Actinides synthetic studies

The equilibria and variety of species discussed above would seem to make studying the specific oxidation states of actinides difficult. However, through careful control of the conditions, oxidation state pure solutions for all of the actinides can be obtained for synthetic, quantitative and/or qualitative studies. 

The silicides of the actinides have not been widely characterized. A uranium sihcide of the form U3Si2 has received the maximum amount of study owing to its nse as a dispersion fuel in research reactors. Another common phase observed for actinide silicides is AnSi2. One of the common synthetic routes to the silicides is the reaction of actinide fluorides, for example, Anp4, with elemental Si at high temperatures. One of the thermodynamic driving forces for this reaction is the release of Sip4. 

Hexavalent. Uranium hexafluoride, UFe, is one of the best-studied uranium compounds in existence due to its importance for uranium isotope separation and large-scale production ( 70 000 tons per year). All of the actinide hexafluorides are extremely corrosive white (U), orange (Np), or dark brown (Pu) crystalline solids, which sublime with ease at room temperature and atmospheric pressure. The synthetic routes into the hexafluorides are given in equation (13). The volatility of the hexafluorides increases in the order Pu < Np < U in the liquid state and Pu < U < Np in the solid state. UFe is soluble in H2O, CCI4, and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The oxidative power of the actinide hexafluorides are in line with the transition metal hexafluorides and the order of reactivity is as follows PuFg > NpFg > UFg > MoFe > WFe. The UFe molecule can also react with metal fluorides to form UF7 and UFg. The same reactivity is not observed for the Np and Pu analogs. 

Based on the leaching data [40,69,83] as well as data on radiation resistance of fast neutron irradiated [84-85] and actinide-doped Synrocs [86-88], as well as individual synthetic phases and their natural analogues study [14-18,89-91] zirconolite- and pyrochlore-based ceramics have been proposed for immobilization of excess plutonium and the other actinides. Moreover, additional phases, which were not part of the Synroc formulation, have been considered because of their chemical durability and radiation resistance (e.g., murataite, zircon, garnet, monazite, britholite). Of particular interest are the zirconate pyrochlores, many of which are extremely radiation resistant [8,92]. Zirconolite-based ceramics. Zirconolite, ideally CaZrTi207, is a major actinide host phase in the Synroc-type ceramics. Study of natural and synthetic samples... 

A well-controlled (and highly accelerated) technique for studying the effects of a-decay damage is to dope synthetic crystals with short half-life actinides, such as Tu (ti/2 = 87.7 years) or (xi/2 = 18.1 years). At typical concentrations of 10 atomic... 

First, the trivalent actinide and lanthanide elements are separated from the other elements in the waste. In the second step, americium and curium are then separated from the lanthanide elements. Experimental studies have largely been laboratory-scale in which synthetic waste solutions and tracer levels of radioactivity were utilized. A few laboratory-scale experiments were made in hot cells on the coextraction of trivalent actinides and lanthanides. The two most promising methods investigated for co-removal of trivalent actinides and lanthanides are ... 

Actinide elements (Z = 89 - 103) include the heaviest natural and most of the synthetic transuranium elements. They form a series of transition elements, characterized by the filling of an inner - the 5f-electron shell. The elements from Ac (Z = 89) to Es (Z = 99) are available in quantities sufficient for solid state studies. Elemental actinides are metallic. The methods of metal preparation and characterization have been improved to yield samples of known purity and crystal structure, sometimes in the form of single crystals. Recent measurements of structural, thermodynamic and electronic properties have emphasized elements in the beginning and in the centre of the actinide series. 

The chemistry required to convert the oxide to other binary compounds is independent of the scale of operation. However, with microscale synthetic methods applied to radioactive materials, successful preparations are achieved more readily by carrying out the chemistry in situ, that is, in such a manner that eliminates, or at least minimizes, the necessity of having to "handle" the sample during or following its synthesis. Thus, actinide compounds are usually prepared in silica capillary tubes which can be flame sealed at the conclusion of a synthesis to provide the desired sample for study in a small volume, quartz container. A special feature of the preparation/vacuum system in the TRL is the capability to interrupt a synthesis, isolate (by means of a stopcock) and remove the sample, examine it in... 

Micro techniques have been very useful in chemical studies of the actinide elements, and particularly so for the study of the halides. Historically, micro methods were required because of the very small amounts of the synthetic actinides originally available for experiment. However, even when large amounts of neptunium, plutonium, and other actinide elements became available, micro methods were found to offer many advantages, and they have therefore continued to be of great service to this day. W. H. Zachariasen was able to obtain and interpret X-ray diffraction patterns on samples of the order of a few micrograms and the... 

The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are (1) a high solubility for actinides and rare earths (10 to 20 wt %) (2) evidence from natural occurrences of good chemical durability (3) an apparent resistance to radiation damage, as natural monazites are seldom found in the metamict state, despite very high alpha-decay event doses (Boatner and Sales 1988). There have been extensive studies of monazite and apatite as potential waste form phases, and a considerable amount of work on a number of synthetic phosphate phases has been completed. 

Dacheux N, Clavier N, Le Coustumer P, Podor R (in press) Immobilization of tetravalent actinides in the TPD structrrre. Proc 10th Inti Ceramics Congress. Vincenzini P (ed) Techna Publishers, Florence, Italy Davis DD, Vance ER, McCarthy GJ (1981) Crystal chemistry and phase relations in the synthetic miner s of ceramic waste forms. II. Studies of uranirrm-containing monazites. In Scientific Basis for Nuclear Waste Management, vol. 3. Moore JG (ed) Plentrm Press, New York, p 197-200 Day DE, Wu Z, Ray CS, Hrma P (1998) Chemically durable iron phosphate glass waste forms. J Non-Crystalline Solids 241 1-12... 

Pu, Am, Cm 5 X 10 M) in the presence of several fission product contaminants in a synthetic waste of PUREX origin. A solution of 0.5 M formic acid and 0.1 M sulfuric acid was utilized as the receiving phase (22,25). Some of the experimental parameters which characterized the process are listed in Table 3. With a series of 20 modules, 99.9 % of Am was recovered from 2 X 10" L of feed solution in 24 hours. In subsequent experiments, a second SLM containing 1 M Primene JM-T in decalin was connected in series to the first SLM to overcome the reduction in strippant effectiveness caused by nitric acid transport into the strippant (29,30), The second helper SLM extracted only nitric acid from the first stripping solution which was 1 M formic acid (plus 0.05 M hydroxylammonium formate in the case of Pu(IV) transport). The stripping solution for the second SLM was 4 M sodium hydroxide. Results of these laboratory-scale studies demonstrate that actinides can be efficiently removed from such effluents to the point that the resulting solution can be considered a non-transuranic waste (<100 nCi/g of disposed form). 

Main Group, Lanthanides and Actinides - In the series of papers Studies on Organolanthanide Complexes several topics have been covered the formation of ion-pair complexes of the type [ Na.phenan3 Cp]LnCl phenan] (phenan = 1,10-phenanthroline), the synthesis and structural chemistry of bismethoxylethylcyclopentadienyl complexes, and furan bridged bis-cyclopentadienyl complexes. A number of synthetic papers have also appeared in the area the synthesis of cyclooctatetraenyl-cyclopentadienyl sandwich complexes, " " the... 

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