Plutonium oxides formation

Equations for the Standard Free Energy of Formation and Partial Molar Free Energies of Atomic Oxygen for Plutonium Oxides (1600-2150 K), cal/mol... 

Thermodynamic Functions of the Gases. To apply Eqs. (1-10), the free energies of formation, Ag , for all gaseous species as a function of temperature are required. Tabulated data were fit by a least-squares procedure to derive an analytical equation for AG° of each vapor species. For the plutonium oxide vapor species, the data calculated from spectroscopic data (3 ) were used for 0(g) and 02(g), the JANAF data (.5) were used and for Pu(g), data from the compilation of Oetting et al. (6) were used. The coefficients of the equations for AG° of the gaseous species are included in Table I. 

Salt Transport Processing (8, 9, 10, 11) The selective transfer of spent fuel constitutents between liquid metals and/or molten salts is being studied for both thorium-uranium and uranium-plutonium oxide and metal fuels. The chemical basis for the separation is the selective partitioning of actinide and fission-product elements between molten salt and liquid alloy phases as determined by the values of the standard free energy of formation of the chlorides of actinide elements and the fission products. Elements to be partitioned are dissolved in one alloy (the donor... 

Radiochemical analyses of PWR primary coolant show that the major fraction of the neptunium and plutonium traces in the coolant is usually associated with the corrosion product suspended solids and that it can be removed from the coolant by filtering. Only in cases of very low corrosion product concentrations in the coolant were significant proportions of the transuranium elements observed to be present in a dissolved (i. e. non-filtrable) form. As yet, it is not known whether mixed oxide formation between magnetite-type oxides and these elements is responsible for this behavior or whether the actinide traces are adsorbed by van de Waals forces onto the large surface areas of the finely dispersed suspended solids. Under constantload operation conditions and as long as no additional fuel rod failures occur, the activity ratio Pu Co in the corrosion products remains virtually constant over time, thus indicating a similar behavior of these different elements in the coolant. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and hahdes. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

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. 

Only the obvious studies of aqueous plutonium photochemistry have been completed, and the results are summarized below. The course of discussion will follow the particular photochemical reactions that have been observed, beginning with the higher oxidation states. This discussion will consider primarily those studies of aqueous plutonium In perchloric acid media but will include one reaction in nitric acid media. Aqueous systems other than perchlorate may affect particular plutonium states by redox reactions and complex formation and could obscure photochemical changes. Detailed experimental studies of plutonium photochemistry in other aqueous systems should also be conducted. 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

Fallout plutonium arrives in natural waters either by direct atmospheric deposition or by erosion and/or dissolution from the land. Although in the past, this plutonium was considered to be in a refractory form due to formation within the fire ball, it seems more likely that most of the plutonium originated in the stratosphere by the decay of 239Np (from 239U formed during the detonation)(4). Deposition occurs predominantly with one or a few atoms incorporated in a raindrop. Investigations by Fukai indicate that collected rain contains soluble plutonium which has oxidation states that are almost totally Pu(V+VI)05). 

Observations of the ratio of oxidized plutonium to reduced plutonium may provide some insight to the observations of erratic formation and lack of equilibration in laboratory solutions at ORNL versus fairly consistent and predictable behavior in oligo-trophic lakes and marine systems. In coastal water and the relatively shallow Lake Michigan, Pu(V) is about 90 percent of the soluble plutonium, but in the upper waters of the open ocean, where it does not interact with the seafloor due to the depths,... 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

C, the rate of reaction tends to be self limiting at hydrogen pressures up to 10 torr. The hydriding technique is used to recover metallic plutonium residues clinging to the walls of ceramic crucibles, and can also be used to recover machining scrap if the feed is free of lubricants or oxides. Mulford and SturdyO4) have found the heat of formation for the reaction... 

The various oxidation states of plutonium exhibit characteristic absorption spectra in the ultraviolet, visible and infrared regions. Each oxidation state is sufficiently distinct that its reaction can be monitored during hydrolysis and complex formation. Various research groups have studied the relationship between oxidation and absorption spectra (6-9). The absorption spectra may respond to complex formation or hydrolysis Nebel (10) has shown that the absorption peak of Pu(IV) shifts from 470 nm to 496 nm when Pu(IV) complexed with two molecules of citrate. 

The Purex process, ie, plutonium uranium reduction extraction, employs an organic phase consisting of 30 wt % TBP dissolved in a kerosene-type diluent. Purification and separation of U and Pu is achieved because of the extractability of U02+2 and Pu(IV) nitrates by TBP and the relative inextractability of Pu(III) and most fission product nitrates. Plutonium nitrate and U02(N03)2 are extracted into the organic phase by the formation of compounds, eg, Pu(N03)4 -2TBP. The plutonium is reduced to Pu(III) by treatment with ferrous sulfamate, hydrazine, or hydroxylamine and is transferred to the aqueous phase U remains in the organic phase. Further purification is achieved by oxidation of Pu(III) to Pu(IV) and re-extraction with TBP. The plutonium is transferred to an aqueous product. Plutonium recovery from the Purex process is ca 99.9 wt % (128). Decontamination factors are 106 — 10s (97,126,129). A flow sheet of the Purex process is shown in Figure 7. 

The plutonium solubility increased in the presence of increased NaN03, NaOH, and NaA102 concentrations. According to the literature, Pu(V) should be the stable oxidation state in alkaline NOjT-NO solutions (8). It has been observed that the solubility of Pu(V) increases as tne NaOH concentration increases (8.11) probably this occured due to formation of the more soluble anionic hydroxide complexes of Pu(V) such as PuO,(OH)2 (11). Sodium nitrate and NaAlO, may have increased Pu(V) solubility through complexation. Sodium nitrate also may have increased plutonium solubility by oxidizing the less soluble Pu(IV), initially present in the tracer solids, to Pu(V). 

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