Pyrochlore-Rich Synroc as a Host for Immobilization of Actinides
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Pyrochlore-Rich Synroc as a Host for Immobilization of Actinides
Jianwen Yang, Baolong Tang and Shanggeng Luo China Institute of Atomic Energy, P.O.Box 275(1), Beijing 102413 Email: [email protected]
ABSTRACT Six different formulations of pyrochlore-rich synroc waste forms were designed to contain 32-50wt% simulated actinides. Their physical properties were determined. Phase identification was carried out by using X-ray diffractometer (XRD), and backscattered electron image (BEI) in scanning electron microscopy (SEM). Phase microanalyses were carried out using energy dispersive spectrometer (EDS) in SEM. Their durability properties were determined by PCT leach tests, the leachate was analyzed by using inductive coupled plasma mass spectroscopy (ICP/MS) and atomic absorption spectroscopy (AAS).
INTRODUCTION Pyrochlore and Zirconolite are the naturally existed stable minerals [1, 2]. It has been approved that zirconolite is the most durable phase in synroc, and also an ideal host for actinides waste [3-6]. Pyrochlore is structurally very similar to zirconolite, it is usually appeared in synroc when the ion substitution of Zr site exceeds 0.5 formula units, and they may have similar properties for accommodating the actinides [3, 4]. But pyrochlore can accommodate more actinides than zirconolite [7], so for this reason the pyrochlore-rich synroc as a host for immobilization of actinides is investigated.
EXPERIMENTAL Six different formulations of pyrochlore-rich synroc to immobilize actinides were designed. Owing to the valance, ion radii and crystal chemical properties analysis, neodymium (Nd) and uranium (U) were used as the simulant for trivalent and tetravalent actinides. The designed synroc contained 32-50wt% simulated actinides. To obtain the pyrochlore phase, the formulation design was mainly according to the solid solution limit of Zr site of zirconolite (Z, CaZrTi2O7). The adopted phase assemblege was 85wt% pyrochlore (Py, stoichiometric formula CaUTi2O7), 5wt% hollandite (H, BaAl2Ti6O16) and 10wt% rutile (R, TiO2). No special ions were added as the valance compensation ions, Ti3+ was used to
self-balance the charge. To maintain chemical flexibility to the slight variations of waste composition and loading, and accommodate any possible residual fission products, two minor phases were also designed in the formula. The composition of the designed py-rich synroc samples are shown in Table I.
Table I. Composition of pyrochlore-rich synroc samples Sample no.
Composition (wt%)
1
2
3
4
5
6
Al2O3 BaO CaO TiO2 ZrO2 Nd2O3 UO2
0.7 1.0 11.2 45.1 9.8 32.2
0.7 1.0 10.4 43.0 4.6 40.2
0.7 1.0 9.8 41.2 47.2
0.7 1.0 10.3 44.4 8.1 3.9 31.5
0.7 1.0 9.4 42.2 2.7 4.9 39.1
0.7 1.0 8.9 41.4 5.3 42.6
Sum.
100.0
99.9
99.9
99.9
100.0
99.9
32.2
40.2
47.2
37.6
46.8
50.9
Waste loading (wt%)
The adopted fabrication process was the standard procedure with the use of a mixture of alkoxides (for Ti, Zr and Al) and hydroxides (for Ba and Ca) followed by calcination at 750°C for 1 hour and hot pressing at
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