25 years after the Chernobyl power plant explosion: Management of nuclear wastes and radionuclide transfer in the environment

Applied Geochemistry (numéro spécial sur le GNR Trasse) / Volume 27, Numéro 7, Pages 1291-1450

The year 2011 was the celebration of the 25th anniversary of the Chernobyl nuclear accident (26 April, 1986). The explosion of nuclear reactor no. 4 from the RBMK nuclear power plant was the worst industrial nuclear accident and resulted in an unprecedented release of radioactive material from a nuclear reactor and adverse consequences for the public and the environment ([Anspaugh et al., 1988] and [MacKenzie, 2000]). Revised estimates of the principal radionuclides released during the course of the Chernobyl accident (decay corrected to 26 April 1986) indicate that a total amount of almost 13,650 PBq (Peta Bq equivalent to 1015 Bq) was released during the course of the Chernobyl accident (I.A.E.A., 2006) including 6533 PBq of rare gases (mostly of 133Xe), 4260 PBq of I (1760 PBq of 131I), 168 PBq of Cs (85 PBq of 137Cs) and 1400 PBq of Te (129mTe and 132Te).

The consequences of the Chernobyl accident on health, especially cancer, have been widely investigated ([Baverstock et al., 1992], [Kazakov et al., 1992] and [Cardis et al., 2006]). It, however, remains a matter of extremely large uncertainties and debates (Williams and Baverstock, 2006). Unfortunately, 2011 was also the year of the Fukushima Dai-ichi power plant accident related to the earthquake and following tsunami (11th March, 2011). An estimate of the amount of radionuclides released into the atmosphere during the accident at Fukushima Dai-ichi was made by IRSN for the period March 12–24, 2011 (I.R.S.N., 2012) with activities of the same order than for Chernobyl for rare gases (6540 PBq) and lower but still very significant activities for I (409 PBq for total I and 197 PBq for 131I), for Cs (58 PBq for total Cs and 21 PBq for 137Cs) and for Te (144 PBq). Total 137Cs deposition over the Japan Islands and the ocean were estimated to be more than 5.6 and 1 Peta Bq respectively (Yasunari et al., 2011).

Both events are also characterized by severe problems of management of highly radioactive materials and liquids immediately after the accident, while trying to control the reactors. In Fukushima Dai-ichi, large volumes of contaminated water were delivered directly to the ocean (Buesseler et al., 2011). The management of other contaminated materials directly on site or close to the site (soils, groundwater, wastes, etc.) is now one of the main issues to be solved. In the Chernobyl case, the situation remained at an extremely high level of danger for several months. In order to allow the workers (liquidators) to extend their time-period on site and to limit potential atmospheric contamination through forest fire, several procedures were undertaken to mitigate on-site radioactivity (Dzhepo and Skalskyy, 2002). In particular, the authorities tried to remove the most significant radioactive sources. These sources included the radioactive materials generated during emergency operations as well as highly contaminated soils and vegetation. The ‘Red Forest’ (pine trees killed by high radioactive levels), covering about 8 km² close to the site was entirely removed. All these wastes were buried by civil defence troops in shallow trenches or covered by soil mounds. Large amounts of various materials were also buried during the shelter construction. According to several studies, 800 repositories have been identified. Only half of these waste repositories have been investigated and it is extremely difficult to define the total amount of waste which could be of the order of 106 m3 ( [Dzhepo and Skalskyy, 2002] and [Bugai et al., 2005]).

It is clear from this that waste related to nuclear accidents represent a specific issue since it results from a crisis management including emergency decisions that can, however, have environmental and social impacts for several decades. One of the most serious problems related to these “emergency-made” repositories concerns the potential transfer of the radionuclides into the environment i.e. towards the hydrosystems and the biosphere in particular through root uptake likely to capture the radionuclides in soils. The summer 2010 wildfires in Russia constituted a serious alert for potential atmospheric transfer of radionuclides incorporated into vegetation.

In order to better appraise the potential for mobilization of radionuclides in ecosystems, geochemical and transfer studies have, therefore, been undertaken aiming in particular at assessing: (i) the source term of the trenches and transformation processes of radioactive particles and (ii) the flows of radionuclides likely to be released into hydrosystems and biosphere. The complexity of the reactions of dissolution, precipitation and retention in a highly permeable medium rich in organic matter and microorganisms is added to the soil complexity which combines an unsaturated zone and an unconfined aquifer with a very low hydraulic gradient at Chernobyl. Hydrogeological modeling coupled to biogeochemical reactions and radionuclide decay was, therefore, essential to predict the transfer of radionuclides to the environment.

In a fundamentally different context, similar scientific issues where geochemistry plays a particular role arise for the management of High Level and Intermediate Level Long-Lived radioactive wastes (HL–ILLL) in a deep geological repository. Indeed, demonstrating the long term safety of deep geological repositories requires assessing the containment capability of the host rock with respect to the HL–ILLL radioactive wastes over several tens to hundreds of thousands of years. This containment capability relates to the specific physical characteristics of the rock and the physico-chemical characteristics of the interstitial fluids as well as their interaction with the rock. The knowledge of geochemistry of the pore fluids in equilibrium with the minerals of the rock as well as of the diffusion and retention properties of the radionuclides in the rock is thus necessary to assess the containment capability of the host formation.

However, though geological settings may be extremely different, similar transport phenomena and bio-physico-chemical reactions need to be considered to assess transfer of radionuclides in natural media. Such similarity justifies applying comparable scientific concepts and modeling tools in order to better understand the contamination in drastically different environments. This was one of the motivations to create the TRASSE research group to which this special issue of applied geochemistry is devoted.

The TRASSE group (Transfer of Radionuclides towards the Soil, the ground and the Ecosystems) was created in 2008, having pluriannual scientific collaboration between IRSN (French national Institute for Radioprotection and Nuclear Safety) and CNRS (French national centre for scientific research). It also includes UIAR (Ukrainian Institute of Agricultural Radiology) and IGS (Ukrainian Institute of Geological Sciences, which depends on the National Ukrainian Science Academy). The TRASSE research group aims to investigate the transport of radionuclides in contaminated soils and address scientific issues related to the containment capability of rocks with regard to the disposal of nuclear wastes. It has also led to the opening of IRSN experimental platforms to the scientific community, i.e. The Chernobyl Platform and the Tournemire experimental site which are described hereafter.