Yang-Il Jung

KAERI’s Development of LWR Accident-Tolerant Fuel

Abstract The Fukushima accident has had a tremendous impact on Japan and the rest of the world in the areas of public health, economy, and nuclear energy policy. Thus, international consensus has been reached that inherent tolerance of nuclear fuel to severe accidents needs to be increased significantly to prevent accidents or to mitigate their consequences. In this respect, several countries have started to develop accident-tolerant fuel (ATF) that can tolerate loss of active cooling for a considerably longer time period than current fuels, while maintaining or improving performance during normal operations and operational transients and also enhancing fuel safety for beyond-design-basis events. The Korea Atomic Energy Research Institute is also developing ATF: surface-coated Zr cladding and metal-ceramic hybrid cladding for the purpose of suppressing hydrogen generation during severe accidents, and microcell UO2 pellets to enhance the retention of highly radioactive and corrosive fission products such as Cs and I, where all UO2 grains are enveloped by thin cell walls that act as chemical traps or physical barriers for the movement of fission products. When the screening of developing fuel materials has been performed through various out-of-pile tests, irradiation tests of the selected materials will be carried out in a research reactor to demonstrate their enhanced accident tolerance.

MICROSTRUCTURE AND MECHANICAL STRENGTH OF SURFACE ODS TREATED ZIRCALOY-4 SHEET USING LASER BEAM SCANNING

The surface modification of engineering materials by laser beam scanning (LBS) allows the improvement of properties in terms of reduced wear, increased corrosion resistance, and better strength. In this study, the laser beam scan method was applied to produce an oxide dispersion strengthened (ODS) structure on a zirconium metal surface. A recrystallized Zircaloy-4 alloy sheet with a thickness of 2 ㎜, and Y₂O₃ particles of 10 ㎛ were selected for ODS treatment using LBS. Through the LBS method, the Y₂O₃ particles were dispersed in the Zircaloy-4 sheet surface at a thickness of 0.4 ㎜, which was about 20% when compared to the initial sheet thickness. The mean size of the dispersive particles was 20 ㎚, and the yield strength of the ODS treated plate at 500oC was increased more than 65 % when compared to the initial state. This strength increase was caused by dispersive Y₂O₃ particles in the matrix and the martensite transformation of Zircaloy-4 matrix by the LBS.

Manufacturing and Examination for ITER Blanket First Wall Small-Scale Mockups With KoHLT-EB in Korea

The ITER first wall (FW) includes beryllium armor tiles joined to a CuCrZr heat sink. The FWs are one of the critical components in an ITER machine with a surface heat flux of 4.7 MW/m2 or above. The small-scale mockup shall be a part of the qualification tests and used to validate the performance of the dominant manufacturing technologies before the production of larger scale components, and this mockup shall be equipped with a hypervapotron heat sink and manufacturing processes developed for a semiprototype design. The small-scale mockup includes 48 beryllium armor tiles (12 mm × 12 mm) capable of withstanding the specified heat flux values. The tile thickness shall be 6 mm to minimize the beryllium surface temperature and evaporation under high thermal loads. The detailed fabrication process of semiprototype small-scale mockups was developed for a qualification test in Korea. For the CuCrZr and stainless steel, the canned materials are processed into an hot isostatic pressing (HIP) device. In the case of beryllium-to-CuCrZr joining, the HIP was conducted at 580°C and 100 MPa. For nondestructive tests of the fabricated semiprototypes, visual and dimension inspections were performed whenever needed during the fabrication process, and ultrasonic tests were performed using ultrasonic probes. Destructive tests for the qualification semiprototype were performed on a small-scale mockup, which was fabricated together with semiprototypes. The Korea heat load test facility using an electron beam system was constructed with an electron gun (maximum electric power of 800 kW) for a high heat flux application with a 300-kW high-voltage power supply and maximum accelerating voltage of 60 kV. This facility was operated to evaluate the performance test of plasma facing components. A cyclic heat flux test will be performed to evaluate the ITER qualification program.

PROPERTIES OF ZR ALLOY CLADDING AFTER SIMULATED LOCA OXIDATION AND WATER QUENCHING

In order to study the cladding properties of zirconium after a loss-of-coolant accident (LOCA)-simulation oxidation and water quenching test, commercial Zircaloy-4 and two kinds of HANA claddings were oxidized at temperatures ranging from 900℃ to 1250℃ and exposed for 300 s, and then cooled to 700℃ before quenching. Microstructural observations were made to evaluate the matrix characteristics with the chemical compositions after the LOCA-simulation test. Ring compression testing was then performed to compare the ductile behaviour of the HANA and Zircaloy-4 claddings. An X-ray diffraction (XRD) analysis was carried out for temperatures ranging from room temperature to 1250℃ for the oxide layer to verify the oxide crystal structure at each oxidation temperature.

Effect of heat-treatment on phase formation and crystallization of sol–gel derived Al2O3, ZrO2–Y2O3, and Ta2O5 oxide coatings

Abstract Various oxides of Al2O3, ZrO2–Y2O3, and Ta2O5 were coated on ferritic–martensitic steel for application as an environmental barrier layer. Sol–gel based coating was investigated to form the oxides by varying the coating parameters, such as the concentration of the precursors, the temperature of the curing, cycles of repeated runs, and additional heat-treatment. The obtained coatings revealed nano-sized granular structures. The surface morphologies were rough in alumina and zirconia, but appeared smooth in tantalum oxide. In the case of alumina and tantalum oxide, coated layers were mostly amorphous after pyrolysis at 750 °C. The crystalline phases were obtained after an additional heat-treatment at 950 °C. In the case of zirconia, a desirable oxide phase was formed when the samples were cured at 750 °C during the coating process. In addition to the heat-treatment after the coating, the repeated coatings were effective in crystallizing the coated layers and forming proper oxides.

Effect of preceramic and Zr coating on impregnation behaviors of SiC ceramic composite

SiC fiber-reinforced ceramic composites were fabricated using a polymer impregnation and pyrolysis process. To develop the low temperature process, the pyrolysis was conducted at 600 °C in air. Both a microstructural observation and a mechanical test were utilized for the evaluation of the impregnation. For the impregnation, two kinds of polycarbosilane having a different degree of cross-linking were used. The level of cross-linking affected the ceramic yield of the composites. The cross-linking under oxygen containing atmosphere resulted in a dense matrix and high density of filling. However, tight bonding between the matrix and fibers in the fully dense composite samples, which was obtained using a cross-linking agent of divinylbenzene, turned out to be deteriorative on the mechanical properties. The physical isolation of fibers from matrix phase in the composites was very important to attain a mechanical ductility. The brittle fracture was alleviated by introducing an interphase coating with metallic Zr. The combination of forming the dense matrix and interphase coating should be a necessary condition for the SiCf/SiC fiber-reinforce composite, and it is practicable by controlling the process parameters.

Effect of Al 2 O 3 Coating as a Corrosion Barrier of Ferritic-Martensitic Steel in Pb-Li Melt

Abstract Corrosion of ferritic-martensitic steel (FMS) Gr. 91 was performed in static Pb-15.7Li melt at 450 °C for up to 3000 h. Preferential grain boundary corrosion along with a homogeneous dissolution was observed. In addition, Al2O3 was coated to prevent the surface recession of FMS. Al2O3 was deposited on FMS using an electron-beam evaporated physical vapor deposition. The as-coated layer was crystallized through a heat-treatment at above 950 °C for 2 h. The alumina coating layer was very stable and effective to prevent the corrosion of FMS. Although Al2O3 was decomposed in 3000 h, the corrosion barrier survived up to 2000 h even in an oxygen-containing harsh environment.

Joining of Be to Ferritic-Martensitic Steels with Diffusion Barrier Interlayer

Abstract A joining of Be to ferritic-martensitic steels (FMS) is an essential process in the fabrication of ITER test blanket module (TBM). The diffusion barrier layers together with the coated interlayer were applied to the HIP joining of Be and FMS in order to develop the interlayer technology for the fabrication of ITER TBM. Multiple layers formed due to an excessive diffusion of elements in the interface region in the absence of a diffusion barrier layer. Such a complicated interface structure consisting of brittle phases in part would be very prone to fracture even at low stress levels. A Cu foil or a HIPed CuCrZr layer applied as a diffusion barrier was effective to retard the diffusion between Be and FMS. It was revealed that the diffusion barrier layers helped to improve the joining properties by reducing the possibility to form diffusion layers in the interface, which made the Be/FMS joint have an appreciable joining strength.

HIP Joining of Tungsten Armor to Ferritic-Martensitic Steel with a Zirconium Interlayer

Abstract Tungsten was joined to ferritic-martensitic steel (FMS) for application in a plasma facing component. Zirconium foil was investigated as an interlayer material for the joining of W to FMS. Repeated hot isostatic pressing (HIP) was conducted for the fabrication of W/FMS joints. The first HIP was performed at 950°C under 100 MPa for 1.5 h (diffusion joining stage), and the second HIP was executed at 750°C under 70 MPa for 2 h (tempering stage). The Zr interlayer formed a sound interface between W and FMS with no observable pores and cracks. The joining strength of W/FMS measured by a shear test was about 54 MPa. Elemental diffusion was observed along the hetero-interfaces of W/Zr and Zr/FMS. At the W/Zr interface, a thin layer of W–Zr inter-phase was observed. At the Zr/FMS interface, no intermetallic compound was formed, however, fine Zr grains featuring body-centered tetragonal lattice structures were formed near the interface.

Effect of Pressurizing during Compaction and Sintering on the Formation of Reaction-Bonded SiC–Ti3SiC2

A reaction-bonded SiC-Ti3SiC2 ceramic composite was produced for use in a ceramic-metal composite cladding tube. The diffusion reaction between TiC and Si was investigated with respect to process pressure. The mole-fraction of TiC and Si was controlled to be 3:2 to obtain a Ti3SiC2 phase in the ceramic composite. Sintering was conducted at 1450 °C where TiC particles could react with melted Si. SiC ceramic composites consisting of Ti3SiC2 and TiSi2 matrix phases were obtained. The formation of the constituent phases was strongly related to the processing pressure. The number of second phases in the SiC-Ti3SiC2 composite was controlled by adjusting the processing pressure. When the powder compacts were not pressurized, no Ti3SiC2 phase was formed. However, the Ti3SiC2 phase was formed under pressurizing during compaction and/or sintering. The higher the pressure the higher the purity of SiC-Ti3SiC2. The dual-phased SiC-Ti3SiC2 composite, however, revealed the decreased resistance to high-temperature oxidation. It is suggested that the incorporation of TiSi2 in the composite increases the oxidation resistance as well as mechanical property. †(Received September 24, 2015)