Actinide impurities

The tetragonal RE, Y, and Sc orthophosphates in particular have been widely used as host media for a variety of solid state chemical, spectroscopic, magnetic resonance, neutron and other studies of rare-earth and actinide impurities. These materials have proved to be ideal hosts for the incorporation of other rare-earth dopants (e.g., Er-doped Lu(P04) for microlaser studies). Doped orthophosphates with desired levels of dopants are desirable for both basic investigations and applications. Unfortunately, there are apparently no available quantitative data on the segregation coefficients for the rare earths in the tetragonal orthophosphates. 

Relativistic Ab Initio Model Potential embedded cluster calculations on the structure and spectroscopy of local defects created by actinide impurity ions in solid hosts are the focus of attention here. They are molecular like calculations which include host embedding effects and electron correlation effects, but also scalar and spin-orbit coupling relativistic effects, all of them compulsory for a detailed understanding of the large manifolds of states of the 5f" the 5f" 6d configurations. The results are aimed at showing the potentiality of Relativistic Quantum Chemistry as a tool for prediction and interpretation in the field of solids doped with heavy element impurities. 

From the theoretical point of view, having shown that we have relativistic Hamiltonians and procedures able to predict with sufficient quality 5/ and 6d states of actinide impurity ions with simple manifolds, this system is specially adequate to monitor their performance when the manifolds become more complex and to substantially show that quantum chemical relativistic ab initio calculations are definitely useful for the interpretation and prediction of physical processes involving the 5f" and 5f" 6d manifolds of actinide impurities. 

In a recent series of articles [179-181], Seijo and Barandiaran have investigated the spectroscopy of several actinide impurities (Pa" - -, and in crystal environments. In particular, they discuss the relative position of the 5 and 5/ " 6i/ manifolds (see also chapter 7 of this book). All calculations use relativistic large-core AIMPs on the actinide centres and on the chlorine ligands. The transferability of these frozen core potentials from the neutral / elements to their cation has been discussed in Ref [182]. The crystal environment is described by the AIMP embedding cluster method. Electron and spin-orbit interactions are treated simultaneously by the three-step spin-fi e-state-shifted method detailed in section 2.2.5, using either MRCI or CASSCF/MS-CASPT2 methods in the spin-fi ee step. The active space includes the 5/ and 6d orbitals of the actinide centre, as well as the Is orbitals in order to avoid the prob-... 

Figure 3 shows a flowsheet for plutonium processing at Rocky Flats. Impure plutonium metal is sent through a molten salt extraction (MSE) process to remove americium. The purified plutonium metal is sent to the foundry. Plutonium metal that does not meet foundry requirements is processed further, either through an aqueous or electrorefining process. The waste chloride salt from MSE is dissolved then the actinides are precipitated with carbonate and redissolved in 7f1 HN03 and finally, the plutonium is recovered by an anion exchange process. 

Mullins, L.J. Christensen, D.C. Babcock, B.R. "Fused Salt Processing of Impure Plutonium Dioxide to High Purity Metal", Los Alamos Nat. Lab. Report LA-9154-MS also Symposium on Actinide Recovery from Waste and Low Grade Sources, ACS, New York City August 23-28, 1981 (in press). 

Glasses are frequently colored with transition-metal ions such as Mn2+, Ni2+, Co2+, lanthanides, or actinides.2 The addition of Co2+ impurities to silica glass leads to a... 

Lisa Townsend, a technician in the Radiochemistry section of the Actinide Analytical Chemistry Group, analyzes bulk components and impurities in plutonium-238 materials used to fabricate heat sources used in space exploration. She utilizes a combination of ion exchange and solvent extraction techniques and determines component concentrations using alpha and gamma radio-counting instrumentation. 

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

Efficient refining of the more volatile actinide metals (Pu, Am, Cm, Bk, and Cf) is achieved by selective vaporization for those (Pu, Am, Cm) available in macro quantities. The metal is sublimed at the lowest possible temperature to avoid co-evaporation of the less volatile impurities and then deposited at the highest possible temperature to allow vaporization of the more volatile impurities. Deposition occurs below the melting point of the metal to avoid potential corrosion of the condenser by the liquid metal. Very good decontamination factors can be obtained for most metallic impurities. However, Ag, Ca, Be, Sn, Dy, and Ho are not separated from Am metal nor are Co, Fe, Cr, Ni, Si, Ge, Gd, Pr, Nd, Sc, Tb, and Lu from Cm and Pu metals. 

Nonmetallic impurities, mostly oxygen, found in actinide metals distilled under a vacuum of 0.1 mPa range from 4000 to 7000 atomic ppm. In a vacuum of 0.1 Pa the nonmetallic impurity content decreases to between 400 and 880 atomic ppm (51, 52). 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

All subsequent preparations of Cf metal have used the method of choice, that is, reduction of californium oxide by La metal and deposition of the vaporized Cf metal (Section II,B) on a Ta collector 10, 30, 32, 45, 91, 97, 120). The apparatus used in this work is pictured schematically in Fig. 16. Complete analysis of Cf metal for cationic and anionic impurities has not been obtained due to the small (milligram) scale of the metal preparations to date. Since Cf is the element of highest atomic number available for measurement of its bulk properties in the metallic state, accurate measurement of its physical properties is important for predicting those of the still heavier actinides. Therefore, further studies of the metallic state of californium are necessary. 

Some properties of berkelium metal have been reported. Thus, its melting point is 986 + 25 °C and its volatility, relative to its congeners, is in the order Cm < Bk < Am < Cf. Its chemical behaviour is described as somewhat similar to Sm, and it does not correspond, as a metal, to Tb or Lu. It reacts with hydrogen at 225 °C to give BkH2, which is isomorphous with other lanthanide and actinide hydrides of the type MH2+ (x < 1). BkO may be formed as an impurity in the production of metallic Bk. 

The different methods of actinide refining are based in part on experience in refining rare earth metals In these methods, actinide metals and their impurities undergo selective phase transitions like evaporation and condensation, melting and dissolution which result in a separation of the constituents of the sample to be purified. 

Efficient purification is achieved by selective evaporation and condensation. This technique is applicable to actinides of medium volatility i.e. Am or Cm The volatile impurities are eUminated by selective condensation of the actinide metal, less volatile impurities are left in the crucible. The efficiency of this refining method is determined by the relative evaporation ratio, which for two elements A and B equals the ratio of their activities at a given temperature. 

Actinide metal samples are characterized by chemical and structure analysis. Multielement analysis by spark source mass spectrometry (SSMS) or inductively coupled argon plasma (ICAP) emission spectroscopy have lowered the detection limit for metallic impurities by 10 within the last two decades. The analysis of O, N, H by vacuum fusion requires large sample, but does not distinguish between bulk and surface of the material. Advanced techniques for surface analysis are being adapted for investigation of radioactive samples (Fig. 11) ... 

However, the intra-atomic Coulomb interaction Uf.f affects the dynamics of f spin and f charge in different ways while the spin fluctuation propagator x(q, co) is enhanced by a factor (1 - U fX°(q, co)) which may exhibit a phase transition as Uy is increased, the charge fluctuation propagator C(q, co) is depressed by a factor (1 -H UffC°(q, co)) In the case of light actinide materials no evidence of charge fluctuation has been found. Most of the theoretical effort for the concentrated case (by opposition to the dilute one-impurity limit) has been done within the Fermi hquid theory Main practical results are a T term in electrical resistivity, scaled to order T/T f where T f is the characteristic spin fluctuation temperature (which is of the order - Tp/S where S is the Stoner enhancement factor (S = 1/1 — IN((iF)) and Tp A/ks is the Fermi temperature of the narrow band). 

Actinide Zeolite Tests. Although less work has been done with actinides than with lanthanides, certain differences in behavior have appeared consistently. It is not known whether these differences, which are always unfavorable, are caused by the actinides themselves or by impurities. Most work was done with 243Am and 241Am although a few tests included 244Cm, which has a much greater a emission than any target of interest. 

Among the lanthanides and actinides there are several metals with the unusual 4P (ABAC) structure, and Sm has the strange 9P (ABAB CBCAC) structure. There are uncertainties of the structures of some of the metals beyond U. For most of these metals only small samples are available, purity is a problem, and in some cases samples are deposited on a filament. Impurities and deposition on another metal can change the structure. 

Protactinium-233 and neptunium-239 diphthalocyanines are prepared from the corresponding thorium-232 and uranium-238 diphthalocyanines by element transformation [6]. The existence of Pa and Np di-Pcs is proven by repeated sublimation of the irradiated parent compounds using platinum gauze to retain the impurities. Neptunium di-Pc is also synthesized on the tracer scale from irradiated uranium metal, using the normal synthetic method for uranium di-Pc (Example 29) [6], Other actinide phthalocyanines are reported [107-114], Their structures, as well as those of 200 metal phthalocyanines and their derivatives, are classified in an excellent recent review [115]. More recent experimental data on actinide phthalocyanines are absent in the available literature. 

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