Changes of radiohydroecological conditions under influence of the "Arch" Installation over the Shelter Object of the Chornobyl NPP

To protect the environment from the impact of the 4th Chornobyl emergency block, a new safe confinement (NSC, hereinafter referred to as "Arch") was constructed and installed in the design position in November 2016. A characteristic feature of the water regime of groundwater and block water at the site of the design position of the "Arch" is the termination of their water supply due to the absence of precipitation. The cessation of atmospheric precipitation infiltration led to a reduction of 2 to 4 times the concentrations of the main ions and the amount of mineralization in the upper part of the aquifer. In the block waters of the "Shelter" object (SO), the concentration of the main ions and mineralization increased by 1.7–8 times. Some water clusters in the premises of the SO have dried up, which has led to a significant increase of a gamma radiation Equivalent Dose. One of the mechanisms for enhancing the migration capacity of radionuclides in groundwater is associated with the formation of a highly alkaline environment with a ph greater than 9.5. High ph values are formed directly upon contact of groundwater with concrete of  the foundations of  structures or the entry into the aquifer of water masses in contact with concrete. However, despite the cessation of intensive inflow into the aquifer of infiltration and block waters, a significant decrease in the ph and volumetric activities of radionuclides other than tritium in groundwater under "Arch" is not happening yet. Concentrations of tritium in groundwater decreased from 900–1,100 to 30–50 Bq/liter. A change in the direction of movement of groundwater from north to north-east led to the formation of new pathways for the distribution of radionuclides with groundwater. So, in the S-10 well, located northeast of the SO, an increase in the concentration of tritium from 3–5 to 30 Bq/l is observed. Outside the "Arch", by separate observation wells, the mineralization and ion concentration increase is 1.4–5 times, possibly due to the contact of the concrete of the pile foundation of the "Arch" with underground waters. The tritium concentrations in the block waters that associated with the facilities, located under reactor increased of 3 to 5 times. An increase in tritium concentrations may be associated with an increase in the density of neutron fluxes in rooms under reactor after the "Arch" was drawn. Increased mineralization leads to an increase in the solubility of block water, which in turn can lead to an additional transfer of radionuclides from the active precipitate into the water of the precipitating clusters. Increasing the volume activities of radionuclides by evaporation and increasing the solubility of block water can lead to the formation of radioactively contaminated water masses, which at the concentration levels of radionuclides will refer to highly active liquid radioactive waste. (author)