Atmosphere Processing Effects on Titanate Ceramics Designed for Plutonium Disposition

Abstract

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.

Radiation damage effects in candidate titanates for Pu disposition: Pyrochlore

“…For the P1 specimens, this target composition suggests a formula Ca 1. 16 [40]. In this formulation, we assumed that Pu(IV) and U(IV) are the relevant oxidation states.…”

Titanate ceramics for immobilisation of uranium-rich radioactive wastes arising from 99Mo production

“…The baseline composition (40 wt% UO 2 , sample 2 in Table 1) showed the sample to contain pyrochlore (71%), rutile (15%), hollandite (10%) and brannerite (UTi 2 O 6 , 4%). The baseline sample was not designed to contain brannerite, however, brannerite is commonly found in pyrochlore-rich ceramics [12,[17][18][19], and the effect of brannerite on the aqueous durability of the waste form will be discussed later. Fig.…”

“…The approximate composition was Ca 1.1 U 0.7 Zr 0.1 Al 0.05-Ti 1.95 O 7 and other workers have reported pyrochlores with Ca contents of >1 f.u. [20][21]. This has been attributed to some of the U being in a valence state of >4+.…”

Radiation damage effects in candidate titanates for Pu disposition: Zirconolite

“…Because we did not have detailed microprobe analyses, we made no attempt to distribute the Pu and U between oxidation states other than four. Several authors have suggested that Pu(III) and U(V) exist in these titanate mineral phases [26][27][28][29][30][31][32][33][34]. Colella et al [31] have demonstrated that natural and synthetic brannerite contain U(V) with IR, XPS, and EELS measurements.…”