Jeong-Yong Park

Out-of-pile and In-pile Perfomance of Advanded Zirconium Alloys (HANA) for High Burn-up Fuel

The performance of the advanced Zr alloys (HANA) for a high burn-up fuel has been evaluated in the out-of-pile and in-pile conditions. The corrosion resistance of the HANA claddings was superior to Zicaloy-4 in a PWR-simulating loop condition. The improved corrosion resistance of the HANA claddings was attributed to the fine distribution of the precipitate. HANA claddings showed a higher creep resistance as compared to Zircaloy-4 from the thermal creep test. The deformation behavior of HANA in a LOCA condition was similar to Zircaloy-4. Threshold ECR value of HANA was higher than the conventional value of 17% in Zircaloy-4, which is mainly due to the fact that the Nb decreases the oxidation rate as well as the hydrogen pickup. Fretting wear test revealed that HANA claddings have a similar wear resistance to Zircaloy-4. From the irradiation test up to burn-up of about 12 GWd/MtU, HANA claddings showed a better corrosion resistance as well as a better creep resistance than Zircaloy-4. The in-pile corrosion resistance of the HANA claddings was improved by 40–50% as compared to Zircaloy-4 on the basis of the oxide thickness measurements.

Effects of ammonia on the alignment of carbon nanotubes in metal-assisted thermal chemical vapor deposition

The effects of ammonia on alignment of carbon nanotubes in an atmospheric pressure thermal chemical vapor deposition assisted by Ni were investigated. It was confirmed that ammonia is critical to the alignment of nanotubes at temperatures of 800–950°C. Both synthesis by C2H2 after ammonia pretreatment and synthesis by NH3/C2H2 mixture flow could establish alignment. Alignment of carbon nanotubes due to ammonia was realized not by its influence on the morphology of the metal particles, but by its role of inhibiting a formation of amorphous carbon during the synthesis, particularly in its initial stage.

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.

Application of Coating Technology on Zirconium-Based Alloy to Decrease High-Temperature Oxidation

Since the Fukushima accident, it has been recognized that a hydrogen-related explosion is one of the major concerns regarding reactor safety during the hightemperature oxidation of zirconium alloys. To decrease the high-temperature oxidation rate of zirconium-based alloy, a coating technology for the zirconium alloy surface was considered. The selection of coating materials was based on the neutron cross-section, thermal conductivity, thermal expansion, melting point, phase transformation behavior, and high-temperature oxidation rate. After consideration of these factors, silicon was selected as a coating material for the first surface coating of zirconium-based alloy. A plasma spray and laser beam scanning were selected for the coating method, as both can be applied to a long tube shape without high-vacuum and high-temperature environments during the coating process. After Si-coated samples on Zircaloy-4 sheet had been prepared via plasma spray and combined plasma spray–laser beam scanning treatments,

INFLUENCE OF ALLOY COMPOSITION ON WORK HARDENING BEHAVIOR OF ZIRCONIUM-BASED ALLOYS

Three types of zirconium base alloy were evaluated to study how their work hardening behavior is affected by alloy composition. Repeated-tensile tests (5% elongation at each test) were performed at room temperature at a strain rate of 1.7 x 10 -3 s -1 for the alloys, which were initially controlled for their microstructure and texture. After considering the yield strength and work hardening exponent (n) variations, it was found that the work hardening behavior of the zirconium base alloys was affected more by the Nb content than the Sn content. The facture mode during the repeated tensile test was followed by the slip deformation of the zirconium structure from the texture and microstructural analysis.

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.

IN-PILE PERFORMANCE OF HANA CLADDING TESTED IN HALDEN REACTOR

An in-pile performance test of HANA claddings was conducted at up to 67 GWD/MTU in the Halden research reactor in Norway over a 6.5 year period. Four types of HANA claddings (HANA-3, HANA-4, HANA-5, and HANA-6) and a reference Zircaloy-4 cladding were used for the in-pile test. The evaluation parameters of the HANA claddings were the corrosion behavior, dimensional changes, hydrogen uptake, and tensile strength after the claddings were tested under the simulated operation conditions of a Korean commercial reactor. The oxide thickness ranged from 15 to 37 mm at a high flux region in the test rods, and all HANA claddings showed corrosion resistance superior to the Zircaloy-4 cladding. The creep-down rate of all HANA claddings was lower than that of the Zircaloy-4 cladding. In addition, the hydrogen content of the HANA claddings ranged from 54 to 96 wppm at the high heat flux region of the test rods, whereas the hydrogen content of the Zircaloy-4 cladding was 119 wppm. The tensile strength of the HANA and Zircaloy-4 claddings was similarly increased when compared to the un-irradiated claddings owing to the radiation-induced hardening.

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.