Plutonium polymers

The depolymerization of the plutonium polymers is a very slow reaction at room temperatures and moderate acid concentrations. It is only upon heating or by the use of very concentrated acid that the depolymerization rate can be made appreciable. Kraus (16) has compared, in two nitric acid media, 6M and 10M, the depolymerization of already formed polymers—one at room temperature and the other at 95 °C. for one hour. The values for the half time of the reactions ranged from 29 minutes for the non-heated, 10M HN03 solution to 730 minutes for the heated, 6M HN03 solution. Lindenbaum and Westfall (22) found that at pH 7.8, a region of environmental interest, the rate of depolymerization was only about 0.04% per hour and at pH 4.0 increased to only 0.18% per hour. 

Irradiation also affects the course of more conventional separation processes. Visible and ultraviolet light have been found to affect plutonium solvent extraction by photochemical reduction of the plutonium (12). Although the results vary somewhat with the conditions, generally plutonium(VI) can be reduced to pluto-nium(IV), and plutonium(IV) to plutonium(III). The reduction appears to take place more readily if the uranyl ion is also present, possibly as a result of photochemical reduction of the uranyl ion and subsequent reduction of plutonium by uranium(IV). Light has also been found to break up the unextractable plutonium polymer that forms in solvent extraction systems (7b,c). The effect of vibrational excitation resulting from infrared laser irradiation has been studied for a number of heterogeneous processes, including solvent extraction (13). 

The presence of DTPA should prevent the formation of plutonium polymer during the transition from pH = 9 to less than pH = 0.3. Above 1 M in hydrogen ion concentration, H5DTPA is no longer very effective as a chelating agent and thus does not affect the Dfs of actinides in the ARALEX process. In addition, DTPA in the Na2C03 scrub also prevents the formation of plutonium hydroxide when macro concentrations of plutonium are present. 

Plutonium reduction. Reduction of plutonium to Pu(III) is completed by adding concentrated hydroxylamine (with hydrazine as holding reductant) to the aqueous raffinate leaving the HC column. The mixture must be held long enough, half an hour or more [B2], to complete the rather slow reduction to Pu(III). To hasten the reaction, the hydroxylamine concentration should be high and the nitric acid concentration as close to 0.3 M as possible without risking plutonium polymer formation. 

Figure 10.38 Plutonium polymer formation limits. (From Mann and Irene [M3].)...

Plutonium polymer. At low acidity and high temperature, plutonium forms a polymer that deposits as an insoluble solid film on the walls of process equipment. Polymer deposition plugs lines, fouls surfaces, and may result in unanticipated accumulation of a critical mass of plutonium. Figure 10.38 summarizes [M3] the results of investigations of the combinations of low acidity and high temperature that must be avoided if plutonium polymer formation is to be prevented. 

As an additional precaution, process equipment in which plutonium polymer might form should be soaked periodically in boiling, concentrated nitric acid. If plutonium is found in solution, the presence of a polymer deposit is indicated. Complete removal may require addition of 0.01 to 0.1 MHF to the hot HNO3. 

M3. Marm, S., and A. R. Irene A Study of Plutonium Polymer Formation and Precipitation as Applied to LMFBR Fuel Reprocessing, Report ORNDTM-2806, Dec. 22, 1969. 

In addition, TOPO has the advantage of being able to sorb the hydrolytic plutonium polymer, which is a common component of plutonium waste streams, whereas anion exchange resins cannot do so [72]. Fortunately, American Cyanamid has introduced a low-cost alternative to TOPO, which also extracts plutonium polymer from nitric acid [73]. This material, called Cyanex 923, is a mixture of trialkyIphosphine oxides, and its low cost makes this approach for the recovery of plutonium more realistic. 

In laboratory animals that received plutonium by intravenous injection, most plutonium was deposited in the liver and skeleton. No differences in distribution between plutonium-238 and plutonium- 239 were reported in mice (Andreozzi et al. 1983) however, Ballou et al. (1967) reported that in rats deposition in the liver and other soft tissues was twice as great after intravenous administration of plutonium-239 than after administration of plutonium-238. In dogs, the concentration of plutonium polymer decreased in the lungs, spleen, and liver with time and increased in the skeleton and kidney (Stevens et al. 1976). 

Experiences with aqueous chemistry and behavior of the transuranium elements obtained in nuclear fuel reprocessing and plutonium processing are only of limited relevance for PWR primary coolants with the extremely low concentrations of these elements in a boric acid—LiOH solution of varying composition. The plutonium polymers which are formed in less acid and neutral solutions and which have been reported to show the highest plate-out potential (e. g. Wilkins and Wisbey,... 

Plutonium(IV) polymer has been examined by infrared spectroscopy (26). One of the prominent features in the infrared spectrum of the polymer is an intense band in the OH stretching region at 3400 cm 1. Upon deuteration, this band shifts to 2400 cm 1. However, it could not be positively assigned to OH vibrations in the polymer due to absorption of water by the KBr pellet. In view of the broad band observed in this same region for I, it now seems likely that the bands observed previously for Pu(IV) polymer are actually due to OH in the polymer. Indeed, we have observed a similar shift in the sharp absorption of U(0H)2S0ir upon deuteration (28). This absorption shifts from 3500 cm 1 to 2600 cm 1. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The study of plutonium hydrolysis is complicated by the formation of oligomers and polymers once the simple mononuclear hydrolysis species start forming. The relative mono-oligomer concentrations are dependent on the plutonium concentration - e.g. the ratio of Pu present as (Pu02)2(0H)22 to that as PuO2(0H)+ is 200 for [Pulx = 0.1 M, 5.6 for 10-1 M and 0.05 for 10 8 M. 

The techniques used in the work have generally been spectroscopic visible-uv for quantitative determinations of species concentrations and infrared-Raman for structural aspects of the polymer. Although the former has often been used in the study of plutonium systems, there has been considerably less usage made of the latter in the actinide hydrolysis mechanisms. 

The implication of these two examples is that the medium in which the Pu(IV) hydrolysis chemistry is studied has a strong bearing on the outcome of the results. In the past, we were content to treat the pure systems and either ignore external interferences (such as the atmosphere) or infer the behavior of mixtures (such as Pu + and U02 " ") based on the known chemistries of the individual species. The example of U02 + interactions with Pu(IV) polymer demonstrates that neither of these approaches is accurate. Therefore, future research efforts will necessarily have to consider plutonium hydrolysis reactions in more detail than has previously been done. 

Reflux Experiments. More recent efforts have been directed at a quantitative evaluation of those parameters that affect polymer growth, namely acidity, plutonium concentration, temperature, and reflux action. The last is an interesting example to illustrate since the admission of low acid condensates or diluents to a Pu(IV) solution causes some polymer formation even when the bulk solution is otherwise acidic enough to prevent any measurable degree of hydrolysis. 

This untimely polymer formation is understood to be caused by the very rapid hydrolysis and aggregation of monomeric Pu(IV) species (at the region of condensate reentry into the hot plutonium solution) to produce hydrous polymers that are not readily depolymerized. At high temperatures such as found under reflux conditions, the polymer rapidly ages through the conversion of hydroxyl- to oxo-bridges ... 

Although the effect of reflux on polymer formation has been recognized for many years, little detailed information is available concerning the extent to which changes in temperature, acidity, and plutonium concentration affect it. 

Table 2 shows the effect of reflux on high concentrations of plutonium (up to 0.55 M) in comparison to nonreflux situations of comparable acidities. Again, the nonreflux solutions did not polymerize above a fixed acid concentration (in this case 3M) whereas all solutions that were refluxed showed the formation of polymer. 

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The primary reason for studying aqueous plutonium photochemistry has been the scientific value. No other aqueous metal system has such a wide range of chemistry four oxidation states can co-exist (III, IV, V, and VI), and the Pu(IV) state can form polymer material. Cation charges on these species range from 1 to 4, and there are molecular as well as metallic ions. A wide variety of anion and chelating complex chemistry applies to the respective oxidation states. Finally, all of this aqueous plutonium chemistry could be affected by the absorption of light, and perhaps new plutonium species could be discovered by photon excitation. 

Visible and UV spectrophotometric techniques are most convenient for studying the polymer and various oxidation states of plutonium. The spectra of the plutonium states and the procedure for resolution of the concentrations were previously described (9 ). Changes in the relative concentrations of the oxidation states and of the polymer generally are determined from corresponding changes in the spectra and a comparison of the changes to standard spectra of the various states. These techniques have been used exclusively for studying the photochemistry of aqueous plutonium. 

Plutonium(IV) polymer is a product of Pu(IV) hydrolysis and is formed in aqueous solutions at low acid concentrations. Depolymerization generally is accomplished by acid reaction to form ionic Pu(IV), but acid degradation of polymer is strongly dependent on the age of the polymer and the conditions under which the polymer was formed (12). Photoenhancement of Pu(IV) depolymerization was first observed with a freshly prepared polymer material in 0.5 HClOh, Fig. 3 (3 ). Depolymerization proceeded in dark conditions until after 140 h, 18% of the polymer remained. Four rather mild 1-h illuminations of identical samples at 5, 25, 52, and 76 h enhanced the depolymerization rates so that only 1% polymer remained after the fourth light exposure (Fig. 3). 

Toth, L. M. Osborne, M. M. "Further Aspects of Pu(IV) Hydrous Polymer Chemistry," Symposium on the Chemistry of Plutonium, American Chemical Society Meeting, Kansas City, Missouri, September 1982. 

Silver was critical of the lack of use by plutonium chemists of a-coefficients. Assuming that Silver was referring to a-coeffi-cients defined as the fraction of the total concentration of a substance that exists as a particular species, he was wrong to say that plutonium chemists have not used them. Phil Horwitz at ANL has used them. Publications from ORNL have reported them to easily show relative concentrations of plutonium species, and L. M. Toth used such a-coefficients as percent of Pu(IV) polymer in his symposium talk Tuesday. Alpha coefficients are a commonly used, simple concept - certainly since Ringbom s article in the Journal of Chemical Education in 1958."... 

Taya A, Hotz G, Seidel A. 1986. Biochemical and electron microscopic studies on binding and transport of americium and plutonium hydroxide polymers in bovine alveolar macrophages and rat lungs. J Aerosol Sci 17(3) 370-375. 

Binary plutonium halides, 79 689 Binary plutonium oxide, 79 688 Binary polymer blends, 20 330-334, 343. See also Binary heterogeneous polymer blends... 

---