Ultrapurification of condensates from evaporators in nuclear power plants for removal of radionuclides with a selective inorganic sorbent

Abstract

Discharges of condensates from evaporators (unbalanced waters), produced during reprocessing of liquid radioactive wastes, are one source of input of radioactive isotopes into the enviro~maent during operation of nuclear power plants. Due to the large volume of such waters, approaches considered involve not only decreasing the level of discharged radioactivity but also bringing the radioactivity of the radionuclides removed with the liquid wastes down to the natural background for water. In practice, for decontamination of low-level liquid waste at nuclear power plants, often sorption methods are used for purification using organic ion-exchange resins. However, in some cases such sorbents have insufficient selectivity relative to cesium isotopes, especially when cations of light alkali metals and ammonium are present in the water to be treated. The Kalinin nuclear power plant treats more than 50 thousand m3/year of unbalanced waters with low level radioactivity (lower than the permissible concentration); in this case, the greatest contribution to the total radioactivity comes from long-lived cesium isotopes (the permissible concentration of 13Cs is 555 Bq/dm3). At the present time, introduction of new effective inorganic sorbents having high selectivity relative to cesium isotopes is important. Introduction of a barrier filter containing such selective sorbents into the standard process flow diagram of the nuclear power plant makes it possible to carry out ultrapurification of unbalanced waters relative to long-lived radionuclides, which increases the ecological safety of the nuclear power plant. A progressive technical solution in this respect is to use selective inorganic sorbents of the Termoksid class, in the form of spherical granules. The scientific and technological principles have been developed for the synthesis of such sorbents and they are produced on a commercial scale in the Termoksid facility near the Beloyarsk nuclear power plant. Different types of sorbents with a high level of homogeneity are obtained by the sol-gel method in the form of spherical granules of size 0.4-1 mm and high mechanical strength [1-6]. In this paper, we describe the results of lengthy life tests for the selective sorbent Termoksid-35 with respect to removal of radionuclides from unbalanced waters of the Kalinin nuclear power plant. The chemical composition of the selective ion exchanger Termoksid-35, in the form of spherical granules, is hexacyanoferrate(II) of a transition metal on an inorganic support. Its basic physicochemical characteristics are the following: