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ISSN : 1738-1894(Print)
ISSN : 2288-5471(Online)
Journal of Nuclear Fuel Cycle and Waste Technology Vol.13 No.Special pp.19-27
DOI : https://doi.org/10.7733/jnfcwt.2015.13.S.19

Pyroprocess Experiments at ENEA Laboratories

Giorgio DE ANGELIS1*, Elio BAICCHI2, Mauro CAPONE3, Carlo FEDELI3, Massimo SEPIELLI3, Giuliano TIRANTI3, Mirko DA ROS4, Francesca GIACOBBO4, Marco GIOLA4, Elena MACERATA4, Mario MARIANI4
1Retired from Italian National Agency for New Technology, Energy and Sustainable Economic Development (ENEA), Casaccia Research Center, Via Anguillarese 301, 00123 S. Maria di Galeria, Roma, Italy
2ENEA, Brasimone Research Center, Bacino del Brasimone, 40032 Camugnano (Bologna), Italy
3ENEA, Casaccia Research Center, Via Anguillarese 301, 00123 S. Maria di Galeria, Roma, Italy
4Politecnico di Milano, Department of Energy, Nuclear Engineering Division, Piazza L. da Vinci 32, 20133 Milano, Italy
Corresponding author. Giorgio DE ANGELIS, Retired from Italian National Agency for New Technology, Energy and Sustainable Economic Development (ENEA), deatro@alice.it, +39-063-973-4219
April 21, 2015 August 26, 2015 October 21, 2015

Abstract

A new facility, known as Pyrel III, has been installed at ENEA laboratories for pyrochemical process studies under inactive conditions. It is a pilot plant which allows electrorefining and electroreduction experiments to be conducted on simulated fuel. The main component of the plant is a zirconia crucible. The crucible is heated by a furnace which is supported in an externally water-cooled well under the floor of a steel glove-box, where an argon atmosphere is maintained by a continual purge of about 10 L·min-1. The vessel is loaded with LiCl-KCl eutectic salt (59-41 mol%) and is currently operated at 460 °C. Several improvements on Pyrel II (the previous operating plant) have been introduced into Pyrel III. They are described in detail, together with the results from the first experimental campaign which used lanthanum metal.

Moreover, studies about the treatment of chloride salt wastes from pyroprocesses have been conducted in parallel. They follow two main routes: on one hand, a matrix termed sodalite, a naturally occurring mineral containing chlorine, has been synthesized from a mix of nepheline, simulated exhausted salts and glass frit; on the other hand, a novel method proposed by Korea Atomic Energy Research Institute (KAERI) is under assessment. The final waste forms have been fully characterized with the support of the Politechnique of Milan, by means of density measurements, thermal analysis, and stereomicroscopy observations, FTIR, XRD, and RAMAN spectra, as well as leach tests under static conditions.


초록


    1.Introduction

    Pyrochemistry is a term referred to process reactions at high temperatures. At present it appears as a promising alternative for the separation of actinides fission products from lanthanides, which is ordinarily carried out in eutectic molten chloride salts, such as LiCl-KCl. This method is expected to be very suitable for advanced nuclear engineering due to its many advantages compared with the hydrometallurgy process currently used for the extraction of lanthanides from metal fuel. They include: radiation stability of molten salts (allowing processing of spent fuels of high radioactivity); absence of neutron moderator such as water; low waste production; non-proliferation requirements.

    Since many years ENEA is involved in research programmes concerning separation of actinides from lanthanides and treatment of related wastes. The facilities recently installed for carrying out experiments in this field are described herein, together with data related to some experiments.

    2.Pyrel III Electrolyzer for Process Studies

    Electrorefining is the key step in the pyroprocess for recovery of actinides (uranium and transuranic elements) by using electrochemical difference among elements in molten LiCl-KCl salt and liquid bismuth under high-purity argon atmosphere at 773 K [1,2]. Chopped spent fuel segments are placed in a perforated steel anode basket and lowered into the electrolyte salt. Noble metals are collected by the liquid bismuth in metal form, while the other elements are dissolved into the salt in the form of their chlorides.

    Actinides are recovered at two different cathodes: a solid steel cathode, at which almost pure uranium is collected; a liquid-bismuth cathode, at which plutonium chloride is reduced and collected with uranium, other transuranic elements, and a small amount of rare-earth fission products. Chemically more active elements (alkali-metal, alkalineearth, and rare-earth elements) remain in the salt (Figure 1).

    A pilot plant termed Pyrel (Pyrometallurgical electrorefiner) III has been installed at ENEA laboratories for pyrochemical process studies under inactive conditions (Figure 2) [3]. This plant is an improvement with respect to previous Pyrel II [4]. The crucible of Pyrel III consists of a zirconia container (125 mm ID x 130 mm high), inserted in a steel vessel (Figure 3) externally heated by a furnace (Figure 4) supported in an externally water-cooled well under the floor of a steel glove-box (about 0.95 m wide, 0.55 m deep x 0.85 m high), where an argon atmosphere is maintained by a continual purge of about 10 L·min-1. The crucible is loaded with 1.39 kg LiCl-KCl eutectic (59-41 mol%) and is currently operated at 460°C, in order to keep the vapour pressure of the salts down to acceptable values. Five perforated disks of a special steel are welded just below the main flange, in order to refrain the heat from reaching the floor of the glove-box (Figure 3). The height of the liquid salt inside the crucible reaches 7.0 cm. The insulated top plate of the electrorefiner has six slots for the stirrer, the thermocouple tube, two reference electrodes, the fuel dissolution basket, and the cathode.

    The oxygen content of the glove-box atmosphere is maintained below 20 ppm, and the moisture below 10 ppm during all the operations.

    The glove-box is fitted with over- and under- pressure tips. It is also equipped with a transfer box (38 cm long, 19 cm OD) to limit the oxygen entrance during posting.

    The chromel-alumel thermocouples allow to record the temperatures corresponding to the salt bath, the gas zone immediately over the salts, the gas zone over the crucible, the internal and external floor of the glove-box, within ± 0.5°C during the experiments.

    The electrodes are connected to an Amel Instruments galvanostat-potentiostat (Model 7050) with data logging via PC. The cell voltage is recorded together with the anode and cathode potentials during each experiment.

    Steel basket anodes (Figure 3) and bar cathodes (Figure 5) are removed from the bath at the end of the experiments and analyzed. The cathode deposit normally contains up to 85-90wt% of salts, together with deposited metal.

    The need for using a zirconia crucible comes from previous experiences with steel crucibles. In spite of the internal coating (Figure 6), made by applying an alumina layer by powder injection plasma spraying process (PIPSP), followed by a layer of silica applied by plasma enhanced chemical vapour deposition (PECVD), the protection proved uneffective after a forthnight of experimental campaign (Figure 7).

    A further device has been added in order to improve the performance of the plant: the gas trapping system, GTS (Figure 8), which allows to “capture„ and remove the gases generated during electrorefining and/or electroreduction experiments, like chlorine, nitrogen, oxygen, carbon oxide and carbon anidride. The GTS consists of a shroud made of an alumina tube connected to a stainless steel tube joined to a Venturi tube in the upper part (Figure 9) [5].

    As the gas removal system will entrain salt vapors, the latter must be removed to prevent plugging and protect downstream equipment. To this end, the stainless steel tube of the shroud is internally equipped with a trap (Figure 10) to condense salt vapors on the cooled surface in a controlled process. The surface is actively cooled by a flux of argon gas which flows inside the trap and then reaches the Venturi tube. So, on one hand, it promotes condensation of the salt vapors, and, on the other hand, it allows to remove gases from the electrorefiner by creating the Venturi effect.

    The gas brought outside the glove-box can be monitored or simply revealed by chemical reactions (Figure 11). In particular, chlorine and carbon anidride pass through two Drechsel bottles and react giving rise to silver chloride and calcium carbonate. Nitrogen gas, on its own, reacts with lithium beads and forms lithium nitride.

    2.1.Experimental Tests

    An experimental campaign has been conducted under inactive conditions by transferring Lanthanum metal from the fuel dissolution basket, FDB, to the solid steel cathode, SSC, according to Figure 12. To this purpose, FDB loaded with La ingots has been immersed into the bath containing lanthanum trichloride, then a direct current has been applied between the electrodes.

    The experiments had a duration of about 23 hours and were made under galvanostatic conditions. Currents from 50 to 100 mA have been applied, with current densities in the range 5.53 - 18.44 mA/cm2/wt% La. The theoretical yield varied from 1.99 to 3.82% and the Faradic yield from 5.07 to 17.59%. Cell potentials (Ecell) were recorded together with anode (Ea) and cathode (Ec) potential. A typical trend is shown in Figure 13. The weight of Lanthanum in the cathode deposit was between 38 and 382 mg, corresponding to a wt% of Lanthanum of 3.81 and 10.42 respectively. Therefore, most of weight deposit is given by deposition of salt (Figure 14). Furthermore, the deposits were characterized by high porosity (see Figure 14). Cathodes of different diameter (8, 12, and 20 mm) were used. The bath of molten salt was normally stirred at 70 revolutions per minute, RPM, during the experiments.

    3.Conditioning of Chloride Salt Wastes

    3.1Conditioning with Sodalite

    Experimental activities on conditioning of chloride salt wastes coming from pyroprocesses have been finalized to demonstrate the feasibility of sodalite synthesis through a process (Pressureless Consolidation) recently proposed by Idaho National Laboratory in USA [6].

    Experiments have been made at a laboratory scale, starting from a homogeneous powder of nepheline, chloride salts and glass frit (Figure 15) [7].

    Such a mix was put into an alumina crucible and slightly pressed with another alumina crucible of a smaller diameter, inside which a stainless steel bar of about 280 g had been inserted. The entire assembly was introduced in a furnace inside an argon-atmosphere glove-box. The furnace temperature was then raised at 10°C/min to 500°C and kept for about one hour, allowing the evaporation of any residual moisture. The temperature was then raised to 925°C for 7 hours. The final waste form (Figure 16) shows a density of 2.382 g/cm3, satisfactory if compared with that reported for pure sodalite (2.1-2.3 g/cm3).

    SEM analyses and stereomicroscopy observations show the absence of both microcracks and macroporosity. As shown in Figure 17, FTIR spectrum of the final sample shows the typical peaks of sodalite at 465 cm-1 [O-Al(Si)- O bending vibrations], 668, 712 and 735 cm-1 [Al(Si)-O-Al( Si) symmetric stretching], and 978 cm-1 [Al(Si)-O-Al (Si) asymmetric stretching]. Coherently, XRD analysis highlighted the presence of sodalite as the main phase and of nepheline in traces.

    Leaching tests under static conditions were performed according to the ASTM C1285-02 procedure [8]. The concentrations of the elements in the leachates, measured by ICP-MS, are reported in Table 1. The leaching behavior of sodalite blended with glass frit is comparable to that of similar glass-based confining matrices reported in the literature. To this purpose Table 1 compares the normalized release with the data by Idaho National Laboratory [9].

    3.2.Conditioning with SAP Matrix

    A novel method proposed by Korea Atomic Energy Research Institute (KAERI) is also applied to the treatment of chloride salt wastes [10,11]. It is based on a matrix, SAP (xSiO2-yAl2O3-zP2O5), synthesized by a conventional solgel process (Figure 18), able to stabilize the volatile salt wastes owing to the formation of metalaluminosilicates, metalaluminophosphates and metalphosphates.

    The addition of a glass powder as a chemical binder and a treatment at around 1150°C gives the final waste form. With this method a higher disposal efficiency and a lower waste volume can be obtained.

    LiCl-KCl melt containing chlorides of alkaline, alkaline- earth metals, and lanthanides has been used to simulate the waste salt. The composite SAP has been prepared by a sol-gel process, using tetraethyl ortosilicate (TEOS), aluminum chloride (AlCl3.6H2O) and phosphoric acid (H3PO4) as sources of Si, Al, and P, respectively. All reagents were dissolved in EtOH/H2O and the mixture, tightly sealed, was placed in an electric oven at 70°C. After a gelling/ageing for 3 days, the transparent hydrogels were dried at 110°C for 2 days and then thermally treated at 600°C for 2 hours. The so-obtained SAP has a density of 2.291 g/cm3. SEM investigations highlighted a morphology similar to that observed for SAP samples reported in the literature [9], as shown in Figure 19 [12].

    A series of final products (SAPs) were reacted with metal chlorides at 950°C for 30 hours inside an Ar-atmosphere glove-box, after mixing them at a SAP/metal chloride mixing ratio of 2.

    XRD (Figure 20), FTIR and Raman spectra of samples at increasing temperatures indicated, for temperatures above 850°C, the presence of peaks which can be assigned to metal-aluminosilicate, metal phosphate and metal-aluminophosphate.

    The reaction products were mixed with glass powder at a mixing ratio of 70 (gel product) : 30 (glass) by weight and heat treated at 1150°C for 4 hours without pressing (Figure 21).

    As expected, XRD analysis indicated an amorphous matrix with a few peaks which can be attributed to aluminum phosphate, while FTIR spectra highlighted the presence of three basic bands characteristic of the silicatephosphate glasses.

    Static leaching tests [8] on the Salts/SAP samples revealed the interesting behavior of such matrix: after 7 days leaching the concentrations of salt elements in the leachates, reported in Table 2, were lower than those obtained with sodalite.

    4.Conclusions

    A new facility has successfully been installed at ENEA laboratories for pyrochemical process studies under inactive conditions. Pyrel III pilot plant allows to perform experiments about electrorefining and electroreduction of simulated fuel. Moreover, an argon-atmosphere glove-box is used for conditioning of chloride salt wastes with sodalite or SAP matrix.

    The data obtained from the experimental activities will give a useful contribution to the comprehension of the phenomena involved and to the improvement of the project design of industrial plants.

    Figure

    JNFCWT-13-19_F1.gif

    Electrorefining step of pyrometallurgical process.

    JNFCWT-13-19_F2.gif

    General view of Pyrel III plant.

    JNFCWT-13-19_F3.gif

    Crucible of the plant (left) and anode basket (right).

    JNFCWT-13-19_F4.gif

    External vessel, heating system, and thermal insulation.

    JNFCWT-13-19_F5.gif

    Steel cathodes of increasing diameter.

    JNFCWT-13-19_F6.gif

    Steel crucible before (top) and after (bottom) internal coating with alumina and silica by means of plasma spraying and plasma enhanced chemical vapour deposition.

    JNFCWT-13-19_F7.gif

    Interior of a steel crucible at the end of an experimental campaign showing the salt layer (left), and the bismuth layer (right) after the partial removal of the salt with hot water.

    JNFCWT-13-19_F8.gif

    Gas trapping system.

    JNFCWT-13-19_F9.gif

    Venturi tube.

    JNFCWT-13-19_F10.gif

    Trap (left) inside the steel shroud as seen from the top (middle) and from the bottom (right).

    JNFCWT-13-19_F11.gif

    Detection of chlorine and carbon anidride (top), and nitrogen (bottom) generated by pyro experiments and transferred outside the glove-box through GTS.

    JNFCWT-13-19_F12.gif

    Schematic of the direct transportation experiment of La ingots from FDB to SSC, also showing the guiding-tube for thermocouples, the two reference electrodes(Ag/AgCl), and the stirrer.

    JNFCWT-13-19_F13.gif

    Typical trend of the potentials recorded during an experiment of direct transportation of Lanthanum from FDB to SSC (Ecell: cell potential; Ea: anode potential; Ec: cathode potential).

    JNFCWT-13-19_F14.gif

    Solid cathode with deposit at the end of an experiment (left); detail at the stereomicroscopy of the cathode deposit, showing high porosity (right).

    JNFCWT-13-19_F15.gif

    Synthesis of sodalite from nepheline.

    JNFCWT-13-19_F16.gif

    Sodalite blended with glass frit after heating at 925°C for 7 hours.

    JNFCWT-13-19_F17.gif

    FITR spectrum of Sodalite blended with glass frit after heating at 925°C for 7 hours.

    JNFCWT-13-19_F18.gif

    Outline of the SAP process.

    JNFCWT-13-19_F19.gif

    Comparison of SEM pictures of SAP samples prepared by KAERI (a) and by ENEA (b).

    JNFCWT-13-19_F20.gif

    XRD spectra of SAP sample (bottom) and of the final product (top).

    JNFCWT-13-19_F21.gif

    Salts/SAP samples obtained after blending with glass frit and heating at 1150°C for 4 hours.

    Table

    Normalized releases (g/m-2) of sodalite samples after 7 days leaching at 90 ± 2°C

    Normalized releases (g/m-2) of SAP samples after 7 days leaching at 90 ± 2°C

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