PSI - Issue 36

Volodymyr Sydorenko et al. / Procedia Structural Integrity 36 (2022) 318–325 Volodymyr Sydorenko, Sergiy Yeremenko, Viola Vambol et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Over the past 35 years, there has been practically no industrial and economic activity in the Chernobyl zone. This contributed to the intensive development and accumulation of natural plant fuel in this area. The bulk is pine forests with a high density of trees, which makes up almost half of the territory of the Exclusion Zone, that is, more than 100 thousand hectares. Rising average annual temperatures in this region have increased the risk of large fires. Climatic anomalies and droughts only exacerbate the problem of fires. During this period, the number of fires in the 30-kilometer zone was about 1400 fires, while 25 thousand hectares of forests and former farmland were burnt. The presence in the biosphere of this territory of radionuclides such as 137Cs and 90Sr (half-lives of 30.1 and 29.1 years, respectively) only complicate the ecological situation in this region. In the forests contaminated around the Chernobyl nuclear power plant, fallout is distributed as follows: about 5% is concentrated in the forest stand, up to 85% in the underlying layer, the rest in the mineral soil layer, and over time, the content of radionuclides increases both in the forest stand and in the mineral soil layer. From 1990 to the present, a significant number of forest fires occurred in the Chernobyl Exclusion Zone (ChEZ), which covered 16.9 thousand hectares of forests and 19.6 thousand hectares of grass flooring. The horizontal migration of radioactive combustion products (RCP) contained in soils after a forest fire leads to pollution or an increase in the density of radiation contamination in those areas that are located near areas undergoing deflation. The intensity and directions of migration are associated with the wind speed, its dynamic characteristics, soil susceptibility to deflation, the degree of its protection from the effects of wind, and the like. At the same time, the danger of secondary radioactive contamination of territories is associated with the density of soil contamination, the effect of wind erosion, and the specific activity of radionuclides in ash. The information on the level of deflation of the RPL after a forest fire is related to the calculations of the deflation modulus. In this regard, a mathematical description of a model for predicting the transport of radionuclides in the air after a forest fire based on deflation parameters, such as radioactivity, maximum wind speed, duration of dust storms, and the like, is of particular interest. The use of this approach contributes to predicting the spread of a forest fire, as well as assessing the impact of a forest fire on the ecological and radiation situation in radioactively contaminated areas in the Chernobyl zone. It should be noted that high temperature and irradiation are the important factors influencing structural integrity therefore the raised problem is interesting also from this perspective 2. Materials and Methods In the course of the research, an overview analytical method was used. In their studies, the authors relied primarily on open sources of information and statistical data that are not closed or for official use. To describe the distribution of radionuclides in the air after a forest fire, the turbulent diffusion model and the Gaussian static model were used (Newstand (1985)). In determining the intensity and direction of migration of radioactive combustion products, a deflationary approach was used. Review of sources As a result of a forest fire with an area of 200 hectares on a radioactively contaminated territory after the Chernobyl accident, up to 70 tons of radioactive ash can be formed, which turns into aerosols of smoke and dust and can be raised to a height of up to 1-3 km according to various estimates and carried over fairly long distances horizontal air flows. Approximately 24% of the total stock of radionuclides accumulated in the Chernobyl zone migrates due to forest fires, while the water (river) runoff (Pripyat river) is approximately 65%; migration due to air masses is about 10%; technogenic migration - 0.5%, biogenic removal - 0.5%. During the combustion of biomass, a large amount of radioactive particles, together with the smoke, rise into the air. In studies, an assessment was made of the secondary radioactive contamination of the area as a result of forest fires. It turned out that as a result of such emergencies in the radioactively contaminated area, there is an increase of almost 104 times in the concentration of radionuclides at the site of a fire in the air compared to background values. Analysis of conventional (Fig. 1) and (Fig. 2) images of the fire indicates in the Chernobyl zone that smoke plumes of forest fires, depending on weather conditions, can be carried over considerable distances. The main carriers of radiation hazard (Yeremenko et al. (2021)), as well as dioxins (Vambol et al. (2016)), during forest fires are solid combustion products - incompletely burnt material and ash. Part of the incompletely burnt material remains