Plutonium stability

Despite the problems of direct experimental evaluation of plutonium stability constants, they are needed in modeling of the behavior of plutonium in reprocessing systems in waste repositories and in geological and environmental media. Actinide analogs such as Am+3, Th+, NpOj and UOj2 can be used with caution for plutonium in the corresponding oxidation states and values for stability constants of these analogues are to be found also in reference 20. 

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

Example 4.6 plutonium stabilization at Los Alamos National Laboratories [4]... 

Currently, the nitric acid used in plutonium stabilization operations at a particular facility is evaporated to remove dissolved solids, assayed for radioactive content and then sent via underground pipe to a low-level waste handling facility. The acid stream is then neutralized with caustic to remove radioactivity and the resulting solids are immobilized in cement as a TRU (trans-uranic waste). The filtrate is then sent for... 

A1.0 L/min stream of a 2 M (22.3 wt% acid) HNO3 solution will be fed to a preheater prior to entering the distillation column. The preheater will raise the temperature of the feed to just below its bubble point. The stream which is to be disposed will be treated to less than 0.007 M HNO3 (450 ppm). The bottoms from the column must be concentrated to 12 M (62.9 wt% acid) HNO3 before it can be reused in the plutonium stabilization process. An azeotrope exists at 15.6 M HNO3 (45 wt%). Find the number of equilibrium stages in the column, assuming the overall efficiency is about 0.7. Solution ... 

The 12 M HNO3 stream which will be re-used for plutonium stabilization will reduce the cost of the process currently the nitrates are discarded as waste. The savings in waste storage and disposal costs is estimated to be US 100,000 annually. 

Phosgene can be employed in a variety of metal-recovery operations, eg, in the recovery of platinum, uranium, plutonium, and niobium (69—73). Phosgene has been proposed for the manufacture of aluminum chloride, beryllium chloride, and boron trichloride (74—76). Phosgene has been patented as a stabilizer, either by itself or in combination with thionyl chloride, for Hquid SO2 (77). 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

T. W. Newton, D. E. Hobart, and P. D. Palmer, The Preparation and Stability of Pure Oxidation States of Neptunium, Plutonium, andMmericium, LAUR-86-967, Los Alamos National Laboratory, Calif., 1986. 

Stability and Electronic Spectrum of Cesium Plutonium Hexafluoride... 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

Measurement of the stability constants of plutonium complexes is hampered by difficulties of maintaining a particular oxidation state. Formation of complexes of Pu+3, except in very acid solutions, is accompanied and often obscured by complexation catalyzed oxidation to Pu+lt. Study of complexation of Pu+lt is often confused by competition with hydrolysis above pH 1-2. 

Stability Constants, Enthalpies, and Entropies of Plutonium(III) and Plutonium(IV) Sulfate Complexes... 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

Prediction of the chemistry of plutonium in near-neutral aqueous media is highly dependent on understanding reactions that may be occurring in such media. One of the most important parameters is the stability and nature of complexes formed by plutonium in its four common oxidation states. Because Pu(III), Pu(IV), and Pu(VI) are readily hydrolysed, complexation reactions generally are studied in mildly to strongly acidic media. Data determined in acid media (and frequently at high concentrations of plutonium) then are used to predict the chemical speciation of plutonium at near-neutral pH and low concentrations of the metal ion. 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

In experiments where Mono Lake water was acidified to remove carbonate and bicarbonate ions and again adjusted to pH 10, more than 90 percent of the soluble plutonium moved to the sediment phase. When carbonate ion concentration was restored, the plutonium returned to solution—strong evidence of the importance of inorganic carbon to solubility in that system(13). Early studies with Lake Michigan water, which has low DOC, had also implicated bicarbonate and carbonate as stabilizing ligands for plutonium at pH 8(14). This latter research characterized the soluble species as mainly anionic in character. 

K3 is the conditional stability constant for the association of plutonium with the solid adsorber. 

The effect of fluoride was further demonstrated by the increase in plutonium solubility in deionized water from about 11 percent to essentially 100 percent by addition of sufficient NaF to raise the fluoride concentration to that of basalt ground water. It is likely that the enhanced solubility of plutonium in waters containing high fluoride concentrations is the result of stabilization of Pu(IV) in solution by formation of fluoride complexes. Normally Pu(IV) is the least soluble of the four... 

The ability of polyvalent cations leached from the glass to suppress the free-fluoride ion concentration in basalt ground water is difficult to assess. Fluoride definitely enhances leaching and is the primary cause of the high concentrations of dissolved plutonium in the basalt ground-water leachate. Once the plutonium is dissolved, however, it is not possible to determine what fraction is stabilized by fluoride as opposed to other species leached from the glass. 

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