After the Accident of the Chernobyl Nuclear Power Plant, which occurred on 26 April 1986, large areas of the Ukraine, Belorussia and the Russian Federation were exposed to radioactive contamination. Currently the main contaminants of these areas are the long-living radionuclides caesium (137Cs), strontium (90Sr) and plutonium (239Pu). Radionuclides of plutonium are found mainly within the 30-km zone around the Chernobyl Power Plant. Radionuclides of strontium have contaminated a number of districts in the Kiev Region (Belorussia) and in the Bryansk Region (Russian Federation). Radionuclides of caesium account for the largest contaminated areas in these states.
In the Russian Federation, the soil surface, in which caesium radionuclide contamination exceeds 37 GBq/km2 (37 Gigabequerel = 3.7×1010 Bq), totals 4.9 million hectares within 15 Regions. The areas in which the radiocaesium contamination density is 555 GBq(= 0.555 TBq)-1,5 TBq/km2 and higher, are mainly in the Bryansk Region (ca.250,000 ha). This Region was the most affected of the Russian Federation. Its forest cover is 1.2 million ha equalling more than 30% of its total land area.
In the most contaminated Regions of the Ukraine, Belarus and the Russian Federation (the Kiev, Zhitomir, Rovmo, Gomel, Mogilev and Bryansk Regions), the prevailing forests are young and middle-aged pine and pine-hardwood stands with high fire danger classes; there are also large areas of drained peatlands covered by forests. In future the degree of forest fire danger is expected to grow due to the accumulating debris, litter and standing dead trees because these forests are not going to be thinned.
During the pre-disaster years 1981-85 the annual number of forest fires in the Regions of these Republics varied from 809 to 3442, covering an area from 165 to 4456 ha. In the year of the disaster a total of 1775 fires burned 2336 ha of forests. In 1987, 803 fires burned on an area of 614 ha. In 1992 the first severe wildfires burned in May. On 1 to 4 May 1992, in the Gomel Region of Belarus, a fire destroyed several dozen hectares of forests and two evacuated villages. On 3 May 1992 the fire spread over to the territory of the Ukraine and penetrated into the 30-km zone of the Chernobyl Power Plant. According to the reports of 9 May 1992, the fire had affected 500 ha, thereof 270 ha of forests. Within the 30-km zone the level of radioactive caesium in aerosols increased 10 times. In the Bryansk Region, 135 forest fires on 93 ha of forest land was recorded by 12 May 1992.
Fires in radioactively contaminated forests turn into a very grave problem. The research required for forest fire prevention and control on areas contaminated by radionuclides, as well as the development of systems to monitor forest fires in these areas, are the responsibility of the All-Russian Research Institute of Chemistry in Forestry.
Fig.1. Contamination of the land area in the vicinity of the Chernobyl nuclear power plant by caesium and strontium radionuclides in 1990 (1 Curie = Bequerel).
This institute is subordinate to the Forest Committee of the R.F. Ministry of Ecology and Natural Resources. The institute studies the nature of forest fires and their consequences under radioactive contamination, focusing on the following factors:
assessment of forest fuels in different parts of the biogeocenoses contaminated by radionuclides;
specific and general radioactivity of the major forest fuel types under different degrees of soil contamination;
specific radioactivity of the combustion products, i.e. ash, partly burned organic matter and aerosols;
characterization of smoke emissions from different types of forest fires;
radioactive characteristics of forest fire smoke plumes;
The institute also works on models of heat and mass transfer of radioactive particles during forest fires and defines characteristics of working conditions of fire fighters.
The research results show that in 1990 the bulk of 137Cs radionuclides was concentrated in the forest litter and upper mineral layer of the soil. In the Bryansk Region, the litter of coniferous forests, that of mixed forests, tree cover and bush-grass cover accounted for 72, 43, 8 and 14 per cent of the radionuclides respectively. By the year 1992, vertical migration of radionuclides caused an increasing contamination of the upper soil layer. Therefore at present a serious danger is posed by surface fires (fires in contaminated litter and humus) and ground fires in contaminated peat.
Incomplete combustion causes additional open sources of ionizing radiation: in the zones where the density of radiocaesium contamination of the soil is 0.6-1.5 TBq/km2 and higher, the specific radioactivity of ash and partly burned litter is 180-1086 kBk/kg).
Another great danger is posed by high-intensity (crowning) fires which develop convective activity and lifting of radionuclides into the atmosphere. To obtain correct quantitative characteristics of the redistribution of radionuclides during forest fires, it is necessary to conduct laboratory and field experiments. These experiments are needed to develop fire behaviour models, especially for heat and mass transfer in the near-ground layer.
The contaminated forest environment has not only affected the working conditions of forest fire fighters and the heavy contamination of fire fighting equipment. The studies of forest fires in radioactively contaminated lands are facing significant technical and financial difficulties, affecting both schedules and scales of research.
From: Sergei I. Dusha-Gudym Head, Forest Fire Laboratory Address: All-Russian Research Institute of Forest Chemistry (VNIIKhleskhoz) VNIIKhleskhoz Zavodskaya Str.10 141250 Ivanteevka, Moscow Region RUSSIAN FEDERATION