Effect of Synthesis Conditions on Phase Composition

Mat. Res. Soc. Symp. Proc. Vol. 663 © 2001 Materials Research Society

Effect of Synthesis Conditions on Phase Composition of Pyrochlore-Brannerite Ceramics S.V. Stefanovsky1, S.V. Yudintsev2, B.S. Nikonov2, M.I. Lapina2 and A.S. Aloy3 1 SIA “Radon”, 7th Rostovskii per., 2/14, Moscow 119121 Russia, Email: [email protected] 2 Institute of Geology of Ore Deposits RAS, Staromonetnii 35, Moscow 109035 Russia 3 V.G. Khlopin Radium Institute, 2nd Murinskii pr. St-Petersburg, Russia ABSTRACT Three melted samples of pyrochlore-brannerite-based ceramics produced under different redox conditions were examined. Two of the samples were produced using cold crucible melting at ~1600 C. The third sample was obtained via melting in a microwave oven at 1700-1800 C. All the samples are composed of major pyrochlore and brannerite phases, and minor rutile and UO2-based solid solution or pseudobrookite phases. Pyrochlore-structured phases predominate in all three samples and account for 50-60% of the total bulk. Two pyrochlore varieties – Ca-pyrochlore (predominant) and Ba-pyrochlore have been found in these samples. The latter phase is more stable at high temperatures than the Ba-hollandite present in sintered pyrochlore-rich Synroc-F ceramics. Decomposition of the Ba-hollandite results in rutile formation in the melted samples. INTRODUCTION Immobilization of radioactive wastes in stable matrices for long-term storage is an important step in a closed nuclear fuel cycle. Currently liquid high level wastes (HLW) from radiochemical facilities are solidified into aluminophosphate or borosilicate glasses. These glasses have low chemical durability and can not guarantee safe long-term isolation of long-lived radionuclides from the biosphere [1]. Various crystalline waste forms have been suggested as alternatives to glass (see, for example, References 2-5), among them Synroc-C, Synroc-D, Synroc-F, and zirconolite-rich ceramics designed for immobilization of commercial and defense HLW, spent fuel, and actinidebearing wastes, respectively. Hot pressing, cold pressing and sintering, and melting routes are being considered for production of these waste forms. Unlike Synroc-C, designed for immobilization of HLW after separation of uranium and plutonium, Synroc-F is meant to incorporate all the constituents of spent nuclear fuel (SNF) [2]. Another variant, Synroc-FA, has been suggested to immobilize SNF components after plutonium separation [6]. Due to the high uranium content in liquid HLW after SNF dissolution (~200 kg/m3) the phase compositions of these waste forms are quite different from all the other Synroc variants where actinides are incorporated in zirconolite, and, to a lessor extent, in perovskite. In Synroc-F the stable phase is pyrochlore-structured Ca-U-Ti-oxide rather than zirconolite, which becomes unstable at a high uranium content. The Synroc-F ceramic produced by hot-pressing (1200-1250°C, 21-37 MPa) is composed of a major pyrochlore (90-95 vol%) phase and minor hollandite and rutile phases. This phase relationship allows incorporation of ~50 wt% SNF.

Pyrochlore ceramics (containing 80-90 vol% of pyrochlore) are considered to be the most promising matrices for immobilization of wastes with a high actinide content, for example, excess weapons plutonium [7-9]. These pyrochlore ceramics have been produced by cold pressing and sintering similar to that used for MOX fuel production. Application of melted ceramics for waste immobilization is impeded by the high process temperatures required. To produce melted waste ceramics, however, cold crucible melting has been proposed [10,11]. This method was especially designed for melting refractory materials [12]. Melting and subsequent crystallization has some advantages over sintering (solid phase synthesis). Cold crucible melting has higher throughput rates, is less sensitive to initial batch quality, and produces melted materials that contain lower amounts of unreacted initial components and metastable intermediate products as compared to sintering [2,4,7,8,13,14]. Since the methods being considered to synthesize the required waste forms differ in temperature, pressure, and redox conditions it can be expected that the properties of ceramics with similar chemical compositions but produced by these different methods should be also variable. The Synroc ceramics prepared by hot pressing, cold pressing and sintering, and cold crucible melting were compared earlier [13,15]. In the present paper phase composition and waste element partitioning in melted, hot-pressed, and sintered pyrochlore-brannerite-based (Synroc-F type) ceramics are compared. Some preliminary data on preparation and characterization of the melted Synroc-F ceramic have been described in our previous work [11]. EXPERIMENTAL The cold crucible melting procedure was described in detail earlier [9,10]. For the current studies, oxide mixtures were melted in the Radon bench-scale unit at temperatures of about 1600°C under oxidizing and reducing conditions followed by controlled cooling for melt crystallization and then natural cooling to room temperature within the furnace chamber. One of the samples (SF-1) was produced in a microwave oven at the V.G. Khlopin Radium Institute at a temperature of about 1700-1800°C. The ceramic samples obtained were examined with X-ray diffraction (XRD) using a DRON-4 diffractometer (Cu Kα – radiation) and scanning and transmission electron microscopy (SEM and TEM) using a JSM-5300 + Link ISIS unit and a JEM-100c + KEVEX-5100 unit, respectively. Data on hot-pressed and sintered samples were obtained from previously published studies [2-9,13]. Hot-pressed Synroc-F ceramic was produced from an oxide mixture doped with 2% metal Ti in nickel capsules at T = 1250°C and P = 500 MPa for 1 hour [2]. Synroc-FA ceramic was prepared by sintering pellets (pre-pressed at 15-25 Mpa) at 1250-1350°C in an inert or reducing atmosphere [6]. The same method was used to produce Pu-containing ceramics: cold pressing of an oxide batch at 14 MPa followed by sintering at 1350°C in argon [7-9]. RESULTS Chemical and phase compositions of the samples produced by cold crucible melting and for the reference hot-pressed and sintered ceramics are given in Table I. XRD patterns of the melted ceramics are very similar (Figure 1). They contain peaks due to three phases: pyrochlore-structured, brannerite, and rutile. The major pyrochlore reflections (in Å), d222 = 2.95-2.96 (100%); d440 = 1.80 (40%); d622 = 1.56 (35%); and d400 = 2.56 (25%), are close to the reflections of rare earth titanates having a pyrochlore lattice (space group Fd3m), especially the reflections for Gd2Ti2O7 (card #23-259 of JCPDS database). Accordingly, the unit

cell dimensions of the pyrochlore in the samples studied and the rare earth titanates are almost the same (Table II). The major brannerite reflections are as follows: d201 = 4.73 (100%); d110 = 3.40-3.41 (100%); d201 = 2.90-2.91 (30%); d111 = 2.75-2.76. These values are in good agreement with the data for synthetic uranium titanate UTi2O6 (monoclinic symmetry, space group B2/m, JCPDS #12-477). The rutile reflections, d100 = 3.25-3.26 (100%); d101 = 2.49-2.50 (80%); and d211 = 1.69 (80%), are close to the reference data for TiO2 (tetragonal symmetry, space group P42/mnm, JCPDS #21-1276). Table I. Chemical (wt%) and phase (vol%) compositions of the pyrochlore ceramics Melted

Hot-pressed

Sintered

Oxides

F-2/1

F-3/1

SF-1

F [2]

FA [6]

CaO UO2

6.6 42.5

9.2 46.6

8.3 42.6

9.5 47.8

No data -

2.0 2.5 ZrO2 Gd2O3 TiO2 45.0 39.7 1.4 0.9 Al2O3 BaO 2.5 1.1 Crystalline Ca-Py + Ca-Py + phases Ba-Py –50, Br Ba-Py –50, – 30, Br – 30, R – 20, R – 10, UO2ss -