Solubility of PuO₂ • xH₂Ο in Alkaline Hanford High-Level Waste Solution

Abstract

Plutonium hydrous oxide /Solubility /Hanford high-level waste / NaOH (aq) SummaryThe solubility of Pu(IV) hydrous oxide, PuO, · χ Η, Ο, in airequilibrated synthetic Hanford high-level waste solutions was determined as a function of NaOH, NaAl(OH)", NaN0 3 , NaNO,, and Na, C0 3 concentrations. The solubility was found to increase with the square of the NaOH chemical activity. The components NaN0 3 and NaNO, increased Pu0 2 · χ Η, Ο solubility by increasing NaOH activity. Aluminate increased solubility by the apparent formation of a 1: 1 complex with the dissolved plutonium species while carbonate increased solubility by forming a 1 : 2 plutonium-carbonate complex. Spectral, electrochemical, and solubility evidence points to the existence of hydroxidecomplexed Pu (V) dissolved species.

Tanks Focus Area Alternative Salt Processing Research and Development Program Plan

“…Literature data indicate plutonium exhibits multiple oxidation states in alkaline aqueous solutions. 11,12 Existing studies do not provide definitive identification of Pu oxidation states in the range of solution compositions that will exist during salt solution processing. Identifying the Pu oxidation states and determining the extent and rate of removal of each oxidation state would decrease the uncertainty in predicting Pu removal behavior under varying waste compositions.…”

Potential for criticality in Hanford tanks resulting from retrieval of tank waste

“…Solubility data for tetravalent actinides (Rai and Ryan 1983, Rai et al 1995) indicate lack of amphoteric behavior, thus the formation of Pu (OH); species in solution appears to be unproven. However, solubility data obtained by Delegard (1987) The stability constants for Pu(1V) hydrolytic species are listed in Table 2.2. The constants indicate that plutonium in the tetravalent state forms more stable complexes than plutonium in tri-, penta-, and hexavalent states.…”

“…Literature is available that supports the choice of a partially crystalline Pu(IV) oxide described as PuO2~xH2O as the solubility-controlling solid in the 2.11 chemical environments inside a Hanford Site tank. Key references where efforts were made to directly determine the nature of the plutonium solid are Rai et al (1980), Delegard (1987), and Carteret (1996o. Other references that provide empirical data on observed plutonium solution concentrations in chemical environments similar to those inside a Hanford Site tank, but do not attempt to directly determine which solid phase is present, include Hobbs et a1.…”

“…Studies conducted by Andelman and Rozzell(l970) showed the bulk of freshly precipitated PuO2*xH2O exists as colloids that cannot be settled through centrifuging. Studies of long-term aging of PuO2*xH2O by Rai et al (1980) and Delegard (1987) showed decreases in solubility, which indicates that particle growth occurred, along with a concomitant increase in crystallinity (longrange structural ordering). Furthur, X-ray diffraction data obtained on aged (over 3 years) PuO2*xH2O precipitates showed the precipitate had crystallized, resulting in particle growth and decreased solubility (Rai et al 1980).…”

Plutonium Solubility in Simulated Savannah River Site Waste Solutions

“…For example, Delegard reported that the solubility of Pu in alkaline solutions was strongly influenced by the concentrations of hydroxide, aluminate and nitrate. [5] Given the wide range of the anion concentrations, binary and ternary complexes could be important in some cases. Nitrate and nitrite can serve as oxidizing and reducing agents that would alter the redox characteristics of the solution and, therefore, influence the oxidation state of the Pu.…”