Speciation of Radionuclides in the Bottom Sediments of Room 012/7 of the NSC – Shelter Object
Authors:   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Odintsov O. O. , Palamar L. A. ,   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Chikur L. B. ,   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine
Published: February 2022

After installation of the Arch of New Safe Confinement (NSC) into design position, precipitations intake in the underroof space of the Shelter object ceased, and water evaporation began from the unorganized accumulations of radioactively contaminated water. As a result of physical and chemical processes of coagulation and deposition of sparingly soluble salts, bottom sediments, containing uranium and radionuclides, formed. The aim of the work is to determine speciation forms of uranium and radionuclides in the bottom sediments, which formed due to the drying out of unorganized accumulations of radioactively contaminated water in room 012/7 of the NSC – Shelter object. Results of the experimental determination of speciation forms of uranium and radionuclides 90Sr, 137Cs, 154Eu, 238Pu, 239+240Pu, 241Am and 244Cm in the bottom sediments of room 012/7 of the NSC – Shelter object are presented. An amount of water-soluble, exchangeable and acid-soluble uranium forms, fission products and transuranium elements in the bottom sediments of room 012/7 of the NSC – Shelter object was determined by the method of consecutive extractions. Content of uranium, 90Sr, 238Pu, 239+240Pu, 241Am and 244Cm in the leaching solutions and insoluble residue was determined by the ion-exchange method. 90Sr activity was determined by beta-radiometric measurements. Activity of 234,235,236,238U, 238Pu, 239+240Pu, 241Am and 244Cm was determined by alpha-spectrometric measurements of uranium, plutonium and americium sources. The concentration of uranium in the bottom sediments of room 012/7 was 1.02 ± 0.26 mg/g. Abundance content of uranium isotopes is 234U – 0.0159, 235U – 1.09, 236U – 0.192 and 238U – 98.71%. Isotope content of uranium in the bottom sediments, on the whole, corresponds to the isotope content of uranium in the irradiated fuel of the Chornobyl NPP 4th unit with an average burnup. The specific activity of 90Sr and 137Cs in the bottom sediments of room 012/7 is in the range of 6∙108−1∙109 Bq/kg. 239+240Pu and 241Am specific activity in the bottom sediments is in the range of 6∙105 – 8∙106 Bq/kg. 90Sr, 137Cs, 154Eu, 238Pu, 239+240Pu, 241Am and 244Сm radionuclides in the bottom sediments are in different physical and chemical forms, which determine their potential mobility. Mainly uranium and 137Cs in the bottom sediments of room 012/7 are in the exchangeable forms. An amount of water-soluble forms of uranium and 137Cs is about 1.5 – 3%. The main amount of 90Sr, more than 60% is in the carbonate forms, which are very soluble in weak acid medium, when pH is 4.8. More than 65% of 238Pu and 239+240Pu in the bottom sediments of room 012/7 are in the acid-soluble forms. Potential mobility of 241Am in the bottom sediments is notably higher than of plutonium in the soluble state, when pH 4.8, transfer more than 40% of 241Am. Relations of activities of radionuclides 137Cs/90Sr, 90Sr/239+240Pu, 241Am/239+240Pu and 44Cm/239+240Pu in the bottom sediments of room 012/7 differ significantly from the similar relations in the fuel containing materials of the Shelter object.