No abstract

Hot isostatic pressing (HIP) is a technology with wide applicability in consolidating calcined intermediate-level and high-level nuclear waste, especially with wastes that are not able to be readily processed by vitrification at reasonable waste loadings. The essential process steps during the HIP cycle will be outlined. We have demonstrated the effective consolidation via HIP technology of a wide variety of tailored glass-ceramic and ceramic waste forms, notably simulated ICPP waste calcines, I sorbed upon zeolite beads, Pu-bearing wastes, inactive Cs/Sr/Rb/Ba mixtures, simulated waste pyroprocessing salts from spent nuclear fuel recycling, Tc, U-rich isotope production waste, and simulated K-basin (Hanford, WA, USA) and Magnox sludges (UK). Can-ceramic interactions have been carefully studied. The principal advantages of the HIP technology include: negligible offgas during the high temperature consolidation step, relatively small footprint, and high waste loadings. As a batch process, the wasteform chemistry can be readily adjusted on a given process line, to deliver wastes into different end states (e.g. direct HIP versus chemically tailored). This flexibility allows the treatment of multiple waste streams on the one process line.

The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-Pass-Flow-Through (SPFT) tests at 70°C. The dissolution of pyrochlore is incongruent with preferential releases of Ca and Gd over Ti, close to stoichiometric releases of U and Ti, and lower releases of Hf and Pu than Ti. Altered pyrochlore and polymorphs of TiO2 (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO2 containing Hf and varying amounts of Gd and Pu. These surface alteration phases are consistent with reported studies of natural samples. The releases of U, Gd, Ca and Ti into solution follow linear kinetics, whereas the releases of Pu and Hf exhibit non-linear behavior. The presence of ∼5% PuO2 and trace amounts of glass does not appear to have an effect on the overall durability of the material. Further, the low Pu release rate and the similar kinetics for Pu and Hf releases limit the possibility of nuclear criticality under repository conditions. Overall, this study provides useful information on the lower bounds of durabilities of the materials.

Baseline formulation titanate ceramics for surplus Pu disposition are based upon a target mineralogy of 95 wt.% pyrochlore (Ca0.89Gd0.22Hf0.23Pu0.22U0.44Ti2O7) plus 5 wt.% Hf-doped rutile (∼ Ti0.9Hf0.1O2), where Ce is used as an analogue for Pu and U, and Th for Pu. Typically, Pu/U, Th/U and Ce/U-baseline samples form major pyrochlore, plus minor brannerite (AnTi2O6) and rutile. Ce/Ce-baseline ceramics were similar but did not form brannerite. Sintering in air produced more U5+ in the ceramics than sintering in Ar. In the pyrochlore the charge compensation for U5+ principally occurred by an increase in Ca2+ and decrease in Ti4+ and Th4+. In the matrix these phase compositional changes result in an increase in brannerite content of the ceramic at the expense of pyrochlore in the air sintered ceramic relative to the Ar sintered sample. Sintering in reducing 3.7% H2 in Ar atmospheres eliminates the brannerite and rutile and results in 2M-zirconolite and perovskite in addition to the major pyrochlore phase. Varying the sintering temperature between 1250°C and 1400°C had little effect on the phase chemistry other than slightly incomplete reactivity of coarse actinide oxide at the lower sintering temperatures and a general increase in density (to ∼ 1350°C) and grain size with increased sintering temperatures. Th4+ appears to be a good analogue for Pu4+ from a crystal chemistry aspect.

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu02. The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low (< 10−4 g m−2 d−1) and comparable rates from the waste form.