Factors influencing the increased Sr radioisotope migration in highly alkaline groundwater at Chornobyl NPP site.

This paper analyses the formation of high Sr concentration in strong alkaline (pH = 9.5-12.5) groundwater using data of the 27 years of observations around the destroyed Chernobyl NPP Unit 4. It appears that the formation of strong alkaline groundwater in different monitoring wells is consistent with the distribution of Sr, pH and main ions. The reason for the increase in Sr concentrations is the process of its migration from the sources of contamination - leaks of highly active water localized in certain premises inside the «Shelter » object. These computational experiments showed that for the groundwater in pH range 9.5-12.4, the concentration of strontium in the form of SrOH  increases and in the form of Sr - decreases. In addition, the fraction of Sr in the form of a soluble neutral complex compound SrCO, which is not sorbed, reaches 14-35%. Increased fractions of Sr in forms of SrOH and SrCO are factors which reduce the isotope ability to be sorbed by soils and therefore increase its migration ability. In strongly alkaline groundwater a sharp increase in Sr volumetric activity may also be caused by ionic strength (IS) increase above 5 mmol/L. Thus, the factors that influence the increase in Sr volumetric activity in strongly alkaline groundwater are the formation of its complex compounds and an increase in ionic strength (IS), which reduces the thickness of the double electric layer and, as a result, reduces the sorption capacity of soils.