Search

Performance tests of mechanical decladding technology for estimating the feeding portions of the recovered fuel fragments to an electrolytic reduction process were conducted in terms of the fuel ro...

Ag-containing 1 hydrophobic aluminosilicate aerogels capable of capturing I2 were prepared by sol-gelation from Na, Al, and Si alkoxides using co-solvent exchange, Ag/Na ion exchange, solvent exchange, silylation-induced surface hydrophobization, and ambient-pressure drying. The Ag contents, Brunauer-Emmett-Teller (BET) surface areas, and pore size distributions of aerogels depended on the solvent of the co-solvent exchange step, with isopropanol affording higher Ag contents and lower BET surface areas than water. This behavior was ascribed to the reaction of isopropanol-insoluble NaOH in gel pores during Ag/Na exchange with aqueous AgNO3 to form water-insoluble Ag2O. The Ag-containing hydrophobic aluminosilicate aerogel modified with hexamethyldisilazane contained meso- and macropores and exhibited a maximum iodine capture capacity of 0.555 g-I/g-sorbent, a BET surface area of 172 m2/g, a specific pore volume of 1.29 cm3/g, and an average pore size of 23.4 nm.

Al2O3-containing silver phosphate glasses were synthesized to investigate the feasibility of phosphate glasses for the immobilization of radioactive iodine (129I) present in spent nuclear fuel. Characterizations were performed by X-ray diffraction, Fourier transformed infrared spectroscopy, and scanning electron microscopy coupled with energy dispersive spectroscopy to examine structures, bonding properties, surface morphology, and elemental distribution of the synthesized glasses. The principal results showed that iodine became more strongly immobilized in the phosphate glasses with the addition of Al2O3, which was confirmed by the decrease of iodine leaching rates with approximately one order of magnitude. The present study would be helpful to decide whether Al2O3-containing silver phosphate glasses could be used as a candidate matrix to incorporate 129I.

A study on an optimal method for the phosphorylation of rare earth chlorides in LiCl–KCl eutectic waste salt generated the pyrochemical process of spent nuclear fuel was performed. A reactor with a pitched four blade impeller was designed to create a homogeneous mixing zone in LiCl–KCl eutectic salt. A phosphorylation test of NdCl 3 in the salt was carried out by changing the operation conditions (operation temperature, stirring rate, agent injection amount). Based on the results of the test, a proper operation condition (450 °C, 300 rpm, 1 eq. of phosphorylation agent) for over a 0.99 conversion ratio of NdCl 3 to NdPO 4 was determined. Under this condition, multi-component rare earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Y) chlorides were effectively converted into phosphate forms. It was confirmed that the existing regeneration process of LiCl–KCl eutectic waste salt can be greatly improved and simplified through these phosphorylation test results.

A minimization of waste salt is one of the most important issues for the optimization of pyroprocessing. The separation of fission products in waste salts and the reuse of purified waste salt are promising strategies for minimizing the waste salt amounts. The phosphate precipitation of lanthanide is currently being considered for eutectic (LiCl–KCl) waste salt purification. In this research, the effects of molten salt temperature (400–550 °C) and reaction time (max. 180 min) upon conversion into the phosphate of lanthanides was investigated using 1 and 3 kg of eutectic salt. The conversion efficiency of lanthanides to molten salt-insoluble precipitates and phosphates was increased with an increase in molten salt temperature and operating time until it attained a specific temperature and time. K3PO4 as a precipitant was more favorable than Li3PO4 in terms of reactivity. To obtain over a 99% overall conversion efficiency, about 30 min was required in the case of using K3PO4 at 450 °C, but about 120 min in t...

The separation characteristics of surrogate rare-earth fission products in a eutectic (LiCl–KCl) molten salt were investigated. This system is based on the eutectic salt used for the pyroprocessing treatment of used nuclear fuel (UNF). The investigation was performed using an integrated rare-earth separation apparatus comprising a precipitation reactor, a solid detachment device, and a layer separation device. To separate rare-earth fission products, a phosphate precipitation method using both Li3PO4 and K3PO4 as a precipitant was performed. The use of an equivalent phosphate precipitant composed of 0.408 molar ratio-K3PO4 and 0.592 molar ratio-Li3PO4 can preserve the original eutectic ratio, LiCl-0.592 molar ratio (or 45.2 wt%), as well as provide a high separation efficiency of over 99.5% under conditions of 550 °C and Ar sparging when using La, Nd, Ce, and Pr chlorides. The mixture of La, Nd, Ce, and Pr phosphate had a typical monoclinic (or monazite) structure, which has been proposed as a reliable host matrix for the permanent disposal of a high-level waste form. To maximize the reusability of purified eutectic waste salt after rare-earth separation, the successive rare-earth separation process, which uses both phosphate precipitation and an oxygen sparging method, were introduced and tested with eight rare-earth (Y, La, Ce, Pr, Nd, Sm, Eu and Gd) chlorides. In the successive rare-earth separation process, the phosphate reaction was terminated within 1 h at 550 °C, and a 4–8 h oxygen sparging time were required to obtain over a 99% separation efficiency at 700–750 °C. The mixture of rare-earth precipitates separated by the successive rare-earth separation process was found to be phosphate, oxychloride, and oxide. Through the successive rare-earth separation process, the eutectic ratio of purified salt maintained its original value, and impurity content including the residual precipitant of purified salt can be minimized.

Recycling of LiCl–KCl eutectic salt wastes containing radioactive rare earth oxychlorides or oxides was studied to recover renewable salts from the salt wastes and to minimize the radioactive wastes by using a vacuum distillation method. Vaporization of the LiCl–KCl eutectic salt was effective above 900 °C and at 5 Torr. The condensations of the vaporized salt were largely dependent on temperature gradient. Based on these results, a recycling system of the salt wastes as a closed loop type was developed to obtain a high efficiency of the salt recovery condition. In this system, it was confirmed that renewable salt was recovered at more than 99 wt.% from the salt wastes, and the changes in temperature and pressure in the system could be utilized to understand the present condition of the system operation.

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 fundamental study on the distillation rate on LiCl-KCl eutectic salt under different vacuums from 66 to 6600 Pa (0.5 to 50 mm Hg) was performed by using both a nonisothermal and an isothermal thermogravimetric (TG) analysis. Based on the nonisothermal TG data, distillation rate equations as a function of the temperature could be derived. Calculated flux by these model flux equations was in agreement with the distillation rate obtained from isothermal TG analysis. A salt distillation operation with a moderated distillation rate of 10- 4 to 10- 5 mol·cm -2 ·s -1 is possible at temperatures of

As an alternative to conventional Group I and II separation methods (such as adding a chemical agent and ion exchange), melt crystallization processes, zone freezing, and layer melt crystallization were tested for the separation (or concentration) of cesium and strontium fission products in a LiCl waste salt generated from an electrolytic reduction process of a spent oxide fuel. In these melt crystallization processes, impurities (CsCl and SrCl 2 ) are concentrated in a small fraction of the LiCl salt by the solubility difference between the melt phase and the crystal phase. As experimental variables, initial molten salt temperature, crucible rising velocity in the zone freezing case, and cooling air flow rate in the layer crystallization case were used. In the zone freezing process, although the operating time is long (1.7 mm/h of crucible rising velocity) when assuming a LiCl salt reuse rate of 90 wt%, >90% separation efficiency for both CsCl and SrCl 2 was shown. In the layer crystallization process, the crystal growth rate strongly affects the crystal structure and therefore the separation efficiency. At a 25 to 30 l/min cooling air flow rate, 700 to 710°C initial molten salt temperature, and

A lab-scale lanthanide precipitation apparatus, which has a 4 kg/batch size, was installed and tested. By using this lab-scale apparatus, cooxidative precipitation experiments of lanthanides were carried out. As lanthanides, 8 rare-earth elements (Y, La, Ce, Pr, Nd, Sm, Eu, and Gd) were used. By a reaction with oxygen, La, Pr, Nd, Sm, Eu, and Gd were converted to their oxychloride form (REOCl) and Ce, Pr, and Y to their oxide form (REO2, RE2O3). Since these lanthanide oxides or oxychlorides are nearly insoluble to eutectic salt, they were all precipitated by a free settling at the bottom, where about 7 h of precipitation time was required. It was found that under the conditions of 750°C salt temperature, a 12 h sparging time, and 5 L/min oxygen sparging rate, all the used rare-earth elements showed over 99% oxidation efficiency. However, in the case of 800°C temperature, they show over 99% conversion efficiencyonly after 6 h.

For the reuse of a waste salt from an electrorefining process of a spent oxide fuel, a separation of rare earth elements by an oxidative precipitation in a LiCl-KCl molten salt was tested without using precipitate agents. From the results obtained from the thermochemical calculations by HSC Chemistry software, the most stable rare earth compounds in the oxygen-used rare earth chlorides system were oxychlorides (EuOCl, NdOCl, PrOCl) and oxides (CeO 2 , PrO 2 ), which coincide well with results of the Gibbs free energy of the reaction. In this study, similar to the thermochemical results, regardless of the sparging time and molten salt temperature, oxychlorides and oxides were formed as a precipitant by a reaction with oxygen. The structure of the rare earth precipitates was divided into two shapes: small cubic (oxide) and large plate-like (tetragonal) structures. The conversion efficiencies of the rare earth elements to their molten salt-insoluble precipitates were increased with the sparging time and temperature, and Ce showed the best reactivity. In the conditions of 650 °C of the molten salt temperature and 420 min of the sparging time, the final conversion efficiencies were over 99.9% for all the investigated rare earth chlorides.

In this work, a new approach to remove fission products including decay heat elements was proposed. This study aims at providing a new way to minimize the amount of waste salt for a repository, while removing the high decay heat fission products [Cs, Sr, Ba, and Y including other rare earth (RE) elements] from the waste salts generated during a chloride pyroprocessing procedure. These elements were removed in consecutive order from the pyroprocessing units. First, Cs could be released in the form of an oxide gas during voloxidation of UO 2 and captured by a fly-ash filter. Then, Sr was recovered in the form of carbonate precipitates from the LiCl waste salt generated during the course of an electoreduction process, by using Li 2 CO 3 . Finally, RE elements plus yttrium in the spent LiCl-KCl waste salt generated during electrorefining were removed in the form of oxides (or oxychlorides) by using an oxygen sparging method. It was confirmed that the removal yields of each element were ∼90% for Cs at ∼1473 K, >99% for Sr at a molar ratio of [Li 2 CO 3 /SrCl 2 = 3], and >99% for the RE elements plus yttrium. Using these successes as a basis, a reference flow sheet for removing the high decay heat elements from pyroprocessing units is presented in this work. Also, a salt regeneration system to minimize the amount of waste salt is proposed in this study.

The carbonate reaction of some alkaline-earth chlorides was investigated by a carbonate agent injection method in LiCl-KCl eutectic salts containing both SrCl2 and BaCl2 and LiCl molten salts containing SrCl2. The effects of the injected molar ratio of a carbonate agent (Li2CO3 or K2CO3) and the temperature (450–750°C) on the conversion efficiencies of the strontium and barium chloride to their carbonates were determined. The forms of strontium and barium carbonate resulting from the carbonate reaction with carbonate agents were identified by XRD and SEM-EDS analyses. In these experiments, the carbonate agent injection method can carbonate strontium and barium chlorides effectively at over 99% under LiCl-KCl eutectic and LiCl molten salt conditions. For LiCl-KCl eutectic molten salts, carbonation efficiency was more favorable in the case of K2CO3 injection than in the case of Li2CO injection, where strontium and barium were carbonated in the form of Ba0.5Sr0.5CO3. For LiCl molten salts, strontium was carb...