Actinides water exchange

Rate Constants and Activation Parameters for Water Exchange on Actinides or Actinyl Aqua Ions... 

Substitution studies on the actinides have been largely confined to those of trans [U(0)2(H20)5]2+. In the solid state [U(0)2(H20)5]2+ is characterized by U—O axial and equatorial distances of 171pm and 227 pm respectively.283 Protonation and oligomerization have, however, precluded a direct study of the water exchange. 

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

Differences in behavior of actinides in the aquatic environment are shown by the data in Table VI. In WOL, we have noted that about 12% of the total Pu in the water column passes a 0.45-vim filter and also passes a 10,000 mol wt membrane filter. Essentially all of this soluble Pu is retained when passed through an anion exchange column. Curium-244 behaves somewhat differently in that about 50% of the activity in WOL water is soluble, but it also behaves anionically. The reasons for a negative charge on Pu and Cm are complex and are not understood, but one possible explanation is the presence of Pu(VI) carbonate complexes, analogous to the soluble uranyl carbonate complexes found in natural waters. Experiments are in progress to determine the valence state of this soluble form of Pu. Another explanation for the observed soluble Pu would be organic complexes. 

NMR results are limited in so far as individual hydration numbers can only be foimd for ions for which the two peaks corresponding to coordinated water and the rest can be observed. This limits the study to ions for which there is slow exchange and means that alkali metal cations, and most of the alkaline earth metal cations are excluded. Nonetheless a considerable nmnber of cations have been studied with unambiguous results. The most common coordination number is six, but cations known to favour tetrahedral or square planar conhgiu ations show a coordination number of four. It is only with large cations such as the lanthanides and actinides that numbers greater than six are found. 

Two principal reactor systems are now in development at Los Alamos. A down-flow reactor for actinide contaminated wastes (ion exchange resins, paper and plastic lab trash) is 1 inch in diameter and 14 inches long. Residence time for waste in the reactor is 30 seconds. The reactor is made of A286 stainless steel with a Ti liner (clean water is run outside the liner). After pyrolysis to transform the waste into pumpable fluid, SCWO occurs at 46 MPa and 540 °C... 

In the case when defective fuel rods are present in the reactor core, the BWR reactor water contains the other fission products and the activation products released from the fuel in concentrations well below those of fission product iodine. This applies as well for fission product cesium, which is retained on the ion exchangers of the reactor water cleanup system with a decontamination factor of about 100. As far as it is known, cesium in the reactor water is present as the Cs ion, whereas large proportions of most of the polyvalent fission products and of the actinides are attached to the corrosion product particles suspended in the water as yet, there is no detailed knowledge on the chemical state of these elements (i. e., adsorbed to the surfaces or incorporated into the Fe203 lattice). It was reported that the strontium isotopes as well as Np appear in the reactor water in the dissolved cationic state, while Tc was found in the reactor water as a dissolved anionic species, most likely Tc04 (Lin and Holloway, 1972). According to James (1988), discrete fuel particles were not detected in the BWR reactor water. 

Another example of an aqueous medium effect is the use of alcohol/water mixtures in several successful ion-exchange-based separation processes for the lanthanides and actinides. A third example, relevant to solvent extraction alone, is the effect of the diluent on the extraction process. The effect of these phenomena, and the reasons for their success (or failure) is the subject of the next section. 

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