Solid Solubilities of Pu, U, Gd and Hf in Candidate Ceramic Nuclear Wasteforms
This goal of this research project was to determine the solid solubility of Pu, U, Gd, and Hf in candidate ceramics for immobilization of high-level nuclear waste. The experimental approach was to saturate each phase by adding more than the solid solubility limit of the given cation, using a nominated substitution scheme, and then analyzing the candidate phase that formed to evaluate the solid solubility limit under firing conditions. Confirmation that the solid solution limit had been reached insofar as other phases rich in the cation of interest was also required. The candidate phases were monazite, titanite, zirconolite, perovskite, apatite, pyrochlore, and brannerite. The valences of Pu and U were typically deduced from the firing atmosphere, and charge balancing in the candidate phase composition as evaluated from electron microscopy, although in some cases it was measured directly by x-ray absorption and diffuse reflectance spectroscopies (for U). Tetravalent Pu and U have restricted (< 0.1 formula units) solid solubility in apatite, titanite, and perovskite. Trivalent Pu has a larger solubility in apatite and perovskite than Pu4+. U3+ appears to be a credible species in reduced perovskite with a solubility of {approximately} 0.25 f.u. as opposed to {approximately} 0.05 f.u. for U4+. Pu4+ is a viable species in monazite and is promoted at lower firing temperatures ({approximately} 800 C) in an air atmosphere. Hf solubility is restricted in apatite, monazite (< 0.1 f.u.), but is {approximately} 0.2 and 0.5 f.u. in brannerite and titanite, respectively. Gd solubility is extended in all phases except for titanite ({approximately} 0.3 f.u.). U5+ was identified by DRS observations of absorption bands in the visible/near infrared photon energy ranges in brannerite and zirconolite, and U4+ in zirconolite was similarly identified.
- Research Organization:
- Australian Nuclear Science and Technology Organization, Menai, NSW 2234, Australia (US)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- FG07-97ER45676
- OSTI ID:
- 781161
- Report Number(s):
- DOE/ER/45676; Project Number 60387; Project Number 60387; TRN: US0102816
- Resource Relation:
- Other Information: PBD: 2 Apr 2001
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
36 MATERIALS SCIENCE
CERAMICS
GADOLINIUM
HAFNIUM
HIGH-LEVEL RADIOACTIVE WASTES
PLUTONIUM
SOLID SOLUTIONS
URANIUM
WASTE FORMS
SOLUBILITY
PHASE STUDIES
SOLID SOLUBILITY
IMMOBILISATION
HIGH-LEVEL
NUCLEAR WASTE
CATION
MONAZITE
TITANITE
ZIRCONOLITE
PEROVSKITE
APATITE
PYROCHLORE
BRANNERITE