Thermochemistry of Hf-Zirconolite, CaHfTi 2 O 7
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ROBERT L. PUTNAM', ALEXANDRA NAVROTSKY', BRIAN F. WOODFIELD", JENNIFER L. SHAPIRO", REBECCA STEVENS", JULIANA BOERIO-GOATES** * Thermochemistry Facility; University of CA, Davis; 4440 Chemistry Annex; Davis, CA 95616 [email protected]; [email protected] ** Department of Chemistry and Biochemistry; Brigham Young University; C 100 BNSN; Provo, UT 84602; [email protected]; [email protected];. [email protected] ABSTRACT
The formation enthalpy, - 3752.3 ± 4.7 kJomol-', of Hf-zirconolite, CaHfTi 20 7, was obtained using high temperature oxide-melt solution calorimetry. Combined with heat capacity data obtained using low temperature adiabatic calorimetry we report the heat capacity (Cp) and for Hf-zirconolite elements) ASO, and AG6, (AH.E .... energetics molarK formation the is made. Zr-zirconolite with oft,Hf-zirconolite K. Comparison to T = 1500 T = 298.15 fromstandard INTRODUCTION
The United States has chosen to utilize a ceramic waste material for the disposition of excess weapons plutonium and uranium. The waste form is expected to contain phases closely related to the naturally occurring minerals zirconolite {(Act,Ca)(Act,Zr)Ti 2 OA} , and/or 4 4 2 pyrochlore (CaActTi 20 7) where Act is an actinide element, primarily U" and Pu1 . These minerals are extremely refractory and have been shown to have retained their natural loadings of actinide elements over periods of up to billions of years in a variety of geologic weathering conditions. 1, 2 An understanding of the thermodynamics of the various phases to be incorporated into a ceramic nuclear waste form can assist in optimizing proposed waste form compositions; for waste loading, for geologic stability, and for manufacturing processes. A database of thermodynamic quantities for each of these systems needs to be obtained durability and stability of zirconolite or other ceramic waste forms can be properly the before assessed for long-term geologic disposal. Unfortunately, the highly refractory nature of these materials, the very property that makes these materials desirable for nuclear waste disposal, results in low solubility and makes obtaining much of the thermochemical data problematic. 3 Recent developments in calorimetric techniques have made it possible to overcome this difficulty. 45 With the recent publication of our study of Zr-zirconolite3' ' we have continued to work to the disposition of related phases other of understanding toward a better thermodynamic weapons plutonium. Among these phases are Hf-zirconolite, CaHfFi 2O2 , because hafnium will be incorporated in the final waste forms as a neutron poison to minimize criticality concerns.6 We have successfully prepared single phase Hf-zirconolite for thermodynamic characterization. EXPERIMENTAL
We have applied two different calorimetric techniques, low-temperature adiabatic calorimetery4 ' 7' 8 and high-temperature oxide melt solution calorimetry 6,1"0 to obtain the formation energetics', heat capacities", and third law entropies" of Hf-zirconolite, CaHfTi 20 7, from T= 0 K t
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