Preparation of analogs of minerals pollucite and sphene by metallothermal self-propagating high-temperature synthesis (SHS) was studied with model systems. These analogs are intended for immobilizing silica gel fixed cesium and strontium radionuclides, respectively. The optimal conditions for SHS were found, and the phase compositions of the reaction products were studied. In synthesis of pollucite, the energy-forming components are Al and Fe2O3, and in synthesis of sphene, Ti and MoO3. The mineral analogs synthesized are distinguished by high hardness and chemical stability. Virtually no cesium is lost by evaporation.

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 W*h/l (1.05*10 16 eV/ml*s) taking into account the electronic density of organic solution. Maximum total dose of gamma-irradiation was 80 W*h/l (1.6* 10 21 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 W*h/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. Our 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.

Radiation and thermal loads on inorganic adsorbents for immobilization of high-level wastes (HLW) and transuranium elements (TUEs) are estimated to predict alterations of physicochemical characteristics of solid matrices and also the behavior of radionuclides during their long-term storage.

It was demonstrated experimentally that the major physicochemical processes occurring in the course of long-term storage of long-lived radionuclides of high-level wastes immobilized on silica in closed tanks are radiation decomposition of adsorbed water and accumulation of new adsorption centers. The former process results in hydrogen evolution into free volume of the tank, increasing the pressure in it, and the latter process, in adsorption of gases from the free volume, decreasing the pressure. The limiting composition of the gas mixture and the limiting total pressure are estimated from the experimental data, and the possibility of self-ignition of a mixture of H-2 and O-2 or of nonchain thermal explosion is evaluated.

An important part of the nuclear fuel cycle management in Russia is solidification of HLW and their safe disposal. Prior to final disposal radioactive waste has to be conditioned and described in detail. Intense research activities have been carried out in recent years at Russian Research Institute for Chemical Technology to develop the method of high temperature adsorption of metals. With this method, radionuclides adsorbed and immobilized in a porous inorganic sorbent, for example, silica gel at temperature 100 - 130{degrees}C followed by calcination of saturated granules at 800 - 1000{degrees}C. Using silica gel for immobilizing radioactive waste offers a wide range of possibilities for different waste streams to meet the requirements of final products. Due to radiolysis of water in silica gel, hydrogen production can not be prevented. In this paper the results of the experimental study of HLW forms behaviors under simulated repository conditions is presented. The experiments performed in this study are directed towards determination of radiation stability of silica gel. It is devoted to investigation the role of ionization damage as well as to acceptability of accelerated testing results for reliable prediction of long-term behavior of final solid material containing beta gamma nuclides. The examinations weremore » supposed to answer the following questions: mechanism of gas-formation; characterization of parameters influencing the rate of gas-formation.« less

The gamma-ray initiated oxidation of dicyclohexano-18-crown-6 (DCH-6) was studied in two-phase system under agitation: nitric acid, aqueous solution and organic solvent. The distribution coefficients of radionuclides, hydrodynamic characteristics (phase separation and interface surface tension), and the radiolysis products after irradiation at a dose rate (60Co) of 1.7 Gy/s were determined. The maximum total dose was 3.0×105 Gy (3.0×107R).