Effect of alkali and alkaline-earth chloride addition on electrolytic reduction of UO2 in LiCl salt bath

The electrolytic reduction process of actinide oxides in a LiCl salt bath at 923 K has been developed for nuclear fuel reprocessing. Since some salt-soluble fission products, such as Cs, Sr and Ba, accumulate in the LiCl salt bath, their effect on UO₂ reduction was investigated. In the experiments, UO₂ specimens were reduced by potential- or current-controlled electrolysis in various LiCl salt baths containing up to 30 mol% of KCl, CsCl, SrCl₂ or BaCl₂. The rate of UO₂ reduction in a LiCl salt bath was considerably decreased by the addition of alkali metal chlorides (KCl and CsCl) and slightly decreased by BaCl₂ addition. SrCl₂ addition had no appreciable effect. It was suggested that the diffusion of O²⁻ ions from the inside of UO₂ specimens to the bulk salt determined the reduction rate during the electrolysis and that the effect of salt composition was related to the solubility of O²⁻ ions in the salt bath.     

Direct Preparation of 35S-Labeled Copper Sulfate

A novel method was developed for preparing ³⁵S-labeled Cu³⁵SO₄·5H₂O directly from neutron irradiated KCl. One gram of neutron irradiated KCl was dissolved in water and passed through a cation exchange resin column in Cu²⁺-form for the purpose of converting KCl to CuCl₂. In the presence of carrier sulfate, the precipitates of ³⁵S-labeled sulfate were separated from the effluent by the addition of ethanol. Factors affecting the recovery and quality of the final products were studied and a standard procedure was devised at optimum conditions.     

Effect of borate concentration on solidification of radioactive wastes by different cements

To investigate the effect of borate on the cementation of radioactive evaporator concentrates, and to provide more data for solidification formula optimization, the simulated borate evaporator concentrates with different borate concentrations (as B) and Na/B ratio (molar ratio) were solidified by sulfoaluminate cement (SAC) and Portland cement (PC), with addition of Ca(OH)₂, zeolite and accelerator or water reducer. The hydration products of solidified matrices were characterized by X-ray diffraction (XRD). The experimental results showed that borate retarded the cement setting for both SAC and PC formulas, and the final setting time prolonged with decrease of Na/B ratio. Borate could enhance the fluidity of the cement mixture. The 28 d compressive strengths of the solidified matrices for both SAC and PC formulas decreased with increase of borate concentration. The XRD patterns suggested that, in the matrices maintained for 28 d, borate did not interfere with the formation of main hydration products of SAC and PC. Borate, in the form of B(OH)⁴⁻, incorporated in ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) as solid solution phase. The formula of SAC and PC developed in this study was effective for cementation of the simulated borate evaporator concentrates. However further optimization was required to reduce retarding effect of higher borate concentrations and to extend the practical feasibility for actual evaporator concentrates.     

Adsorption behavior of radioactive cesium by non-mica minerals

We studied the adsorption behavior of radioactive cesium (Cs) by the non-mica minerals kaolinite, halloysite, chlorite, montmorillonite, mordenite, MnO₂, TiO₂, Al₂O₃, and FeOOH to elucidate the environmental behavior of radioactive Cs fallout from the Fukushima Daiichi Nuclear Power Plant in the Tohoku region of Japan. The adsorption and desorption experiments of Cs on the minerals were carried out at the Cs concentrations 1 × 10^?4, 1 × 10^?5, and 2 × 10^?9 mole L^?1 at pH 5.5. The desorption of Cs from the minerals was examined using 0.1 mole L^?1 LiCl, NaCl, KCl, RbCl, and CsCl solutions. The sequential desorption was examined using a 0.1 mole L^?1 LiCl solution, a 1 mole L^?1 KCl solution, and a 1 mole L^?1 HCl solution. The distribution coefficient (K _d) for the minerals at the Cs concentration 10^?9 mole L^?1 was in the order of mordenite > illite > montmorillonite, sericite, MnO₂, kaolinite, and halloysite > chlorite, TiO₂, Al₂O₃, and FeOOH, differing from the order observed at higher Cs concentrations. After the sequential desorption by the three reagent solutions, the residual fraction of Cs was higher at the Cs concentration 10^?9 mole L^?1 than at higher concentrations. Approximately 40%, 40%, 50%, and 25% of the adsorbed Cs were residual in montmorillonite, mordenite, MnO₂, and kaolinite, respectively, after the sequential desorption. These results strongly suggest that (1) radioactive Cs at 10^?9 mole L^?1 is more strongly associated with the non-mica minerals than at higher concentrations of 1 × 10^?4 and 1 × 10^?5 mole L^?1, and (2) the non-mica minerals montmorillonite, mordenite, kaolinite, and MnO₂ contributed to the fixation of the radioactive Cs fallout on Fukushima soil.     

Surface Alteration of Pollucite under Hydrothermal Conditions

In connection with studies for evaluating the leachability of Cs from solidified radio-active waste, the surface alteration of pollucite was studied under hydrothermal conditions. Pollucite exposed to NaCl or KCl at 300°C formed on its surface a precipitate phase of analcime in the case of NaCl or leucite in the case of KCl. Cubo-octahedral analcime built up not only on the outermost surface of sample but also in the altered layer underneath. This crystal penetration was attributed to precipitation and isomorphous substitution of Na by Cs in the pollucite. The leachability of Cs proved to increase in keeping with salt concentration.

Exposure to CaCl₂ or to MgCl₂ formed a thick precipitate layer of anorthite in the case of CaCl₂ or clinochlore in the case of MgCl₂ over a thin reaction layer. This precipitate layer had the effect of inhibiting the migration of Cs. Si and Al to restrain Cs leaching into solution, which caused the amount of Cs leached to level off beyond 0.01 mol·dm^?3 salt concentration in the case of CaCl₂, and beyond 0.01 mol·dm^?3 in the case of MgCl₂.     

Study of the properties of ammonium polyuranate precipitates and UO<Subscript>2</Subscript> powders and pellets obtained with different technologies

The results of a differential-thermal analysis are used to compare the properties of ammonia polyuranate precipitates, UO₂ powders and pellets, obtained by different methods as well as metallic uranium. It is found that the phase NH₃·3UO₃·5H₂O forms in regular precipitation of ammonium polyuranate. When using nanotechnology, the phases NH₃·2UO₃·3H₂O and 4NH₃·6UO₃·8H₂O are also present in the precipitate. UO₂ powder prepared from such precipitate has high activity, since all phase transformations in it occur at a lower temperature. Modified fuel pellets of uranium dioxide, which are obtained by means of nanotechnology or mechanical addition of ammonia-containing reagents to powder, differ from the standard powders by a lower rate and more complex mechanism of oxidation, similar to metallic uranium. Modified UO₂ fuel pellets fabricated at the Physics and Power-Engineering Institute, are now undergoing tests in the BOR-60 reactor. After tests on the irradiated new modified fuel have been completed, it will be possible to judge its reliability.     

Corrosion behavior of ceramic structural materials in an electrolytic reduction process

The electrolytic reduction of spent oxide fuel involves the liberation of oxygen in a molten LiCl electrolyte, which results in a chemically aggressive environment that is too corrosive for typical alloying structural materials. Therefore, the choice of the optimum material for the processing equipment that handles molten salt is critical. We investigated the corrosion behaviors of CaO-stabilized ZrO₂ (CSZ) and mullite (Al₆Si₂O₁₃) at 650°C for 168 h in molten (1, 3) wt% Li₂O–LiCl. The as-received and tested specimens were examined by scanning electron microscopy/X-ray energy dispersive spectrometry and X-ray diffraction. CSZ showed a much better hot-corrosion resistance in the presence of Li₂O–LiCl molten salt than mullite. The surface corrosion layers of mullite consisted of LiAlSiO₄ in 1 wt% Li₂O–LiCl, and a LiAlO₂ phase appeared as the Li₂O concentration increased to 3 wt%. Furthermore, Li₂SiO₃ was the only corrosion product observed at 3 wt% Li₂O–LiCl. The surface corrosion layers of CSZ were composed mainly of tetragonal-ZrO₂ with partial monoclinic-ZrO₂ in 1 wt% Li₂O–LiCl, and a Li₂ZrO₃ phase appeared at 3 wt% Li₂O–LiCl. There was no corrosion product detached from the surface for those specimens. CSZ was beneficial for increasing the hot-corrosion resistance of the structural materials that handle high-temperature molten salts containing Li₂O.     

Chlorination-Volatilization Processing of Irradiated Uranium Dioxide, (I)

A chlorination-distillation process for irradiated U0₂ fuel treatment is presented, which utilizes the difference in vapor pressure between the chlorides of uranium and those of F.P. Good recovery of uranium and decontamination of F.P. were not obtained with single distillation. Improvement was seen with the adoption of a process in which most of the F.P. were removed from the irradiated U0₂ fuel through a sorption-desorption process using a BaCl₂ bed. The present report describes the behavior of uranium chlorides in this process.

The vapor of uranium chlorides formed by chlorinating U0₂ powder with a mixed gas of Ar and CCl₄ vapor was passed through the bed whose temperature varied along its length from 500° to 100°C, under which conditions the chloride vapor was completely sorbed on the bed. The uranium chlorides thus sorbed did not desorb in a mixed gas stream of Ar and CCl₄ vapor at bed temperatures below 480°C, but when the bed was heated to 480°~560°C, and Cl₂ gas added to the gas stream, the chlorides were completely desorbed in the form of vapor of higher uranium chlorides.     

Reaction Characteristics of β-UO3 in Aqueous NH4NO3

The influences of reaction time, temperature and NH₄NO₃ concentration on the reaction of solid β-UO₃ with aqueous NH₄NO₃ were experimentally studied. Both type I(UO₃·2H₂O) and type II(3UO₃·NH₃·5H₂O) uranyl compounds were observed in the X-ray diagrams of reaction product. High temperature (80°C) and long reaction time (2 h) favor the formation of type II uranyl compounds and generation of uranyl nitrate. Based on the experimental results, two possible models were proposed. The model that assumed pseudohomogeneous reactions conforms to the experimental data better than the model that assumed noncatalytical solid-fluid heterogeneous chemical reaction. The dependence of uranyl nitrate generation rate on reaction time, temperature and NH₄NO₃ concentration can be calculated from the pseudohomogeneous reaction model.     

Electrochemical and spectrophotometric study on trivalent neodymium ion in molten binary mixtures of LiCl and alkali earth chlorides

The thermodynamic stability of Nd(III) complexes was studied by electrochemical techniques for molten binary mixtures composed of LiCl and an alkali earth chloride, CaCl₂, SrCl₂, or BaCl₂ at 923 K. The Gibbs free energy change of formation of Nd(III) in the melts was determined. The obtained values showed a good correlation with the polarizing power of solvent cations. The Nd(III) complex was more stable in the melts with low polarizing power. The electronic absorption spectrum of the hypersensitive f-f transition of Nd³⁺ was investigated to obtain information about the structural change of the complex. The estimated oscillator strength of the ⁴G_5/2,²G_7/2 ← ⁴I_9/2 transition and the degree of the energy splitting in electronic energy levels showed different trends upon the polarizing power for each of added alkali earth chloride. This indicates that the coordination environments further from second neighbors could have significant impact on the electronic energy levels of Nd(III) in the mixtures including alkali earth cations.     

Thermodynamic investigation of the possibility of using uranium dioxide to obtain hydrogen and oxygen from water

The equilibrium composition in the system U–O–H at temperatures 300–3000 K is calculated using thermodynamic functions. It is shown that the decomposition of water in H₂ and O₂ can be achieved in steps: first with only hydrogen being released at 1200–1500 K and then only oxygen at 2100–2500 K. When uranium dioxide interacts with water, mainly two uranium oxides are formed – U₄O₉ and UO₃; nine gaseous components can have appreciable partial pressure: UO, UO₂, UO₃, UO₂(OH)₂, H₂O, H₂, H, OH, HO₂. Translated from Atomnaya énergiya,Vol. 106, No. 2, pp. 76–79, February, 2009.     

Study on Redox Equilibrium of UO2 2+/U2 + in Molten NaCl-2CsCl by UV-Vis Spectrophotometry

In order to enhance the understanding of the redox equilibriums of uranyl ions in molten NaCl-2CsCl eutectic salt UV-Vis absorption spectrophotometry measurements were performed for UO₂ ²⁺ in molten NaCl-2CsCl at 923 K under simultaneous electrolytic control of their ratio. A prominent absorption band at 395 nm was assigned to UO₂ ⁺, and its molar absorptivity was determined to be 832±27 mol^-1·l·cm^-1. From the dependence of the rest potential of the melt on the spectrophotometrically determined ratio of [UO₂ ²⁺]/[UO₂ ⁺], the standard redox potential of the couple UO₂ ²⁺/UO₂ ⁺ was determined to be ?0.903±0.007 V vs. Cl₂/Cl^—.     

Determination of the composition and instability constants of complex Pu+3oxalate ions

The solubility of Pu₂(C₂O₄)₃ · 9H₂O in aqueous solutions of K₂C₂O₄ of various concentrations (0.01–2.4 moles /liter) has been determined at constant ionic strength of the solution at 20. It was found that Pu⁺³ complexes are formed in these solutions. It was found from the results of Pu₂(C₂O₄)₃ · 9H₂O solubility determinations that in the region of K₂C₂O₄ concentrations studied the following complex ions are formed [Pu(C₂O₄)₂]^?, [Pu (C₂O₄)₃]^?3 and [Pu (C₂O₄)₄]^?5, the total instability constants of which are 4.9 · 10^?10; 4.10 · 10^?10 and 11.9 · 10^?11 respectively. The solubility of Pu₂(C₂O₄)₃ · 9H₂O in aqueous (NH₄)₂C₂O₄ solutions has also been determined in the range of ammonium oxalate concentrations from 0.07 to 0.7 mole/liter at 70 °. It is shown that the composition of the complex ions under these conditions corresponds to [Pu(C₂O₄)₂]^?, [Pu(C₂O₄)₃]^?3 and [Pu(C₂O₄)₄]^?5. The calculated total instability constants of these complex ions are 11.6 · 10^?9; 5.6 · 10^?9 and 2.5 · 10^?9 respectively. The heats of formation of complex Pu⁺³ oxalate ions have been calculated for the reaction Pu⁺³ + nC₂O₄ ^?2 ?[Pu(C₂O₄)_n]^3?2n Δ¯Q for the [Pu(C₂O₄)₂]^? ion is 1300 cal., for [Pu(C₂O₄)₃]^?3, 1200 cal., and for [Pu(C₂O₄)₄]^?5, 1300 cal.     

Study on a recovery of rare earth oxides from a LiCl-KCl-RECl3 system

Radioactive rare earth chlorides in waste LiCl-KCl molten salts have to be separated as a stable form to minimize waste volume and to achieve stable solidification. In this work, thermal behavior of rare earth chlorides (CeCl₃, GdCl₃, NdCl₃, PrCl₃) was investigated in an oxygen condition to recover rare earth oxides from a LiCl-KCl-RECl₃ system. The rare earth chlorides in the LiCl-KCl molten salts were smoothly converted to an oxychloride form at a higher temperature than 650 °C, except for CeCl₃. CeCl₃ was totally converted to an oxide from at a higher temperature than 450 °C. The rare earth oxychlorides (GdOCl, NdOCl, PrOCl) were effectively converted to oxide forms at a higher temperature than 1100 °C. It was confirmed that rare earth oxides can be recovered from a LiCl-KCl-RECl₃ system without impurity generation.     

A simulation of radiolysis of chloride solutions containing ferrous ion

ABSTRACT

To investigate the effect of dissolved species from steels on the radiolysis processes of Cl^?, radiolysis simulations of solutions containing both Cl^? and Fe²⁺ were carried out. The results showed that the generation of radiolytic products (H₂O₂, O₂ and H₂) increased mainly by the addition of Fe²⁺, and the concentrations of H₂O₂ and O₂ increased with increasing dose rate. Moreover, radiolysis of Fe²⁺ solutions also induced noticeable pH drop due to the hydrolysis of Fe³⁺. This pH drop enhanced the reactivity of Cl^? with ^?OH, which induced additional generation of H₂O₂ and O₂. These results show that low concentrations of Cl^? (1 × 10^?3 mol/dm³ = 35 mg/kg) in the presence of Fe²⁺ could influence the generation of H₂O₂ and O₂ during water radiolysis. On the other hand, it is considered that these effects of Fe²⁺ and Cl^? on water radiolysis are less important for corrosion of steels due to the low concentrations of H₂O₂ and O₂ generated if the concentrations of these additives and dose rate are sufficiently low. The other process, such as dissolution of iron enhanced by FeOOH, might predominantly induce corrosion under the conditions of solutions with low concentrations of H₂O₂ and O₂.     

Preparation of potassium iron(III) hexacyanoferrate(II) supported on activated carbon and Cs uptake performance of the adsorbent

Synthesis of potassium iron(III) hexacyanoferrate(II) (K/Fe-Fe(CN)₆) in the pores of activated carbon (AC) was attempted by impregnating AC with K₄[Fe(CN)₆] and FeCl₃, and the Cs uptake performance of the resulting adsorbent was examined. K/Fe-Fe(CN)₆ supported on AC was prepared by varying the reaction conditions such as the supplied amounts and molar ratios of the reagents, and the Cs uptake performance was optimized. The impregnated product was characterized by XRD, EPMA, and porosimetry to elucidate the condition to which Fe₄[Fe(CN)₆]₃ was filled in the AC pores. The K/Fe-Fe(CN)₆-on-AC was immersed in seawater containing 0.075 mmol · dm^?3 Cs and agitated for 1 day to obtain the Cs uptake. The Cs uptake was large at pH < 10 but decreased abruptly at pH > 10.5. The maximum Cs uptake was 10.4 μmol · g^?1 at the equilibrium Cs concentration of 49 μmol · dm^?3 and the distribution coefficient was 45.5 dm³ · g^?1 at the equilibrium concentration of 0.015 μmol · dm^?3, respectively. When K/Fe-Fe(CN)₆-on-AC was immersed in Cs-containing seawater, K⁺ ions in the adsorbent were completely exchanged for Na⁺ ions in seawater, and the added Cs⁺ ions were then substituted for the Na⁺ ions in the adsorbent.     

Effect of carbide on Al2O3/B4C burnable poison sintered in Ar

Al₂O₃–14 wt%B₄C composites with 0–1 wt% C addition were sintered in Ar at 1550–1650 °C. The influence of the C additive on the B loss, densification behavior, and microstructure of the Al₂O₃–B₄C composites were investigated. The results show that there are B₂O₃, H₃BO₃ and Al₁₈B₄O₃₃ exist between Al₂O₃ and B₄C interface, which result in B loss because of B₂O₃'s high vapor tension at above 1500 °C. The presence of Al₁₈B₄O₃₃ grains formed by chemical reaction of Al₂O₃ with surface oxides on B₄C inhibit the densification of pellets by reducing the specific free surface energy of the Al₂O₃. However, the added C eliminates those oxides to reduce B loss because of its higher activity than B₄C, and it also coarsens Al₂O₃ grain although the density of pellets is decreased by gas products.     

On the electrochemical formation of Pu–Al alloys in molten LiCl–KCl

Properties of Pu–Al alloys were investigated in connection with development of pyrochemical methods for reprocessing of spent nuclear fuel. Electroseparation techniques in molten LiCl–KCl are being developed in ITU to group-selectively recover actinides from the mixture with fission products. In the process, actinides are electrochemically reduced on solid aluminium cathodes, forming solid actinide–aluminium alloys. This article is focused on electro-chemical characterisation of Pu–Al alloys in molten LiCl–KCl, on electrodeposition of Pu on solid Al electrodes and on determination of chemical composition and structure of the formed alloys. Cyclic voltammetry and chronopotentiometry were used to study Pu–Al alloys in the temperature range 400–550 °C. Pu is reduced to metal in one reduction step Pu³⁺/Pu⁰ on an inert W electrode. On a reactive Al electrode, the reduction of Pu³⁺ to Pu⁰ occurs at a more positive potential due to formation of Pu–Al alloys. The open circuit potential technique was used to identify the alloys formed. Stable deposits were obtained by potentiostatic electrolyses of LiCl–KCl–PuCl₃ melts on Al plates. XRD and SEM–EDX analyses were used to characterise the alloys, which were composed mainly of PuAl₄ with some PuAl₃. In addition, the preparation of PuCl₃ containing salt by carbochlorination of PuO₂ is described.     

Characterization of Cement Alteration Process by Transmission Electron Microscopy with High Spatial Resolution

Application tests for advanced TEM analysis techniques were carried out to study the cement alteration processes associated with water penetration at high spatial resolution. Prior to TEM analysis, we measured the changes in the penetration coefficient and determined the characteristics of the penetrating water in order to gain a fuller understanding of the overall process. These experiments revealed that the process begins with the preferential dissolution of Ca(OH)₂. After most of the Ca(OH)₂ is dissolved out, the penetration coefficient increases, while the pH value of the water decreases. It has been demonstrated that scanning transmission electron microscope (STEM) techniques are quite useful for determining local structures and compositions in the cement at sub-micron meter spatial resolution. The preferential dissolution of Ca ions results in refinement of cement grains. When the Ca/Si ratio decreases to 1.05, most grains have a round shape in the sub-micron range. X-ray mapping suggests the formation of 3CaO·A1₂O₃· xSiO₂·(6-2x)H₂O (x=0-3). Ettringite has been mostly dissolved out. But Mg ions remain still in form of brucite. When Ca/Si reaches 0.91, the morphology has changed to a mixture of fibers and granules. The fibers have been identified as a mixture of Calcium Silicate Hydrate Gel and silica gel. Quantitative EDX composition analyses have demonstrated that the granules are altered products of hydrogrossular, 3CaO·A1₂O₃·2SiO₂·2H₂O, which have been predicted by previous theoretical studies. It is also been shown that hydrotalcite with Mg and Al has been also formed. The results thus obtained are in principle in accordance with the process predicted by previously proposed thermodynamic models.     

Oxidation-Reduction Properties of Mixed Oxides in Lanthanum-Uranium-Oxygen System

The oxidation-reduction properties and the phase relations of mixed oxides in the system La-U-O were examined by means of thermogravimetry and X-ray diffraction method. The O/M ratios of the mixed oxides La_yU_1-y O_2±x, prepared at 1,650°C in vacuo by the reaction of La₂O₃ and UO₂, were expressed as 2–0.5 y except 0.2≤y≤0.4. The oxides were the single fluorite phase at y0.7. These oxides of 0.2≤y≤0.6 were oxidation-susceptible in air even at room temperature. For the oxides heated or prepared in air at 1,000°C, the O/M ratio vs. y relationship was composed of three line with the turning points, 2.26 at y = 0.27 and 1.92 at y = 0.70. The fluorite phase was observed in the regions of 0.3≤y≤0.45 and 0.7≤y≤0.9, and the rhombohedral phase was in ?0.55≤y-0.7. The O/M ratios of the oxides reduced with hydrogen were nearly 2 at y ≤0.2 and decreased with increasing y at y >0.2. The oxide of y =0.5 showed the curious property: The O/M ratio had a minimum when the oxide reduced with hydrogen was oxidized in air.