ISSN 00405795, Theoretical Foundations of Chemical Engineering, 2010, Vol. 44, No. 4, pp. 592–599. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.A. Avramenko, S.Yu. Bratskaya, A.V. Voit, V.G. Dobrzhanskiy, A.M. Egorin, P.A. Zadorozhniy, V.Yu. Mayorov, V.I. Sergienko, 2009, published in Khimicheskaya Tekhnologiya, 2009, Vol. 10, No. 5, pp. 307–416.
RARE, DISPERSED, AND RADIOACTIVE ELEMENT CHEMISTRY AND TECHNOLOGY
Implementation of the ContinuousFlow Hydrothermal Technology of the Treatment of Concentrated Liquid Radioactive Wastesat Nuclear Power Plants V. A. Avramenko, S. Yu. Bratskaya, A. V. Voit, V. G. Dobrzhanskiy, A. M. Egorin, P. A. Zadorozhniy, V. Yu. Mayorov, and V. I. Sergienko Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia email:
[email protected] Received May 28, 2008
Abstract—In this work, consideration is given to the feasibility of using hydrothermal oxidation for the destruction of organic 60Co complexes during the course of the treatment of mediumlevel liquid radioactive wastes with a high salt content—evaporator concentrate in the reactor water cleanup system—formed at nuclear power plants (NPPs). It has been shown that hydrothermal oxidation makes it possible to effectively solve the problem of the selective extraction of the radionuclides of transition metals (60Co, 54Mn) with a minimum volume of solid radioactive wastes being formed. The results of laboratory experiments and pilot tests of the hydrothermal oxidation installation at the Novovoronezhskaya and the Kurskaya NPPs are pre sented. The general scheme of the hydrothermal technology of processing the evaporator concentrate at nuclear power plants is proposed. Key words: hydrothermal technology, sorption, liquid radioactive wastes, environmental safety. DOI: 10.1134/S0040579510040421
INTRODUCTION The selective decontamination of the evaporator concentrate in the reactor water cleanup systems at nuclear power plants offers a significant advantage over the nonselective methods of the solidification of liquid radioactive wastes (cementation, bituminiza tion, and vitrification) that are currently used at nuclear power plants. This advantage is associated, above all, with a decrease in the volumes of solid radio active wastes transported into storage. The selective decontamination of the evaporator concentrate from radionuclides was used for the first time at the Loviisa NPP (Finland) [1], but the quality of decontamina tion did not allow for the decontaminated evaporator concentrate to be considered as nonradioactive. The only technology currently implemented that makes it possible to completely decontaminate the evaporator concentrate according to the requirements of radia tion safety standards is the technology of the ion selective decontamination of evaporator concentrate [2, 3] implemented at the Kol’skaya NPP. However, there are a number of restrictions for this technology related, above all, to the stage of ozone treatment that is used in the technology for oxidizing the organic component of evaporator concentrate. These restric tions are caused by the very high total content of vari ous organic compounds in the evaporator concentrate (oxidation susceptibility of the evaporator concentrate
of up to 50 g of O2 per dm3) and the very low rate of the oxidation of Me–EDTA complexes by ozone [4]. The use of hydrothermal, including supercritical, methods for solving problems related to the processing of radioactive wastes was discussed long ago [5]. Actual works in this direction began in the 1990s in Los Alamos [6], when oxidation by supercritical water was used for solving problems of the handling of radio active wastes. The most considerable achievements in this direction have been made in oxidizing organic extractive agents contaminated by radionuclides [7, 8]. In addition, hydrothermal methods were used for oxidizing organic wastes (mainly radiationcontami nated spent ionexchange resins) and other organic wastes under subcritical conditions [9]. At the same time, experience in processing the evaporator concen trate at nuclear power plants is very limited [10]. The aim of the present work is to develop a new hydrothermal method of evaporator concentrate pro cessing that eliminates the stage of ozone treatment of evaporator concentrate, and to test this technology for the processing of real evaporator concentrate at nuclear power plants equipped with reactors of various types. EXPERIMENTAL The thermolysis and oxidation of ethylene diamine tetraacetic acid (EDTA), its salts, and EDTA com
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From the cold water supply system
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Fig. 1. Schematic diagram of the laboratory facility used for hydrothermal oxidation: DV1 is the discharge vessel containing the simulated solution; DV2 is the vessel with the oxidant solution; GF1 and GF2 are gauze filters; HP1 and HP2 are dosing high pressure pumps; NRV1 and NRV2 are nonreturn valves; HR is the heat regulator; AVR is the automatic voltage regulator; PT is the singlephase power transformer; RHO is the reactor for hydrothermal oxidation; WCO is the water cooler; TV is the throttling valve; RV is the receiving vessel.
plexes with transition metals were carried out at a con tinuousflow hydrothermal oxidation installation consisting of two chromatographic highpressure pumps and one flowtype reactor with a volume of 10 ml heated by an electric furnace. A schematic dia gram of the installation is shown in Fig. 1. Solutions containing organic complexes of transi tion metals and corrosive radionuclides were fed by chromatographic highpressure pumps (Shimadzu LC–20) into the reactor, whose temperature is con trolled by means of a thermocouple placed immedi ately at the outflow of the solution from the reactor. Depending on the problems to be solved, in this work, reactors composed of stainless steel, titanium alloy, or stainless steel impregnated by Teflon were used. After the reactor, the solution was fed into the heat exchanger and tapped off into the collector through a throttling valve (Swagelok #SS6R3AMM). The rate of feed of the solution and the oxidant was controlled in the range from 0.02 to 10 ml/min, and at the reactor volume equal to 10 ml, the average time of residence of the solutions in the reactor varied from 30 s to 250 min. The analysis of the solutions was carried out by means of capillary electrophoresis (the capillary elec trophoresis system Agilent CE).
In this work, 0.01N solutions of disodium salt of EDTA (Trilon B) of analytically pure grade and 6% solutions of hydrogen peroxide of chemically pure grade were used. In the studies of the process of liquid phase oxidation, the reactor temperature and the rate of delivery of the components were changed. The content of heavy metal ions in the solution was determined by means of a Solaar AA 6M atomic absorption spectrophotometer. In order to avoid errors caused by the presence of nanosized particles of the oxides in the solution, and, as a consequence, by the slow atomization of the components of the solution, the solutions being analyzed were boiled beforehand together with a mixture of nitric and hydrochloric acids (3 : 1) for 1 h. The activity of the solutions containing radionu clides 57Co, 60Co, and 137Cs was determined by means of an Aspekt 1S gamma spectrometer with a NaI(T1) detector of 63 × 63 mm. The sizes of the oxide particles formed during the course of the destruction of the complexes and their ζpotentials were determined by means of a device for measuring the size and electrophoretic potential of the particles by the methods of photoncorrelation spec troscopy and laser Doppler electrophoresis (Zetasizer Nano ZS, Malvern).
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Table 1. Chemical and radiochemical composition of the evaporator concentrate at nuclear power plants [13] Typical values of parameters for various nuclear power plants Parameters NPP with VVER400 reactors NPP with VVER1000 reactors NPP with RBMK reactors pH NH3, kg/m3 H3BO3, kg/m3 Na, kg/m3 NO3, kg/m3 Susceptibility to oxidation, kg O2/m3 Specific activity, GBq/m3 Activity of 137Cs, GBq/m3 Activity of 60Co, GBq/m3
11.5–13.5 – 920–200 40–150 10–60 10–20 1–10 1–10 0.1–1.0
Sorption experiments were carried out in the static mode in accordance with the conditions recommended in [11]. In the work, commercially available chelating sor bents (Purolite S920 (thiouron functional groups), Amberlite IRC–718 (iminodiacetate functional groups), and Duolite C467 (aminophosphate functional groups)), as well as an experimental specimen of a sorbent selective as to the chalcophile element, were used; the method of obtaining this sorbent is described in [12]. A description of the pilot facility used for the hydro thermal processing of the evaporator concentrate at nuclear power plants is given in the next section. RESULTS AND DISCUSSION The evaporator concentrate in the reactor water cleanup systems of nuclear power plants is highly con centrated brine (with a salt content ranging from 150 to 400 g/l), in which all reagents used for nuclear energy production are concentrated. The chemical and radiochemical composition of the evaporator concentrate at nuclear power plants equipped with VVER and RBMK reactors is given in Table 1. The evaporator concentrates at power units equipped with reactors of the RBMK type differ in that they do not contain compounds of boric acid, and sodium nitrate is their main component. Another essential distinction of the evaporator concentrates at power units with RBMK reactors is either a very small Table 2. Comparison between the coefficients of the distribu tion of cobalt from simulated solutions containing EDTA by various sorbents Sorbent
Kd
Purolite S920 Amberlite IRC718 Duolite C467 FSH
2.1 1.4 1.5 3.2
11.5–13.5 – 80–200 40–200 20–170 10–40 1–10 1–10 0.1–1
11.5–12.5 0.01–0.2 – 60–90 130–240 10–50 1–10 0.1–5.0 0.01–0.5
content or a lack of salts and complexes of EDTA that are used for flushing the steam generators in pressuri zed water reactors. The radiochemical composition of the evaporator concentrate held for a sufficiently long time is deter mined by radionuclides 137Cs and 60Co. For decon tamination of the evaporator concentrate from cesium radionuclides, there is a sufficient number of selective sorbents based on ferrocyanide that ensure decontam ination to the level of activity required by radiation safety standards [14]. At the same time, for the cobalt radionuclides present in the evaporator concentrate in the form of complexes with organic ligands (oxalic acid for the evaporator concentrate in RBMK reactors and EDTA for the evaporator concentrate in VVER reactors), the situation is more complicated. Table 2 shows the data on the decontamination of the simu lated solutions containing ETDA from cobalt radio nuclides by means of various sorbents selective in regards to transition metals. It can be seen that the problem of the sorption extraction of cobalt from the evaporator concentrate containing its complexes is unlikely to be handled effi ciently. On one hand, the analysis of numerous attempts to use sorption methods for the extraction of Co–EDTA complexes, for example, [15–17], dem onstrates that in regards to the actual evaporator con centrates containing, in addition to anionic Co– EDTA complexes, enormous amount of anions of nonradioactive salts and organic anions (for example, surfactants), the sorption methods have shown very little promise. On the other hand, it is well known that EDTA itself and its components with transition metals are extremely labile at high temperatures [18, 19]. Figure 2 illustrates the hydrothermal destruction of EDTA complexes directly in the solution and upon their oxidation by hydrogen peroxide. As a result of thermolysis and oxidation destruc tion, oxides of cobalt and iron in the form of Me2O3 or Me3O4 are formed, depending on the conditions of
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0.6 0.4 0.2
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2.5
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595
1000 100 2
10 1
1 0.1 0.01 200
250
300 350 400 Temperature, °C
450
500
Fig. 3. A plot of the dependence of the concentration of cobalt and iron on the thermolysis of Me–EDTA com plexes at various temperatures: (1) determination of con centration directly in solution; (2) the same, after boiling with HNO3 + HCl.
Fig. 2. Thermolysis and oxidation of Fe–EDTA com plexes by hydrogen peroxide at 250°C; the average resi dence time of the solution in the reactor is 13 min: (1) total oxidizability of solution, and (2) EDTA–Fe content.
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Concentration of Fe and Co, mg/ml
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2000 4000 6000 8000 nM After 360 min
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Fig. 4. Image of magnetic particles at various stages of the oxidation of Co–EDTA complexes obtained by means of an atomic force microscope.
thermodestruction and oxidation. It can be seen that the destruction of complexes takes place at sufficiently high temperatures without the introduction of oxi dants too; however, as is shown in Fig. 3, a rather con siderable amount of cobalt still remains in the solu tion. The reasons for this are, most probably, both the formation of complexes of radionuclides by thermoly sis products and the stabilization of the formed nano sized particles of oxides by thermolysis products; in this case, special filtering of the solution is required. At the same time, during the hydrothermal oxidation of Co–EDTA complexes, because of the full oxidation of the destruction products, it is possible to attain a high degree of solution decontamination from cobalt radi onuclides. An even higher effect can be attained dur ing the hydrothermal oxidation of Co–EDTA and Fe–EDTA solutions with their concurrent filtration
through the magnetite bed under hydrothermal condi tions. In this case, the growth of crystals of mixed oxides of iron and cobalt with a spinel structure at the granules of the filter bed takes place (Figs. 4, 5). In [20], it was noted that the magnetite precipita tion upon the thermal destruction of EDTA com plexes is much more active on the metal–solution interface than in the volume of solution. One of rea sons for this is the formation of micro and nanosized particles of metal oxides stabilized by products of EDTA decomposition. Figure 6 shows the distribution of particles formed upon the continuousflow ther molysis of EDTA complexes of iron and cobalt, and the average size of the particles at various solution flow rates in the reactor. At a low solution flow rate, not only the rather full destruction of EDTA complexes, but also the oxida tion of the destruction products take place, and this
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Intensity
596 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1
2 5
10
20
30
40 2Θ, °
50
60
70
Fig. 5. Xray diffraction patterns of the oxides resulting from the hydrothermal oxidation of EDTA complexes of cobalt (1) and iron (2).
results in the full crystallization of oxides on the sur face of the filtering bed (and, correspondingly, in high coefficients of decontamination from 57Co radionu clides (Fig. 7, curves 1, 2)). At the same time, with an increase in the flow rate through the filtering bed, oxides are formed in the vol ume of the solution, and thereafter they are partially filtered on the bed of the oxides (without crystalliza tion on the surface of the latter). After the passing of a certain amount of the solution under such conditions,
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Activity, Bq/l
Stable particles of cobalt oxide, %
the accumulated particles of oxides are washed away by the flow in the form of a suspension, and this results in a sharp increase in the activity of a solution contain ing such a suspension (see Fig. 7, curves 3, 4). The minimum filtration rate at which the accumu lation of oxides from the volume of the solution pointed out above takes place (“critical rate”) depends on the temperature of the thermal oxidation (Fig. 8). The considerable advantage of hydrothermal oxi dation, as compared to other methods of oxidation
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Fig. 6. Distribution of particles of cobalt oxide that are sta ble in the solution and that were obtained during the course of thermolysis of Co–EDTA complexes (1, 2) and their oxidation by 0.1% hydrogen peroxide (3); (1) the temper ature is 340°C, and average residence time in the reactor is 6.5 min; (2, 3) the temperature is 250°C, and average resi dence time in the reactor is 30 min.
101 0
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200 300 Volume, ml
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Fig. 7. Decontamination from 57Co during the hydrother mal oxidation of the mixture of Co–EDTA and Fe–EDTA complexes by a 1% solution of hydrogen peroxide during filtration through a magnetite bed and a temperature of 250°C and various flow rates, ml/min: (1) 1.0; (2) 2.0; (3) 4.0; (4) 6.0; and (5) activity of the initial solution.
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Flow rate, ml/min
2.4 2.2 2.0 1.8 1.6 1.4 0 190
200
210
220 230 240 Temperature, °C
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Fig. 8. Dependence of the critical flow rate on the temper ature.
9 Relative volume of deposit, %
destruction, is the very small volume of the deposit produced, since, in this case, crystalline oxides are formed, as opposed to the amorphous hydroxides formed upon the oxidation of complexes at a low tem perature. In addition, a very important value is the high rate of destruction of Co–EDTA complexes, which makes it possible for researchers to limit them selves to a small size of continuousflow hydrothermal facilities so as to provide the proper productivity of the industrial process. Figure 9 gives data on the oxidation time and the amount of deposit being formed for vari ous methods of the oxidation of 60Co EDTA in the evaporator concentrate at nuclear power plants. Experiments on ozonolysis, electrochemical, and hydrothermal oxidation were carried out by the present authors [21], while data on the oxidation by an electric arc were taken from published works [22]. A comparison between various methods of oxida tion suggests that the hydrothermal method of the destruction of 60Co–EDTA complexes is rather prom ising. In particular, the rate of hydrothermal oxidation is three orders of magnitude higher than the ozonation rate, while the amount of deposit formed upon the hydrothermal oxidation is one order of magnitude less than that of the deposit produced by ozonation. Oxide ceramics formed as a result of hydrothermal oxidation are characterized by a very low leachability of cobalt radionuclides (less than 10–6 g/(cm2 day)), and this is very important for ensuring the environ mental safety of the longterm storage of radioactive wastes. Bench tests of the hydrothermal technology of cleaning the evaporator concentrate at nuclear power plants were carried out at power units nos. 1 and 2 of the Novovoronezhskaya NPP in 2006, and at the first construction stage of the Kurskaya NPP in 2007. The capacity of the facilities designed for the approbation of hydrothermal technology reached 15 dm3/h. Their schematic diagram is shown in Fig. 10. The evaporator concentrate is fed to the prelimi nary ferrocyanide filter by means of a pump fitted with a mechanical filter at the inlet. The evaporator con centrate decontaminated from cesium radionuclides by means of a highpressure pump is fed successively through the heat recuperator and the electric heater to the reactor with a volume of 1 dm3 partially filled with a filtering/catalytic material. The same pump feeds hydrogen peroxide into the reactor. The temperature in the reactor is controlled in the range from 150 to 250°C. As a result of the hydrothermal oxidation, oxides of iron and other metals contained in the evap orator concentrate are crystallized on the reactor load. As this occurs, the efficient (up to 104) decontamina tion of the evaporator concentrate from radionuclides of transition metals (mainly from 60Co) takes place. The decontaminated evaporator concentrate passes through the recuperator and the heat exchanger and is introduced through the throttling device, maintaining a pressure of 100 atm in the receiving vessel from
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Time, min Fig. 9. Comparison between various methods of the oxida tion of the evaporator concentrate at nuclear power plants in regards to the rate of formation and volume of the deposit being formed: (1) oxidation of 15 mg of O3/l at the temperature, °C, of (1) 20 and (2) 60; (3) electrochemical oxidation of the platinum anode; (4) oxidation by electric arc; and (5) hydrothermal oxidation by hydrogen peroxide on magnetite at 250°C.
which it is fed by the pump to the ferrocyanide filter for the final decontamination from cesium radionuclides. The implementation of hydrothermal oxidation makes it possible to employ the preliminary decon tamination of the evaporator concentrate from cesium radionuclides with the use of ferrocyanide sorbents, since the further hydrothermal oxidation destroys the ferrocyanide colloids that are formed upon the sorp tion in the alkali medium and allows for the perform ing of the advanced decontamination of the evapora tor concentrate on ferrocyanide sorbents as well. Such a process would be impossible without hydrothermal oxidation. However, the use of the twostage scheme of decontamination of the evaporator concentrate from cesium radionuclides allows one to extend by
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1 3 Initial liquid radioactive waste (137Cs + 60 Co)
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LRW (137Cs
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+ 60Co)
Power source N = 5 kVA
Fig. 10. Schematic diagram of the plant for the thermal processing of the evaporator concentrate at nuclear power plants: (1) vessel with initial solution; (2) pump for the supply of liquid radioactive waste; (3) filter for 137Cs; (4) doubleplunger pump; (5) vessel with the oxidant; (6) pressure surge compensator; (7) electronic manome ter; (8) contact pressure gage; (9) electric regulator of the heating of the solution and the pressure in the system; (10) reactors; (11) electric furnaces; and (12) LRWto LRW heat exchanger.
several times the effective service life of ferrocyanide sorbents and increase the safety of the hydrothermal process. Bench tests demonstrated, the following:
—the feasibility of using various oxides as catalysts for the oxidation process; —the impact of the temperature and concentra tion of hydrogen peroxide on the quality of evaporator concentrate decontamination; —the degree of radiation contamination of various parts of the equipment; and —the feasibility of the advanced decontamination of the evaporator concentrate from cesium radionu clides after its hydrothermal processing. In Tables 3 and 4, the results of the hydrothermal oxidation of the evaporator concentrate on benches with a capacity of up to 15 l/h are shown, with the full decontamination of the evaporator concentrate from radionuclides of cesium, cobalt, and manganese. It can be seen that the implementation of the hydro thermal oxidation of the evaporator concentrate will allow for the performance of the infallible decontami nation of the evaporator concentrate from main long lived radionuclides of cesium and cobalt in such a way that the total activity of the radionuclides contained in the evaporator concentrate will not exceed the radia tion safety standards. In accordance with the test results, a basic diagram of the hydrothermal processing of the evaporator con centrate was developed (Fig. 11). The use of the cementation of the decontaminated evaporator concentrate (nonradioactive industrial wastes) will make it possible to reduce the environ
Table 3. Processing of the evaporator concentrate in reactors of the VVER type (power units nos. 1 and 2 of the Novovor onezhskaya NPP) Activity of decontami Reactor Flow rate of Hydrogen Activity of evaporator Activity of evaporator nated evaporator con temperature, evaporator peroxide concentrate on filter 1 as concentrate on the filter 60 centrate as to Co, concentrate, dm3/h concentration, % to 137Cs, Bq/dm3 2 as to 137Cs, Bq/dm3 °C Bq/dm3 250 200 190 210
5 10 15 7
2,5 1.4 1.5 2,5
