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Plutonium Incorporation in Phosphate and Titanate Ceramics for Minor Actinide Containment

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Plutonium Incorporation in Phosphate and Titanate Ceramics for Minor Actinide Containment Journal of Nuclear Materials 352 (2006) 233–240 www.elsevier.com/locate/jnucmat Plutonium incorporation in phosphate and titanate ceramics for minor actinide containment X. Deschanels a,*, V. Picot a, B. Glorieux b, F. Jorion a, S. Peuget a, D. Roudil a, C. Je´gou a, V. Broudic a, J.N. Cachia a, T. Advocat a, C. Den Auwer a, C. Fillet a, J.P. Coutures b, C. Hennig c, A. Scheinost c a DIEC/LMPA, CEA/DEN Valrho Marcoule, BP 17171, 30207 Bagnols-sur-Ce`ze cedex, France b PROMES-CNRS Tecnosud, Rambla de la Thermodynamique, 66100 Perpignan, France c ROBL-CRG, ESRF, BP 220, 38043 Grenoble, France Abstract Two ceramics, zirconolite and a monazite–brabantite solid solution (MBss) were studied for the immobilization of minor actinides (Np, Am, Cm) produced by reprocessing spent fuel. Monoclinic zirconolite (CaZrTi2O7) is a fluorite deriv- ative structure and is the primary actinide host phase in Synroc (a titanate composite). Monazite (LnPO4, where Ln = La, Ce, Nd, Gd, etc.) is a monoclinic orthophosphate containing trivalent cations, and brabantite (Ca0.5An0.5PO4) is an iso- structural monazite compound containing tetravalent cations (An = Th and U). The nominal composition of the ceramics studied in this work is (Ca0.87Pu0.13)Zr(Al0. 26Ti1.74)O7 for zirconolite and (Ca0.09Pu0.09La0.73Th0.09)PO4 for the monazite– brabantite solid solution. These formulas correspond to 10 wt% PuO2 loading in each material. XANES spectroscopy showed that the plutonium is tetravalent in zirconolite and trivalent in MBss. Thorium, another tetravalent cation, can be incorporated at 10 wt% ThO2 in MBss. Aluminum and calcium balance the excess cationic charge resulting from the incorporation of Pu(IV) in zirconolite and Th(IV) in brabantite, respectively. The relative density of the pellets exceeded 90% of theoretical density. The samples exhibited a homogeneous microstructure even if some minor phases, representing less than 2% of the surface area, were detected. The two ceramics are compared in terms of actinide loading, and prelimin- ary results on their long-term behavior are discussed. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Je; 61.82Àd 1. Introduction can be considered following enhanced chemical separation during spent fuel reprocessing. The refer- The minor actinides including neptunium, cur- ence option, transmutation by neutron bombard- ium, americium are highly radiotoxic. Two options ment in nuclear facilities, would reduce the initial radiotoxic inventory. The second option would be * Corresponding author. Tel.: +33 4 66 79 60 87; fax: +33 4 66 to incorporate the separated actinides into an inor- 79 77 08. ganic matrix ensuring long-term stability [1]. In this E-mail address: [email protected] (X. Deschanels). context, zirconolite and a monazite–brabantite solid 0022-3115/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2006.02.059 234 X. Deschanels et al. / Journal of Nuclear Materials 352 (2006) 233–240 solution (MBss) were selected to quantify their quantity of added aluminum was determined by performance under self-irradiation, notably under assuming the plutonium was at tetravalent oxidation active conditions. Plutonium was used to simulate state, resulting in the nominal composition the behavior of the minor actinides: its long-lived (Ca0.87Pu0.13)Zr(Al0.26Ti1.74)O7. isotope 239Pu (half-life: 24 000 years) was used to verify the loading capacity of these elements in the 2.2. Monazite–brabantite solid solution proposed structures, and its short-lived isotope 238 Pu (87 years) to accumulate irradiation damage Monazite (MPO4) has a monoclinic structure in a short time. In addition, plutonium is found at (space group P21/n). It consists of series of alternat- the same oxidation states (III and IV) as the minor ing tetrahedral PO4 sites and MO9 polyhedra that actinides Am(III), Cm(III), and Np(III) or Np(IV) may contain trivalent or tetravalent elements. In in oxide matrices, making it a suitable surrogate this case, loading involves the following substitu- for these elements. tion: An4+ +Ca2+ =2M3+ (where An represents Previous work [2–7] has already shown the an actinide), yielding a brabantite structure. The capacity of the zirconolite and the monazite matrix solid solution between the two isostructural to accommodate actinide loading. This study com- compounds LaPO4 (monazite) and Ca0.5Th0.5PO4 pares the performance of the two matrices in terms (brabantite) is continuous [12]. Plutonium, ameri- of minor actinide loading based on the results cium and curium are incorporated in this structure obtained with plutonium-239. Preliminary results at trivalent state [13–16]. It should also be possible on the irradiation behavior of the materials are also under certain conditions to reduce Np to trivalent discussed. oxidation state [10], but Np(IV) is generally observed [17]. 2. Plutonium loading schemes in the test matrices A monazite–brabantite solid solution (MBss) with a nominal composition of (Ca0.09Pu0.09- 2.1. Zirconolite La0.73Th0.09)PO4 corresponding to the incorpora- tion of Pu(III) was selected for this study because The structure of zirconolite-2M is monoclinic it is suitable for simultaneously loading trivalent (space group C2/c) with the molecular formula (Pu) and tetravalent (Th) actinides. CaZrTi2O7. Tetravalent elements including the actinides (Np(IV), Pu(IV)) can be incorporated 3. Experimental not only at zirconium sites but also calcium sites with charge compensation provided by substituting 3.1. Fabrication of plutonium-239 doped ceramics aluminum for titanium. Trivalent elements includ- ing the actinides (Am(III), Cm(III), Pu(III), The fabrication process is basically the same for Np(III)) can also be incorporated at the calcium site both MBss and zirconolite ceramics doped with plu- with the same type of charge compensation (addi- tonium-239 (Table 1). The process was developed in tion of aluminum at the titanium site) [8]. Depend- a glove box. ing on the redox conditions and the incorporation The inactive precursor used to synthesize zircon- scheme, plutonium may be found at trivalent or tet- olite was fabricated by a sol–gel process described ravalent oxidation state in this structure [9–11]. in reference [18]. The monazite–brabantite solid Previous work [1] showed that the crystal structure solution was fabricated by an oxide process. The of zirconolite-2M is conserved between 0.15 and 0.16 precursors were ammonium dihydrogenphosphate 4+ 4+ unit Pu per formula unit at the Zr site; the zircon- (NH4H2PO4), calcium carbonate, thorium oxide olite-4M superstructure appears at higher values. To and lanthanum oxide [19]. The operating conditions prevent this structural change, which could affect the for each process step are indicated in Table 1. The performance of the conditioning matrix, we chose to precursors and the calcined powder were co-milled incorporate Pu at the calcium site. Under these con- in a Retsch MM200 mixer-mill with zirconia grind- ditions, the limit for the monoclinic structure is about ing jar and balls. For zirconolite the milling opera- 0.2 unit Pu3+ per formula unit for materials produced tion was carried out in isopropanol, but in view of under reducing conditions [9]. This limit is the same the difficulties encountered (the grinding jars were for loading Pu(IV) in this way under oxidizing condi- not sufficiently leaktight), the MBss was milled tions [9]. In fabricating the ceramics for this study, the dry. The pellets were sintered in air. X. Deschanels et al. / Journal of Nuclear Materials 352 (2006) 233–240 235 Table 1 Fabrication processes for zirconolite and monazite–brabantite pellets doped with 239Pu Process steps Zirconolite Monazite–brabantite Blending and grinding of 180 min in isopropanol 3 · (15 min at 20 Hz) dry precursors with PuO2 Calcining 120 min at 1523 K 10 h at 1523 K Milling of calcined powder 180 min 5 · (20 min at 25 Hz) Uniaxial press compaction 80 MPa (8 mm dia · 10 mm high, 120 MPa (8 mm dia · M 1.5 g) 10 mm high, M 1.5 g) Sintering 15 h at 1723 K 10 h at 1723 K 3.2. Pellet analysis described above exceeded 92% (Table 2). X-ray dif- fraction analysis of a sample pellet showed that the Polished cross sections of these samples were desired phase was synthesized in both cases. No examined by scanning electron microscopy (SEM) secondary phase could be detected using this coupled with X-ray microanalysis (EDX) to ensure technique. their chemical homogeneity. The ceramics were also A more thorough examination by scanning elec- examined using a Cameca SX50 electron micro- tron microscopy and X-ray microanalysis (EDX) probe. The standards used for these analyses were revealed precipitates of a mixed titanium and zirco- a borosilicate glass specimen containing 2 wt% plu- nium oxide (Fig. 1) containing minimal quantities tonium for Pu determination (Ma line), CaSiO4 or of plutonium in the zirconolite, and precipitates with CaTiSiO5 for Ca and O (Ka line), ZrSiO4, CaTiSiO5 a PuO2 core surrounded by a Pu, P, O phase in the for Ti (Ka line), KAlSi3O8, GaP (Ka line), LaF3 (La MBss (Fig. 2). This suggests that plutonium oxide line), Th (Ma line). X-ray diffraction analysis was also used to deter- mine the presence of any secondary phases. The Table 2 239 examination was carried out using a Seifert diffrac- Characteristics of Pu-doped monazite–brabantite and zircon- tometer in Bragg–Brentano geometry. A silicon stan- olite pellets 239 239 dard was used to adjust the device geometry to Characteristics Pu zirconolite Pu monazite ensure an angular position variation of less than –brabantite 0.02° with respect to the theoretical positions of the Pellets Number 19 19 main peaks in the compound. The source corre- 3 Density (g cmÀ ) 4.39 ± 0.12 5.12 ± 0.04 sponded to the Ka line of copper; a monochromator Relative density (%) 92 ± 2.7 96 ± 1 was used to eliminate the K lines and spurious radi- b Crystal parameters ation due to the radioactive environment of the glove a(A˚ ) 12.431 (7) 6.8101 (1) box.
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