is greater when the concentration of radioactive atoms is high and when air pollution persists over time.

Wet deposition forms as a result of rain or snowfall. Raindrops and snowflakes wash radioactive particles out of the air. They carry them to the ground, resulting in much more intense deposition than dry deposition.

With moisture, radioelements penetrate the soil, leaving residual contamination that produces an ambient dose rate. Part of the deposit remains where it formed, while another part runs off over the surface and reaches watercourses. This phenomenon is mainly caused by rainfall. Runoff down slopes or across hard surfaces, such as roofs and roads, results in an uneven distribution of contamination, with some areas less affected and others much more heavily contaminated.

Ground deposits: what are the dangers?

Ground deposits are long-lasting and present two dangers: the lesser is chronic external exposure due to ambient radiation; the more serious is internal exposure through the ingestion of food produced from contaminated crops, mainly leafy vegetables and fresh milk.

Iodine-131 is responsible for the most dangerous contamination, fortunately of short duration because it disappears completely after three months. During the Chernobyl accident, the delay by the Soviet authorities in taking appropriate measures and informing the population resulted in thyroid cancers among children and adolescents, particularly through the consumption of contaminated milk.

Today, virtually the only deposits remaining on the ground are caesium deposits in the most contaminated areas near Chernobyl and Fukushima. The most contaminated areas of Chernobyl, with ground deposits of caesium-137 exceeding 555 kBq/m2, are located in Belarus: more than 7,000 km2, representing 3.4% of the country’s territory. At Fukushima, caesium-137 is accompanied by caesium-134, which has a half-life of 2 years and had not had time to decay significantly. The deposits are concentrated in a 20 km by 50 km strip northwest of the power plant, covering approximately 1,000 km2. In this area, the combined activities of the two isotopes exceeded 600 kBq/m2.

Although the kilobecquerel or kBq is a small unit, activities of several hundred kBq remain considerable. Some of this radioactivity passes into plants. Food products originating from contaminated areas must be checked and monitored. However, measurements such as those carried out by IRSN show that absorption through the roots, which follows absorption through the leaves during the first year, is much less significant in subsequent years and continues to decrease.

In the long term, the main exposure is due to the penetrating gamma rays emitted by caesium-137. In the most contaminated areas around Fukushima, where annual exposure would reach 50 millisieverts (mSv), the return of the population is not envisaged, even after decontamination. In areas where the potential external dose is below 20 mSv/year, residents have been allowed to return.