Il-Hyun Kim

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.