Leaching Behavior of Zirconolite-Rich Synroc Used to Immobilize High-Fired Plutonium Oxide
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inide wastes have been produced and they have excellent leach resistances [4]. Zirconolite can also incorporate Gd, Hf and Sm as neutron absorbers for criticality control [4]. Recent considerations on the disposition of weapons-grade Pu have targeted the immobilisation route as being more appropriate for the disposal of 'high-fired' Pu0 2 which contains significant levels of impurities. This approach requires the waste form being considered for Pu immobilisation to be able to handle these impurities as well as the 'high-fired' Pu0 2 and neutron absorbers to relieve criticality concerns. This present work reports on the demonstration of the immobilisation of 'high-fired' PuO 2 and neutron absorbers in a zirconolite-rich version of Synroc. EXPERIMENTAL The Synroc was designed to contain 80 wt% of Pu-doped zirconolite plus 10 wt% each of Bahollandite and rutile. Equimolar amounts of Gd and Hf, relative to Pu, were included in the zirconolite phase as neutron absorbers. The formulation was designed to incorporate the absorbers and Pu into the zirconolite phase with Pu on the Ca site, Hf on the Zr site and for Gd to be incorporated equally on the Ca and Zr sites. Preliminary non-radioactive experiments were performed using CeO 2 as a simulant for Pu0 2 on an equimolar basis. Table I gives the composition of the Pu-doped and Ce-doped formulations. The Ce-doped formulation was prepared by cold pressing 3 to 4 g of oxide-route powder, which had been calcined for 1 hour at 750'C and ball-milled, in a 19 mm diameter steel 161
Mat. Res. Soc. Symp. Proc. Vol. 506 01998 Materials Research Society
die at = 100 MPa. Sintering was carried out in a muffle furnace for 4 hours in air at 1325'C. The pellets were heated at 2°C min1 to 600'C, then at 5'C min-' to the sintering temperature and were then cooled at a rate of 5'C mrin. The cooling and heating rate schedules were not optimised as part of this work. The pellets produced had a 'fired' density of 4.82 g cm-3 and an open porosity less than 0.1%. Table I: Nominal batch compositions for Pu-doped and Ce-doped formulations on an oxide basis Composition (wt%) Ce-doped Pu-doped 4.23 6.86 2.33 2.26 6.50 5.93
Al20 3 BaO CaO TiO2
42.97
40.12
ZrO 2 HfO 2 Gd 20 3
14.29 11.08 9.54 9.06
13.03 10.10 8.69
CeO 2 PuO 2
-
13
'High-fired' PuO 2 was produced by drying a nitric acid solution of Pu and calcining it in air for 2 hours at 1000°C. For the Pu-doped formulation slightly more A120 3 was incorporated into the formulation following earlier work in which the A120 3 content of the PuO 2-doped material was similar to that of the CeO 2 -doped sample. It was also made by the oxide route, with calcination and ball-milling treatment similar to those for the Ce-doped batch. Pellets, = 0.5 g, were prepared by cold pressing in an 11 mm diameter steel die at = 50 MPa. Sintering was carried out in a similar manner to the Ce-doped batch except that a sintering temperature of 1375°C was used. The pellets produced had a 'fired' density of 4.82 g cm-3 and an open porosity of 0.2%. Powder X-ray di
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