Selective removal of radionuclides from the bottoms residue of evaporation equipment used in nuclear power plants has an enormous advantage over the conventional methods currently being used to condition liquid radwastes (cementing, bituminization). The advantage is primarily due to the decrease in the volume of the conditioned wastes put into solid radwaste repositories. Selective purification was first used at the Lovisa nuclear power plant (Finland) [1], but the quality of this operation was too low to consider the purified bottoms residue as being nonradioactive. The removal of radionuclides of corrosion origin, primarily cobalt, remained unsolved. The only currently used technology for completely cleaning bottoms reside to NRB-99 requirements is ion-selective purification used at the Kola nuclear power plant [2]. However, it has limitations associated with ozonization, which is used to oxidize the organic component.The objectives of the present work were to develop a new technology for reprocessing bottoms residue, which does not use ozonization, and to test it on real wastes from nuclear power plants with different types of reactors.The main purpose of oxidation is to destroy the intracomplex compounds of radionuclides of corrosion origin, first and foremost, 60 Co, which form strong complexes with some organic ligands. For example, the stability constant of the 60 Co complex with ethylenediaminetetraacetate (used to decontaminate and wash equipment) [Co(III)-EDTA] -is 10 43 . The formation rate of this complex is 10 4 slower than that of free EDTA molecules [3]. This leads to substantial overconsumption of ozone during the oxidation of the bottoms residue, making ozonization ineffective. Increasing the oxidation temperature decreases the time required to destroy the organic complexes [4,5], so that the processes proceeding at a temperature much higher than the boiling temperature of the bottoms residue at an appropriate pressure (hydrothermal processes) can be used to solve the problem of oxidation more quickly using hydrogen peroxide. Comparing different oxidation technologies shows that the hydrothermal method of destroying 60 Co-EDTA complexes has promise (Fig. 1). Specifically, the rate of hydrothermal oxidation is more than 1000 times higher than that of ozonization and the volume of the residue formed with hydrothermal oxidation of the residue is 10 times smaller than for ozonization. The latter effect is due the fact that crystalline oxides of heavy metals, first and foremost, iron, whose density is higher than that of hydroxides of the same metals released at low temperature, are formed during hydrothermal oxidation.The oxide ceramic formed as a result of hydrothermal oxidation is characterized by low leaching of cobalt (less than 10 -6 g/(cm 2 ·day)), which is an important indicator of the ecological safety of long-time storage of the radioactive wastes.Bench testing of the hydrothermal technological for purifying bottoms residue formed in nuclear power plants was performed for the first two units...
A novel technology was developed for treatment of evaporator concentrates produced as a result of operation of evaporation devices comprising the main component of special water purification systems of nuclear power plants (NPP). The developed technology includes a hydrothermal (T = 250–300°C and P = 80–120 bar) processing of evaporator concentrates in oxidation medium in order to destruct stable organic complexes of cobalt radionuclides and remove these radionuclides by oxide materials formed during such a processing. The cesium radionuclides contained in evaporator concentrates are removed by a conventional method — through application of one of the developed composite sorbents with ferrocyanides of transition metals used as active agents. Extensive laboratory studies of the processes occurring in evaporator concentrates under hydrothermal conditions were performed. It was shown that hydrothermal oxidation of evaporator concentrates has a number of advantages as compared to traditional oxidation methods (ozonation, photocatalytic, electrochemical and plasma oxidation). A laboratory installation was built for the flow-type hydrothermal oxidation of NPP evaporator concentrates. The obtained experimental results showed good prospects for the developed method application. On the basis of the results obtained, a pilot installation of productivity up to 15 l/hour was developed and built in order to work out the technology of evaporator concentrates hydrothermal treatment. The pilot tests of the hydrothermal technology for evaporator concentrates hydrothermal treatment were performed for 6 months in 2006 at the 1st reactor unit of the Novovoronezhskaya NPP (Voronezh Region, Russia). Optimal technological regimes were determined, and estimations of the economic soundness of the technology were made. The advantages of the presented technology in terms of management of concentrated liquid radioactive wastes (LRW) at nuclear cycle facilities, as compared to other methods applicable for this type of LRW, were demonstrated. Application of the hydrothermal technology in the system of NPP LRW management enables one to reduce substantially the volume of solid radioactive waste sent for final disposal.
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