Ikuo Ioka

Influence of Impurities on Intergranular Corrosion of Extra High Purity Austenitic Stainless Steels

An intergranular corrosion is observed in austenitic stainless steels exposed to high temperature, concentrated nitric acid (HNO3 ) solution with highly oxidizing ions. It is an important degradation mechanism of austenitic stainless steels for use in a nuclear fuel reprocessing plant. The intergranular corrosion is caused by the segregation of impurities to grain boundaries and the resultant formation of active sites. Extra High Purity (EHP™) austenitic stainless steel was developed with conducting the new multiple refined melting in order to suppress the total harmful impurities less than 100ppm. The intergranular corrosion behavior of EHP alloys with various impurities was examined in boiling HNO3 solution with highly oxidizing ions to find a correlation between the intergranular corrosion and the impurities of EHP alloys. A good correlation was confirmed between the degree of intergranular corrosion and the corrosion rate. The relationships between the corrosion rate and the impurities content of EHP alloys was determined using a multiple regression analysis. The influence on corrosion rate became small in order of B, P, Si, C, S and Mn. It was important to control B in intergranular corrosion behavior of EHP alloys.Copyright

Susceptibility of Intergranular Corrosion for Extra High Purity Austenitic Stainless Steel in Nitric Acid

Austenitic stainless steels suffer intergranular attack in boiling nitric acid with oxidants. The intergranular corrosion is mainly caused by the segregation of impurities to grain. An extra high purity austenitic stainless steel (EHP alloys) was developed with conducting the new multiple refined melting technique in order to suppress the total harmful impurities less than 100ppm. The basically corrosion behavior of type 310 EHP alloy with respect to nitric acid solution with highly oxidizing ions was investigated. The straining, aging and recrystallizing (SAR) treated type 310 EHP alloy showed superior corrosion resistance for intergranular attack. The segregated boron along the grain boundaries was one of main factor of intergranular corrosion from fission track etching results. The SAR treatment was effective to restrain the intergranular attack for type 310 EHP alloy with B less than 7ppm.Copyright

Recent activities in the field of nuclear materials and nuclear fuels

The research and development of nuclear materials and fuels is aggressively advanced for the improvement of reliability or performance of nuclear systems, such as light water reactor, fast breeder reactor (FBR), fusion reactor and so on. Many of outstanding researches are presented by the Journal of Nuclear Science and Technology. The latest activity in nuclear materials and fuels is introduced in this summary by surveying the recent journal. Several ceramic materials possess many superior properties for fusion reactors or high-level radioactive waste forms. Though thermal property is one of the most important factors, neutron-irradiated ceramics show severe degradation in thermal diffusivity [1–3]. A positron annihilation lifetime (PAL) has been measured to investigate the irradiation defects which primarily controlled thermal diffusivity in ceramics [4]. The correlation between PAL and thermal diffusivity was clarified with a-Al2O3 and AlN in doses of 0.01–0.02 dPa [5]. Nowadays, ceramics as alternative waste forms of glasses have been developed to immobilize actinide elements extracted from high-level radioactive nuclear wastes. Thermal properties of some appropriate ceramics (Y6WO12, Yb6WO12, Y6UO12) were reported as the waste form [6–8]. Since ceramic materials have the excellent characteristic, it is expected that these activities will be promoted more. There have been considerable researches of zirconium alloys for the cladding tube, low-alloy steels for the reactor pressure vessel and stainless steels for the core structural materials. The behavior of hydrogen diffusion in zirconium oxide was reported [9] because the hydrogen in zirconium oxide affected the corrosion resistance of the cladding tube made of zirconium alloy. The hydrogen diffusion behavior in monoclinic and tetragonal zirconium oxides was estimated by performing electronic state calculation [10,11]. As a result, it was estimated that compression stress reduced