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A chemical trap in the form of Ba-Al-Si-O that captures gaseous cesium and iodine has been developed for application to UO2 nuclear fuel. A key issue for better trapping efficiency is to secure the ability to capture individual fission gases, in this case for cesium and iodine. This can be done through chemical trapping with the presence of chemical compounds that spontaneous react with them. In this study, based on thermodynamic estimations, a promising composition of 1BaO-1Al2O3-1SiO2 (mole ratio) is suggested for amphoteric trapping. The chemical design was confirmed experimentally under simulated conditions approximating an actual fuel environment. In each experiment for cesium and iodine, the trapping reaction was confirmed to occur at 650 °C in a reducing atmosphere. This study provides a methodology for compositional design and empirical confirmation to shed light on an accident-tolerant fuel development strategy that reduces the release of fission gases.

As cesium-trapping agents in an accident-tolerant fuel (ATF) that reduces fission gas release (FGR), Al–Si–O and Zr–Si–O (resulting in Cs–Al–Si–O and Cs–Zr–Si–O) are considered promising due to their good cesium retention ability up to high temperatures. However, in previous research, there is a lack of experimental data to compare them and determine which material is superior. In this study, the cesium retention ability of these materials was assessed by observing their weight loss behaviors up to 1750 °C. Other cesium compounds, such as pollucite (CsAlSi2O6) and Cs–Si–O possibly formed in the real fuel environment were also compared. As a result, Cs–Al–Si–O (CsAlSiO4) exhibited the most favorable result. It released cesium at higher temperatures and showed lower loss rate: only 2.36% of captured cesium was released at 1750 °C when sufficiently homogenized at 1050 °C. The real nuclear fuel environment in the light water reactor (LWR) was taken into account for the interpretation of comparative results, demonstrating that Al–Si–O is the most advantageous trapping agent in terms of high-temperature cesium retention ability.

A reduction process in the head-end for pyroprocessing has been adopted to avoid oxidation attack on the molybdenum crucible during sintering. The reduction process is employed to reduce U3O8 pellets to UO2 prior to sintering. This allows elimination of the oxygen source, which causes oxidation attack during sintering, thereby permitting the use of a metallic crucible. However, little densification occurs due to the low reduction temperature limited by the INCONEL crucible. Consequently, the amount of material scraps from the pellets increases, thus creating an additional processing burden due to its high radioactivity. To reduce the amount of scraps, densification should be enhanced. This study suggests a simple atmospheric control strategy and clarifies its effects. With the atmospheric control, a higher bulk density and better attrition resistance were obtained in comparison to without this strategy. This can be explained in terms of O/U ratio dependent diffusion kinetics during the reduction of U3O8 to UO2.

As candidates for accident-tolerant fuels, ceramic microcell fuels, which are distinguished by their peculiar microstructures, are being developed; these fuels have UO2 grains surrounded by cell walls. They contribute to nuclear fuel safety by retention of fission products within the UO2 pellet, reducing rod pressure and incidence of SCC failure. Cesium, a hazardous fission product in terms of amount and radioactivity, can be captured by chemical reactions with ceramic cell materials. The capture-ability of cesium therefore depends on the thermodynamics of the capturing reaction. Conversely, compositional design of cell materials should be based on thermodynamic predictions. This study proposes thermodynamic calculations to evaluate the cesium capture-ability of three ceramic microcell compositions: Si-Ti-O, Si-Cr-O and Si-Al-O. Prior to the calculations, the chemical and physical states of the cesium and the cell materials were defined. Then, the reactivity was evaluated by calculating the cesium potential (ΔGCs) and oxygen potential (ΔG02) under simulated LWR circumstances of normal operation. Based on the results, cesium capture is expected to be spontaneous in all cell compositions, providing a basis for the compositional design of ceramic microcell fuels as well as a facile way for evaluating cesium capture.

The grain growth behavior of 0.95(Na0.5Bi0.5)TiO3&ndash, 0.05BaTiO3 (mole fraction, NBT&ndash, 5BTdid not appear in any of the NBT&ndash, 5BT samples with excess Bi2O3. The amount of liquid phase increased as the amount of Bi2O3 increased. Therefore, the rate of grain growth could be decreased by the increasing the distance for the diffusion of atoms. These observations allowed us to conclude that the growth of Bi2O3-excess NBT&ndash, 5BT grains is governed by the growth of facet planes via the two-dimensional nucleation grain growth mechanism during changing grain shape and amount of liquid.

Hydrogen sulfide (H2S) emitted from the petroleum refinery process, landfills, or biogas purification is severely corrosive and has harmful health effects on the human body when inhaled at even small concentrations. Selective catalytic oxidation using V2O5–TiO2 is considered to be a benchmark technology for removing H2S. This paper presents a novel preparation method to synthesize V2O5–TiO2 nanocatalysts to improve desulfurization performance. Compared with the conventional sol-gel or wet-impregnation method, the integrated sol-gel and hydrothermal synthesis allows for a substantial increase in the specific surface area, which is mainly attributed to the highly reduced sizes of the primary particles. The enhanced surface area contributes to effective dispersion of the vanadium (V) species on the TiO2 support, by which the interactions between V and titanium (Ti) are strengthened. Accordingly, the redox properties of the catalysts are improved with increased concentrations of the chemisorbed oxygen species on the catalyst surfaces. Finally, the properties of the V2O5–TiO2 nanocatalysts are improved by the new preparation conditions, resulting in significantly enhanced catalytic activities in the selective oxidation of H2S: the best catalyst exhibits excellent activity even at 50 °C. It was also observed that the temperature during the hydrothermal reaction severely affected the catalytic performance of the V2O5–TiO2 nanocatalysts. This study suggests the importance of the preparation conditions for manipulating the physicochemical properties and performances of catalysts.

The purpose of this study is to investigate the influences of atmospheric gas and temperature while preparing V2O5/TiO2 catalysts to find a suitable heat-treatment method to improve catalytic performance during the process of H2S removal. The catalysts prepared by wet-impregnation were heat-treated at different temperatures (400 or 600 ℃) under various atmospheres (Air, N2, or H2). The catalytic tests demonstrated that the catalyst heat-treated at 400 ℃ under N2 atmosphere (N-400) possessed excellent catalytic activities regarding H2S conversion (96.4%) and sulfur yield (89.1%). The characterization results revealed that the mild reducing condition employed for N-400 led to the formation of partially reduced V2O5 crystals and a strong V-Ti interaction owing to the anatase TiO2 phase, resulting in the high oxygen vacancies on the catalyst surface. However, severe reducing conditions (H2 or N2 with 600 ℃) or the higher temperature (600 ℃) induced highly reduced V2O5−x or rutile TiO2 related to a weak V-Ti interaction, respectively, which facilitated lower oxygen vacancies. This study is the first to demonstrate the significance of a precisely controlled heat-treatment to enhance catalytic performance for H2S removal.

KAERI의 PRIDE 시설에서 공학규모의 전해환원용 원료물질인 UO2 다공성펠렛 제조를 위해 공정과 장치를 최적화시킨 내 용을 다루었다. UO2 분말과 별도로 attrition 밀링된 대용산화물 분말을 출발분말로, 정밀 칭량을 통해 사용후핵연료 조성을 모사하였다(Simfuel). Simfuel 분말은 각각 tumbling mixer로 혼합하여 균질화 하고, rotary press로 성형하여 furnace를 이 용해 소결하였다. 4% H2-Ar 분위기에서 1450℃ 24시간 고온 열처리하여 제조된 소결펠렛은 6.89 g·cm-3의 벌크밀도를 가 지며 이는 후속 전해환원 공정의 요구에 부합한다. 소결된 다공성펠렛의 미세구조 관찰을 통해 다공성 기지상과 함께 산화/ 금속 석출물이 관찰되어 사용후핵연료의 상이 모사됨을 확인하였다. 본 결과는 향후 공학규모 이상의 파이로 연구를 위한 UO2 다공성펠렛 제조에 중요한 기초자료로 활용 될 것이다.

A simple way to use a commercial Al 2 O 3 -based refractory in a sintering furnace was developed for the head-end system in pyroprocessing. The key was to suppress the intrinsic chemical reactivity of the mullite phase in a refractory with gaseous Cs released from spent fuel in a sintering furnace. Owing to the excellent thermodynamic and thermal stability of Y 2 O 3 , it was deposited on commercial refractories as air-plasma-sprayed coatings. The coated samples maintained their physical integrity without severe delamination after repeated heat treatment at 1450 °C for 12 h in an atmosphere of Ar and 4% H 2 . The chemical reactivity of the coated samples was also confirmed to be satisfactory through a reaction experiment using Cs 2 CO 3 powder. In addition, the coated surfaces were resistant against accidental contamination owing to the formation of non-Cs-reactive yttrium silicate rather than Cs-reactive mullite.

The electrolytic reduction process in pyroprocessing of used fuel requires that fuel fragments be pre-treated to remove cesium and iodine. The effect of high temperatures and fuel fragment size on release of cesium was investigated over 10 h at a temperature of 1300–1400 °C for fuel fragments with an O/U ratio of 2.2. Re-fragmentation of the original fragments was observed as changes in cesium release count rates as fuel was heated from 484 to 1329 °C. The release of cesium is highly dependent on the size of fuel fragments and most of the cesium is released as the temperature is held at the maximum target value.

Ten electrolytic reduction or oxide reduction (OR) runs of a 0.6 kg scale-simulated oxide fuel in a Li 2 O-LiCl molten salt at 650 °C were conducted using metal anode shrouds. During this procedure, an anode shroud surrounds a platinum anode and discharges hot oxygen gas from the salt to outside of the OR apparatus, thereby preventing corrosion of the apparatus. In this study, a number of anode shrouds made of various metals were tested. Each metallic anode shroud consisted of a lower porous shroud for the salt phase and an upper nonporous shroud for the gas phase. A stainless steel (STS) wire mesh with five-ply layer was a material commonly used for the lower porous shroud for the OR runs. The metals tested for the upper nonporous shroud in the different OR runs are STS, nickel, and platinum- or silver-lined nickel. The lower porous shroud showed no significant damage during two consecutive OR runs, but exhibited signs of damage from three or more runs due to thermal stress. The upper nonporous shrouds made up of either platinum- or silver-lined nickel showed excellent corrosion resistance to hot oxygen gas while STS or nickel without any platinum or silver lining exhibited poor corrosion resistance.

We present our findings that uranium (U) metal prepared by using the electrolytic reduction process for U oxide (UO 2 ) in a Li 2 O–LiCl salt can be reoxidized into UO 2 through the reaction between the U metal and Li 2 O in LiCl. Two salt types were used for immersion of the U metal: one was the salt used for electrolytic reduction, and the other was applied to the unused LiCl salts with various concentrations of Li 2 O and Li metal. Our results revealed that the degree of reoxidation increases with the increasing Li 2 O concentration in LiCl and that the presence of the Li metal in LiCl suppresses the reoxidation of the U metal.

A voloxidizer with a double reactor capable of processing several tens of kilograms of HM/batch of nuclear spent fuel has been developed for the decladding and voloxidation of rod-cuts into hulls and pellets through the conversion of UO2 pellets to U3O8 powder. In this study, we optimized the engineering design of this voloxidizer to improve its hull-recovery ratio. First, we tested the oxidation performance of the device prototype and evaluated the effectiveness of various mechanical and chemical voloxidizing methods. On the basis of the results, we selected the screw-and-rotation method for the double rotary drum. Next, we derived a theoretical equation for calculating the optimal reactor volume for various rod-cut weights and lengths and then validated the equation using centimeter-scale acryl reactors and hulls. Subsequently, we modularized the main components such as the heater, utility, motor, reactor, valve, and structure. The double reactor was subject to preliminary separation tests of hulls and powder. Moreover, the hull-separation performance of the voloxidizer reactor was tested at a loading of 50 kg HM/batch. Finally, the remote assembling and disassembling possibility of the modules were experimentally optimized.

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li2O–LiCl electrolyte for the electrochemical reduction of a simulated oxide fuel (simfuel). First, the electrochemical behaviors of Nd2O3, NiO, and Nd2O3–NiO were studied by cyclic voltammetry and voltage control electrolysis. Then, the electrochemical reduction of the simfuel containing UO2 and rare earth oxides (Nd2O3, La2O3, and CeO2) was conducted in molten LiCl salt with 1 wt.% Li2O via the co-reduction of NiO. The extent of reduction of the rare earth oxides was found to be significantly improved. Keywords: Pyroprocessing, Electrochemical reduction, Molten salt, Spent oxide fuel, Rare earth oxides

In this study, the thermal detachment of rare earth (RE)-rich particles adhered to RE-poor particles separated by the high-temperature oxidation of (U,RE)3O8 powder was investigated by repeated oxidation after reduction at low temperature using air and ozone as oxidizing agents. The RE-rich particles detached from RE-poor particles were collected using a magnet of 0.8 T. The relative degree for the detachment and collection of RE-rich particles from RE-poor particles was evaluated by the observation of particle morphologies by SEM. As most of the RE-poor U3O8 particles during oxidation at low temperature were broken into plates, the detachment of RE-rich particles from RE-poor particles increased with increasing RE-rich particle sizes dependent on the oxidation temperature of (U,RE)3O8 powder. The detachment of the RE-rich particles increased with decreasing oxidation temperature from 500 to 360 °C under air. Under ozone, the (U,RE)O2 pellets, RE-rich and poor UO2 particles, and RE-rich U4O9 particles were not oxidized, while the RE-poor U3O8 particles were converted to the RE-poor UO3 particles with very low magnetic susceptibility. The collection of RE-rich particles increased with increasing magnetic susceptibility difference between the RE-rich and RE-poor particles.

A series of electrolytic reduction experiments were carried out using a simulated oxide spent fuel to investigate the reduction behavior of elements in a mixed oxide condition and the fates of elements in the reduction process with 1.0 wt% Li2O-LiCl. It was found out that 155% of the theoretical charge was enough to reduce the simulated. Te and Eu were expected to possibly exist in the precipitate and on the anode surface, whereas Ba and Sr showed apparent dissolution behaviors. Rare earths showed relatively low metal fractions from 28.2 to 34.0% except for Y. And the solubility of rare earths was observed to be low due to the low concentration of Li2O. The reduction of U was successful as expected showing 99.8% of a metal fraction. Also it was shown that the reduction of ZrO2 would be effective when a relatively small amount was included in a metal oxide mixture.

In this study, the feasibility of yttria-stabilized zirconia (YSZ) was investigated for use as a ceramic material, which can be commonly used for both electrolytic reduction and electrorefining. First, the stability of YSZ in salts for electrolytic reduction and electrorefining was examined. Then, its stability was demonstrated by a series of pyroprocessing tests, such as electrolytic reduction, LiCl distillation, electrorefining, and LiCl KCl distillation, using a single stainless steel wire mesh basket containing fuel and YSZ. A single basket was used by its transportation from one test to subsequent tests without the requirements for unloading.

The electrolytic reduction rate of porous UO2 pellets in a LiCl salt was investigated for various applied charges. The degree of reduction (α) value was evaluated from the ratios of cross-sectional areas of the reduced and oxide parts. An analysis of the experimental results revealed that the first-order reaction model is the best geometry function to describe the reduction reaction. An electrolytic reduction rate equation was proposed using the first-order model, although it was available in a limited region of (0≤α≤0.56). A power law based reaction rate equation was also suggested for the whole range of α, and the reaction time for a complete reduction, estimated using the power law equation, was confirmed through the experimental results. Changes in the Li-Li2O concentration around the reduced pellets for various applied charges were also measured, which increased up to 23 wt% with increasing α.

The effect of sintering aids on the grain growth of simulated DUPIC fuel pellets with burn-ups was investigated using SIMFUEL pellets as a surrogate for actual spent PWR fuel avoiding the high radioactivity of spent fuel. In this study, titania and niobia were used as sintering aids, and SIMFUEL pellets with burn-ups of 35 GWd/tU and 60 GWd/tU were used. The sintering aids significantly improved the grain sizes of the sintered pellets. Moreover, metallic precipitates, which inhibit the movement of the grain boundary, were included even at the interior of grains due to an increase of grain boundary mobility caused by the addition of sintering aids. Titania is more effective than niobia in grain growth, while niobia is more effective than titania in densification.

The effect of oxidation on dielectric-temperature behavior of core/shell grained BaTiO3 was studied in an attempt to reveal the prime governing parameter of dielectric-temperature behavior. BaTiO3 samples doped with 3 mol% Y2O3 or 3 mol% Dy2O3 with the same effective charges were sintered in a reducing atmosphere (Po2

The platinum anode for the electrolytic reduction process is generally surrounded by a nonporous ceramic shroud with an open bottom to offer a path for O2 gas produced on the anode surface and prevent the corrosion of the electrolytic reducer. However, the O 2- ions generated from the cathode are transported only in a limited fashion through the open bottom of the anode shroud because the nonporous shroud hinders the transport of the O 2- ions to the anode surface, which leads to a decrease in the current density and an increase in the operation time of the process. In the present study, we demonstrate the electrolytic reduction of 1 kg-uranium oxide (UO2) using the porous shroud to investigate its long-term stability. The UO2 with the size of 1~4 mm and the den

In conventional van der Waals type equation of state (EOS) modeling, the denominator of the attractive contribution is regarded as being a necessarily factorable quadratic polynomial for the closed-form expression of the corresponding Helmholtz free energy. This study evaluates the effect of the opposite case, an irreducible quadratic polynomial, on the description of the volumetric behavior of real fluid through a comparison with the critical isotherm data, coexistence curve, and PVT isotherm data. In a generalized cubic EOS, an irreducible quadratic polynomial was found to yield an improved description of the flattened region around the critical density while a nonclosed form of the Helmholtz free energy results. For an improved volumetric description using an irreducible quadratic polynomial and a derivation of the closed form of the Helmholtz free energy, we developed a quintic EOS containing seven parameters, two of which are determined by correlating the critical isotherm data and others, by regress...

The reduction behavior of U 3 O 8 during sintering in an Ar atmosphere has been investigated in the temperature range between 1200 and 1500 °C. The changes in stoichiometry (O/U ratio) and dimension of the U 3 O 8 pellets were measured by TGA and TMA, respectively. During the reduction at 1400 and 1500 °C, the reduction rate was decreased abruptly at the O/U ratio below about 2.25 while a constant reduction rate was observed at 1200 and 1300 °C. The retardation was accompanied by a significant decrease of the activation energy for the reduction process, possibly due to the shift of the rate determining step. The sintered density, however, was similar irrespective of the sintering temperature. The microstructural observation by SEM demonstrated that the pores did not contribute to the densification due to their coalescence and entrapment in the grain boundaries, triple junctions, and inside the grains, resulting in the porous microstructures.

Following the Fukushima accident in March 2011, Korea Atomic Energy Research Institute (KAERI) has developed micro-cell fuel pellets for the purpose of reducing gas release, both stable and radioactive, not only under accident conditions, but during normal operation as well. The micro-cell pellet is composed of many cells covered with a ceramic or metallic wall material, whose function is either to lower or block the movement of gas atoms out of the cells, thereby reducing gas release out of the pellet. Based on both a Halden irradiation test of the micro-cell pellets up to 16 MWd/kgU and experience of fission gas release (FGR) in the conventional UO2 pellet, a simplified approach is proposed to estimate FGR in the micro-cell pellets when it is irradiated with a constant linear power of 30 kW/m up to 60 MWd/kgU under normal operating conditions. Considering the lack of measured data on diffusivity in the micro-cell pellets, sensitivity studies for FGR were performed for two cases: diffusivity in the micro-cell pellet is the same as and 10 times higher than in the UO2 pellet, respectively. Even for the 10 times higher diffusivity, FGR in the ceramic micro-cell pellet is at least comparable to or lower than in the UO2 pellet, and FGR in the metallic micro-cell pellet is clearly lower than in the UO2 pellet, due to its lower fuel temperature and the impact of its walls on gas movement. In real situations, the degree of reduction of FGR in the micro-cell pellets would be determined by the fraction of perfect walls and the magnitude of gas diffusivity. Until PIE results become available for the irradiated micro-cell pellets, the current approach provides only rough estimates and therefore its results need to be taken as preliminary.

The mechanism of core/shell formation during sintering in BaTiO3-based systems was studied in (Mg, Y)-doped BaTiO3. The effect of ball milling time on core size and shell thickness was first observed. The core size was similar irrespective of ball milling time whereas the shell thickness increased with increasing ball milling time. The measured powder size after ball milling suggested that the cores were from the larger BaTiO3 particles and the shells formed via dissolution of smaller particles and precipitation of dissolved material, in contrast with the interpretation of the results of a previous investigation. To identify the core/shell formation mechanism, bi-layer samples with different chemical compositions, 94BaTiO3–2Y2O3–2MgO–2SiO2 (mol%) (BT–YMS) and 98BaTiO3–2SiO2 (mol%) (BT–S), and different grain sizes were prepared. The morphology of the newly formed shell layer and the shape of an {111} twin across the interface between a core and a shell confirmed that the formation mechanism of the core/shell structure during sintering is the dissolution and precipitation of material rather than solid-state diffusion of solutes into BaTiO3.

The changes in solid/liquid interface structure and grain growth behavior with oxygen partial pressure ( P O 2 ) were systematically studied during liquid-phase sintering of 8TiO2/2SiO2-added BaTiO3. As the P O 2 of the sintering atmosphere increased, the grain boundaries and solid/liquid interfaces showed increased faceting, indicating an increase in step free energy. This increase in P O 2 and step free energy caused a change in grain growth behavior as a function of sintering time. When samples were sintered in H2, abnormal grain growth (AGG) occurred from the beginning, resulting in a coarse microstructure with a large average grain size. With increasing P O 2 , the incubation time necessary for AGG also increased. Finally, for samples sintered in air, AGG did not occur even after 100 h. These changes in incubation time for abnormal grain growth demonstrate the effect of changing the step free energy on the microstructural development during liquid phase sintering of ceramic systems.

In the pyroprocess integrated inactive demonstration (PRIDE) facility at the Korea Atomic Energy Research Institute (KAERI), UO2porous pellets were fabricated as a feed material for electrolytic reduction on an engineering scale of 30 kg-U/batch. To increase the batch size, we designed and modified the corresponding equipment for unit processes based on ceramic processing. In the course of pellet fabrication, the correlation between the green density and sintered density was investigated within a compaction pressure range of 106–206 MPa, in terms of the optimization of processing parameters. Analysis of the microstructures of the produced UO2porous pellets suggested that the pellets were suitable for feed material in the subsequent electrolytic reduction process in pyroprocessing. This research puts forth modifications to the process and equipment to allow the safe mass production of UO2porous pellets; we believe these results will have immense practical interest.

Two sets of (Mg,Y)-doped BaTiO3 samples were prepared to investigate the effects of the core/shell volumetric ratio on the dielectric-temperature behavior of BaTiO3: one set with samples of the same grain size but different core sizes and the other with samples of the same core size but different shell thicknesses. The microstructural variation of the samples was characterized and their dielectric properties were measured. For both sets of samples, the temperature stability of the dielectric properties was generally improved with a reduction of the volumetric shell ratio regardless of the grain and core sizes. There existed, however, a limit of the reduction; for the studied range, shell thickness of one third of the core radius appeared to be an optimum thickness for the given amounts of dopants. It was concluded that the volumetric shell ratio should be optimized so as not to exceed a specific limit, for our case two thirds of the grain volume, to secure temperature stability of the dielectric properties of BaTiO3.

Core/shell-grained BaTiO3 samples were prepared with addition of rare earth elements. The core/shell interface was semi-coherent, and many misfit dislocations formed in Dy-doped samples. In contrast, a coherent interface and few dislocations were observed in Ho- and Er-doped samples. Dy-doped samples exhibited poor temperature stability, showing a peak with no frequency dispersion. Ho- and Er-doped samples exhibited a broad curve with frequency dispersion. This improved temperature stability is attributed to the coherency strain, which leads to the formation of polar nano-regions in the shell. Coherency at the core/shell interface is critical to improve the temperature stability of core/shell-structured BaTiO3.