Chernobyl, April 26 1986: At 01.24 AM a steam explosion destroys the 2000 tonne graphite cover of NO. 4, the newest reactor at the Chernobyl power plant. The explosion ejects parts of the reactor 1000 meters into the air. The atmospheric oxygen then ignites the graphite and three seconds later, a thermal explosion ejects radioactive fuel.
Estimates show that the explosion was identical to 10 tonnes of TNT, making the Chernobyl disaster the worst nuclear power plant accident in history.
Closed for humans
– About 10 Exabecquerels (a billion billion Becquerels) of radioactive fission by-products were released from the Chernobyl accident in 1986, says scientist Nikolaos Evangeliou from NILU – Norwegian Institute of Air Research. He is a radiochemist who has been studying release, transport and deposition of Chernobyl since 2003, and he has also conducted radioecological analyses for the recent accident in Fukushima, Japan.
– Most of these by-products were short-lived radionuclides, noble gases and iodine, Evangeliou continues.
– Among the most long-lived ones, Strontium-90 (90Sr) and Cesium-137 (137Cs) attached to fine aerosol particles and travelled over long distances. But the most dangerous Plutonium isotopes (238Pu, 239Pu and 240Pu) and Americium-241 attached to coarser particles and did not travel very far. But, far enough to make the nearby villages of Chernobyl inhabitable for humans, and it will stay like that for thousands of years.
Changing nature
Chernobyl and areas around were quickly evacuated, and since April 1986, there is hardly any activity in the so-called Chernobyl Exclusion Zone (CEZ).
– This attracted numerous animals to live and reproduce in the CEZ, and by now the area looks like a jungle without any human activity, Evangeliou describes.
– Being there in 2013, I remember it as an eerily silent place, where even the smallest noise makes you worried. I saw deer and wild horses running around, but the abundant wildlife does not mean that radiation have not had a negative effect.
Evangeliou’s colleagues T. A. Mousseau (University of South Carolina) and A. P. Møller (Université Paris Sud) have been working in the area since 1991. They have recorded several mutations in both animals and plants. Specifically, the abundance of birds went down by more than 66%, while communities of bumblebees, butterflies, spiders, grasshoppers, dragonflies also declined. Møller also found that migratory birds (barn swallows) in the area have smaller brains and lower IQs, whereas cataracts, albinism, tumors and other mutations have been recorded.
Gathering lost information
Immediately after the accident, the official organisations pushed the then Soviet Union to conduct measurements all over its territory. The same was done in all European countries, in the frame of a program called Radioactivity Environmental Monitoring.
– About 500,000 observations of deposition of Cesium-137 were gathered and interpolated on a map known as Atlas, says Evangeliou. – About 300,000 of these were from Former Soviet Union countries. However, the data are not accessible even nowadays. Only 5,000 measurements from Europe are available online for download, and some may have been entirely lost.
Still, with help from contacts made while working in Chernobyl, and with additional help from Møller’s group, Evangeliou has managed to enrich this database to 12,000 observations. Most of the new additions are from Ukraine, Belarus and Russia.
– The data gives a deposition similar to Atlas, which proves the quality of the database. All measurements and the gridded map are available through the website http://radio.nilu.no, and the whole study will soon be published in Environmental Pollution, Evangeliou says.
The high-resolution gridded map that Evangeliou and his colleagues have made to accompany the interpolated database can be used for any kind of assessments for human and non-human biota.
For instance, the map can be combined with ecological modelling tools to identify the impact on animal and plant populations remotely, without the need of additional measurements in these highly contaminated regions.
Calculating backwards
– At the time of the accident, the estimation of how much radioactive material was released from the accident was based on core inventory calculations from the damaged reactor, explains Evangeliou. – This is the reason that the releases from Chernobyl are associated with at least a 50% uncertainty.
However, nowadays, the increased computational resources have made the development of inverse modelling tools possible.
Inverse means going to the opposite direction. In the modelling context, it is the process of calculating from a set of observations the causal factors that produced them. In the case of Chernobyl, terrestrial observations are used to calculate how much radioactive material was emitted in the atmosphere when the accident took place.
– Such tools can identify source-terms quantitatively, and with smaller uncertainties, says Evangeliou. – We currently use this updated dataset for the source identification of Chernobyl, and it shows very interesting results.
Fire as a main threat
What made the greatest impression on Evangeliou when visiting Chernobyl in 2013 was how lack of human activity has changed the landscape. Tree-cover was about 53% before the disaster, but it is more than 70% nowadays, and the rest is agricultural land.
This wild growth could also affect the area in the future. According to Evangeliou’s previous research in France, climate change is expected to increase temperature in the CEZ. It will also reduce precipitation until 2100, and the risk for drought is significant even today.
– This will increase the accumulation of carbon litter in the area and the burned area along the mid-latitudes. Hence, the radioactive forests of Chernobyl will become more vulnerable to forest fires, says Evangeliou. – Forest fires near Chernobyl produce radioactive smoke. Because all organic material in contaminated areas contain radioactive material, any fire will disperse radionuclides in a way that will depend on surface heat flux released by the fire, prevailing wind and patterns of rainfall causing deposition of radioactive material elsewhere.
Scientists have also found that using firewood for cooking and heating in private houses causes considerable levels of radioactivity indoors. Likewise, burning of dried potato stalks during fall causes the levels of radiation in the fields to rise many times higher than radiation levels due to contamination from the original catastrophe in 1986. Thus, people still living in the areas around Chernobyl need to be educated, so that they can make better choices, concludes Evangeliou.
