Hafnium Hydroxide Complexation and Solubility: the Impact of Hydrolysis Reactions on the Disposition of Weapons-Grade Pl
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13.84 ± 0.54 25.71 + 0.39 36.24 + 0.88 45.52 ± 0.80
The solubility product of Hf(OH) 4 (s) is determined in 0.1 M NaC10 4 by measuring the in solution that is in equilibrium with an excess of hafnium hydroxide solid under hafnium total an argon atmosphere. The total Hf concentration is determined by ICP-AES. The solubility product is determined using the stability constants measured for the H f hydrolysis products in 0.1 M NaCl0 4. The precipitate examined is confirmed to be a hydroxide by IR spectroscopy. For Hf(OH) 4 (s) in 0.1 M NaC10 4, the solubility product is log
Ksp
(Hf(OH) 4 (s)) = -51.8 + 0.5
The solubility and stability constants determined are used, along with literature values for plutonium solubility and complexation constants, to examine the behavior of hafnium and plutonium under the conditions expected at Yucca Mountain. INTRODUCTION The current U.S. strategy for the disposition of surplus weapons-grade plutonium recovered from dismantled nuclear weapons calls for the development of two dispositioning options: burning the WGPu as fuel in commercial nuclear reactors and directly disposing of the WGPu in the high level waste repository in some immobilized form. The current formulation of the WGPu immobilization host phase consists primarily of pyrochlore with smaller amounts of hafnium-zirconolite, rutile, and brannerite. In this formulation, the zirconium is completely replaced by hafnium, uranium, plutonium, and gadolinium. The hafnium, along with gadolinium, is present in molar amounts equal to that of the WGPu to serve as a long-term
1025 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
criticality control element for the geological disposal of immobilized WGPu [1,2]. As such, the behavior of hafnium under environmental conditions needs to be better understood. The first step in understanding the behavior of hafnium in the environment is to first understand the interaction of hafnium with the hydroxide ion. The work described in this paper covers the investigation of the hydrolysis constants for hafnium as well as the determination of the solubility product of hafnium hydroxide. The stability constants for hafnium hydroxide complexes are determined by potentiometric titration at varying ionic strengths. The infinite dilution values (I, = 0) of the stability constants are determined from these values using the Specific Ion Interaction theory (SIT) [3]. The solubility of Hf(OH) 4 is determined in 0.1 M NaC1O 4 by solubility experiments. Table I contains values for the stability constants for hafnium hydroxide species found in the literature. The references listed in Table I are compilations and contain the references for the actual experiment. The solubility products shown from [4] are calculated from the hydrolysis product using a log Kw - 13.995. All of the constants listed are at an ionic strength of 0.
Table I: Stability Constants for the Hf+/OH System from the Literature Reference log 3 Species [5] 13.7 Hf(OH) 3 7/[Hf]*[OH-] [4] 13.75 Hf(OH) 3 7[Hf 4 i]*[OH-] [5] 25
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