Equilibrium plutonium

Table 3 equilibrium plutonium composition in the multi-recycling process in a CAPRA... 

One of the contributing factors to a thermal.reactor temperature coefficient is that due to different temperature dependence of fission and nonfission cross sections. It is the purpose of this work to examine this coefficient for the BER,.both initially and after equilibrium plutonium-239 has been attained. While the numerical results are valid only for BER, the qualitative aspects of the problem are applicable to similar reactors. 

Vapor Pressures and Vapor Compositions in Equilibrium with Hypostoichiometric Plutonium Dioxide at High Temperatures... 

Vapor pressures and vapor compositions in equilibrium with a hypostoichiometric plutonium dioxide condensed phase have been calculated for the temperature range 1500 I H 4000 K. Thermodynamic functions for the condensed phase and for each of the gaseous species were combined with an oxygen-potential model, which we extended from the solid into the liquid region to obtain the partial pressures of O2, 0, Pu, PuO and Pu02 as functions of temperature and of condensed phase composition. The calculated oxygen pressures increase rapidly as stoichiometry is approached. At least part of this increase is a consequence of the exclusion of Pu +... 

One of the most Important thermophysical properties of reactor fuel In reactor safety analysis Is vapor pressure, for which data are needed for temperatures above 3000 K. We have recently completed an analysis of the vapor pressure and vapor composition In equilibrium with the hypostolchiometric uranium dioxide condensed phase (1 ), and we present here a similar analysis for the plutonium/oxygen (Pu/0) system. 

Blackburn also included in his model an equilibrium between trivalent and divalent plutonium. Because our interest lies principally in the region for which x < 0.1, the divalent ion concentration is relatively unimportant (21). 

Table III. Ionization Potentials (IP) of the Molecules and Atoms In Equilibrium with a Plutonium Dioxide Condensed Phase (1 eV molecule- - - 23.06 kcal mol- =...

An alternative way to view the oxygen enrichment of the vapor relative to the condensed phase Is to calculate the oxygen-to-plutonium ratio of the gas, R(gas), with Eq. (2). The value of R(gas) exceeds that of the condensed phase with which It Is In equilibrium by a large amount. Like the U/0 system, this oxygen enrichment of the vapor relative to the condensed phase Is Increasing with temperature. One Implication of these results Is that the condensed-phase and vapor-phase compositions will depend upon the extent of vaporization of a sample with overall composition given by 0/Pu = 2 - x. 

Equilibrium with the Hypostoichiometric (0/M = 1.96) Dioxides of Plutonium and Uranium... 

Measurement of the 0/Pu ratio of the vapor in equilibrium with plutonium dioxide liquid. 

Green, D. W. Fink, J. K. Leibowitz, L. "Vapor Pressures- and Vapor Compositions in Equilibrium with Hypostoichiometric Uranium-Plutonium Dioxide at High Temperatures," presented at the 8th European Conference on Thermophysical Properties, Baden-Baden, September 27 - October 1, 1982 to be published in High Temperatures-High Pressures. 

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. 

After observing the photochemical reduction of Pu(VI) and Pu(IV), it seems obvious that reaction (3) should be light-sensitive. However, it is not obvious how photons would affect the equilibrium concentrations of the plutonium species. The experimental results [3,4] are very interesting and are described below, but a complete explanation is yet to be developed. 

After dark-condition equilibrium was established, as indicated by the visible spectra, the photo-shift in equilibrium was observed to be completely reversed when the illumination ceased. This photogalvanic effect maintained a mass balance in the system, with no reagent consumed or generated during the dark-light-dark cycle. This observation suggested that the plutonium system in the proper network of a concentration cell... 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Rain in equilibrium with atmospheric C02, but uncontaminated by industrial emissions, should have a pH of 5.7. However, atmospheric pollution from burning fossil fuels has resulted in acid rain of pH as low as 3.5 (24). If this condition continues for a long time, it may lead to a change in groundwater composition, which may considerably change the migration of plutonium in nature. 

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. 

A general conclusion from the review of the distribution of plutonium between different compartments of the ecosystem was that the enrichment of plutonium from water to food was fairly well compensated for by man s metabolic discrimination against plutonium. Therefore, under the conditions described above, it may be concluded that plutonium from a nuclear waste repository in deep granite bedrock is not likely to reach man in concentrations exceeding permissible levels. However, considering the uncertainties in the input equilibrium constants, the site-specific Kd-values and the very approximate transport equation, the effects of the decay products, etc. — as well as the crude assumptions in the above example — extensive research efforts are needed before the safety of a nuclear waste repository can be scientifically proven. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

Eh-pH diagram for the speciation of plutonium in equilibrium with Pu02 in water (10). 

The mechanisms by which Pu(IV) is oxidized in aquatic environments is not entirely clear. At Oak Ridge, laboratory experiments have shown that oxidation occurs when small volumes of unhydrolyzed Pu(IV) species (i.e., Pu(IV) in strong acid solution as a citric acid complex or in 45 percent Na2Coj) are added to large volumes of neutral-to-alkaline solutions(23). In repeated experiments, the ratios of oxidized to reduced species were not reproducible after dilution/hydrolysis, nor did the ratios of the oxidation states come to any equilibrium concentrations after two months of observation. These results indicate that rapid oxidation probably occurs at some step in the hydrolysis of reduced plutonium, but that this oxidation was not experimentally controllable. The subsequent failure of the various experimental solutions to converge to similar high ratios of Pu(V+VI)/Pu(III+IV) demonstrated that the rate of oxidation is extremely slow after Pu(IV) hydrolysis reactions are complete. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

This PUCI3 also acts as a salt-phase buffer to prevent dissolution of trace impurities in the metal feed by forcing the anode equilibrium to favor production (retention) of trace impurities as metals, instead of permitting oxidation of the impurities to ions. Metallic impurities in the feed fall into two classes, those more electropositive and those less electropositive than plutonium. Since the cell is operated at temperatures above the melting point of all the feed components, and both the liquid anode and salt are well mixed by a mechanical stirrer, chemical equlibrium is established between all impurities and the plutonium in the salt even before current is applied to the cell. Thus, impurities more electropositive than the liquid plutonium anode will be oxidized by Pu+3 and be taken up by the salt phase, while impurities in the electrolyte salt less electropositive than plutonium will be reduced by plutonium metal and be collected in the anode. 

NaCl-KCl-PuCl3-MgCl2, under near-equilibrium conditions. Virtually all of the impurities concentrate in the anode. Of the impurities usually present in plutonium, only americium concentrates in the salt. 

Since the water movement will be very slow compared with the rate at which the wastes dissolve, we are concerned first and foremost with equilibrium solubility. Also, if only to relate behaviour on the geological time scale to that on the laboratory time scale, we will need to know about the mechanisms and kinetics of dissolution and leaching. The waste forms envisaged at present are glass blocks containing separated fission products and residual actinides fused into the glass and, alternatively, the uranium dioxide matrix of the used fuel containing unseparated fission products and plutonium. In the... 

Even though the solubility product of Pu(OH)4 is 1 X 10 56, some Pu4+ must remain in solution as the equilibrium is established. The monomeric Pu(OH)4 and the very low molecular weight polymeric species are able to pass through an ultrafilter, and Lindenbaum and West-fall (22) found that as much as 5% of the hydrolysis species remained ultrafilterable after 72 hours at pH 11. These unfilterable species may be either true radiocolloids or pseudocolloids. The latter likely occur as a result of minute impurities in the solutions which act as nuclei on which the polymeric or ionic species adsorb (14). However, this point has been the subject of extensive debate (36, 37, 39), and opinions vary as to whether pseudocolloids form in this manner, or in fact whether there are such species at all. In general the term colloidal plutonium will be used throughout this paper to indicate all of the insoluble plutonium hydrolysis products and polymeric species of colloidal size. 

Table I. Selected Equilibrium Constants for Aqueous Plutonium Reactions0...

Table II. Standard Reduction Potentials (at 25°C.) for Various Plutonium Hydroxides in Equilibrium with Pu(OH)4 (21)...

For the reduction of Pu02(0H)2 to Pu(OH)4, if Eh of the solution is less than +0.51 minus 0.059 pH, the Pu(OH)4, is the stable species. Pu(OH)3 cannot exist as a solid form in equilibrium with water because if Eh becomes less than zero minus 0.059 pH, water would be reduced to hydrogen. The pentavalent hydroxide form, PuQ2(OH), is unstable in both acidic and basic aqueous solutions (21) and converts to either the hexavalent or the tetra or trivalent forms because of its E°. Thus, if Eh is greater than 0.51 — 0.059 pH, PuC OHJa will be the stable form, while below this Eh Pu(OH)4 will be, these being the only stable plutonium hydroxides likely to be found in equilibrium with water. The question remains as to the likely Eh to be encountered in natural waters or even under laboratory conditions. Baas Becking, Kaplan, and Moore (1) have collected many reports of Eh values in natural waters. In an... 

However, these solubility relationships will change with Eh—e.g.9 as Eh becomes more positive, the Pu3+ line will be lower, while those for Pu(OH)3+ and Pu02+ will be raised. Also the presence of carbonate will raise the solubility line for Pu(IV) significantly. Nevertheless, in the neutral region of pH Figure 2 indicates that at the Eh used here, a typical environmental value, the solubility of plutonium in equilibrium with solid Pu(OH)4 is quite low. 

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