Radiation stability of dibenzo-21-crown-7 used for solvent extraction of cesium-137

Crown ether Dibenzo-21-crown-7 (DB-7) has demonstrated a high capability for removing cesium-137 both from acidic and alkaline solution. However, there is a very important problem concerning changes in physical and chemical properties of such mixtures as a result of high irradiation. High radiation doses may cause a decrease in extraction efficiency and selectivity, formation of hazardous radiolysis products, disturbance hydrodynamics, and even decay of extractant. It is necessary to estimate such changes before extraction testing on hot solutions. A special procedure, earlier developed in the RRICT (Russian Ministry for Atomic Energy), was used for testing of radiation and chemical stability of extraction mixtures. This method is based upon continuous control of extraction parameters--distribution coefficients of the specified radionuclides, their hold-up in organic phase, of hydrodynamic properties--kinetics of separation of mixed phases, surface tension, etc., and formation of radiolysis products. The dose rate, determined using a ferrosulfate dosimeter was 1.7 mltimess/l (1.05times1016 eV/mltimess) taking into account the electronic density of organic solution. Maximum total dose of gamma-irradiation was 80 Wtimesh/l (1.6times1021 eV/ml). The specimens of irradiated organic solution--DB-7 in the mixture of fluorinated alcohol (FA) and higher alcohol--after phase separation were analyzed to determine the hydrodynamic parameters. Both organic and aqueous phases were analyzed to estimate the decay of extractant and the composition of radiolysis products. The primary emphasis for the first radiolytic experiments was given to the estimation of the FA decay by means of measuring the fluoride-ion concentration in the aqueous phase. It was shown that this value increases through the dose of 60 Wtimesh/l and then decreases again. Such behavior may be attributed to an interaction of the fluoride ion with nitric acid and formation of volatile or organic phase extractable compounds. Tests performed on real HLW acid solutions in P/A Mayak also demonstrated that solutions of DB-7 in fluorinated alcohols have a high resistance to radiolysis