Chiaki Kato

Characterization and storage of radioactive zeolite waste

For the safe storage of zeolite wastes generated by the treatment of radioactive saline water at the Fukushima Daiichi Nuclear Power Station, this study investigated the fundamental properties of herschelite adsorbent and evaluated its adsorption vessel for hydrogen production and corrosion. The hydrogen produced by the herschelite sample is oxidized by radicals as it diffuses to the water surface and thus depends on the samples water level and dissolved species. The hydrogen production rate of herschelite submerged in seawater or pure water may be evaluated by accounting for the water depth. From the obtained fundamental properties, the hydrogen concentration of a reference vessel (decay heat = 504 W) with or without residual pure water was evaluated by thermal–hydraulic analysis. The maximum hydrogen concentration was below the lower explosive limit (4%). The steady-state corrosion potential of a stainless steel 316L increased with the absorbed dose rate, but the increase was repressed in the presence of herschelite. The temperature and absorbed dose at the bottom of the 504 W vessel were determined as 60 °C and 750 Gy/h, respectively. Under these conditions, localized corrosion of a herschelite-contacted 316L vessel would not immediately occur at Cl− concentrations of 20,000 ppm.

Corrosion Phenomenon of Stainless Steel in Boiling Nitric Acid Solution Using Large-Scale Mock-Up of Reduced Pressurized Evaporator

To evaluate the component life in a spent nuclear fuel reprocessing plant, a large-scale mock-up test apparatus of a reduced pressurized thermosiphon evaporator was constructed, and the corrosion mechanism of a heat transfer tube made of ultralow carbon type 304 stainless steel in boiling nitric acid solution was studied. The corrosion tests were conducted for about 36,000 h, and changes in the corrosion amount and rate in the test duration were discussed. The relationships between the amount of corrosion and tube surface temperature and heat flux were investigated, and the corrosion propagation mechanism considering intergranular penetration was studied based on the observations of morphologies of corrosion surfaces and the measurements of intergranular penetration depths. After a long duration, the increases in the corrosion amount and rate saturated when intergranular penetration and grain dropping occurred by turns. This result means that a linear estimation can be applied to the life prediction for corrosion. Three portions of the tube were observed, and the amounts of corrosion were different among the three portions, but no difference in the morphology of intergranular corrosion existed. The amount of corrosion was affected by both tube surface temperature and heat flux. A large amount of corrosion could be observed in both the boiling starting portion and the top, where high tube surface temperature and heat flux were observed.

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

In Situ Electrochemical Study on Crevice Environment of Stainless Steel in High Temperature Water

In situ electrochemical impedance spectroscopy measurement within crevice of stainless steel in 288 °C water has been conducted to analyze crevice water chemistry. Small sensors (\( {\varphi} {\sim} 250\,\upmu{\text{m}}) \) measured local solution electrical conductivity κ crev, polarization resistance and electrochemical corrosion potential. Real-time response of the κ crev as functions of bulk water conductivity and dissolved oxygen (DO) concentration has been quantitatively analyzed. The κ crev differ more than an order of magnitude depending on the oxygen potential inside the crevice. The κ crev increased with addition of small amount of bulk DO (e.g. 30 ppb). The maximum κ crev was observed with DO of 32,000 ppb and became more than 100 times higher than that of bulk water. The effect of geometrical factors on the crevice environment was also found to play an important role in the water chemistry inside.

The effect of crystal textures on the anodic oxidization of zirconium in a boiling nitric acid solution

ABSTRACT The effects of crystal textures and the potentials in the anodic oxidation of zirconium in a boiling nitric acid solution were investigated to study the stress corrosion cracking of zirconium in nitric acid solutions. The test specimen was machined such that the specimen surface was parallel to the rolling surface, arranged with a (0002) crystal texture. The potentials applied for the anodic oxidation of zirconium were set at 1.2, 1.4, and 1.5 V against a saturated KCl–Ag/AgCl electrode (SSE) in boiling 6 M HNO3. The growth of the zirconium oxide film dramatically changed depending on the applied potential at a closed depassivation potential (1.47 V vs. SSE in this study). At 1.5 V, the zirconium oxide film rapidly grows, and its growth exhibits cyclic oxidation kinetics in accordance with a nearly cubic rate law. The zirconium oxide film grows according to the quantity of electric charge and the growth rate does not depend on the crystal texture in the pretransition region before the cyclic oxidation kinetics. However, the growth and cracking under the thick oxide film depend on the crystal texture in the transition region. On the normal direction side, the oxide film thickness decreases on average since some areas of the thick oxide film are separated from the specimen surface owing to the cracks in the thick oxide. On the rolling direction (RD) side, no cracks in the thick oxide film are observed, but cracks are found under the thick oxide film, which deeply propagate in metal matrix along the RD without an external stress. The cracks under the thick oxide film propagate to the center of the oxide layer. The crystal orientation relationship between the oxide layer and the zirconium matrix is (0002)Zr//(111)ZrO2, and the cracks in the oxide layer propagate in the (0002)Zr plane in the zirconium matrix. The oxide layer consists of string-like zirconium oxide and zirconium hydride. The string-like zirconium oxide contains orthorhombic ZrO2 in addition to monoclinic ZrO2. It is not well known why the cracks propagate along the (0002)Zr plane under the thick oxide film. As one assumption for the mechanism of crack initiation and propagation without an external stress, it is considered that the oxidizing zirconium hydrides precipitated in the (0002)Zr plane near the interface of the thick oxide film and the matrix. Then, the phase transformation from orthorhombic ZrO2 to monoclinic ZrO2 in the oxide layer causes the crack propagation in the (0002) plane.

Study on Corrosion of Stainless Steel in Boiling Nitric Acid Under Heat Transfer Conditions

In PUREX process for spent fuel reprocessing plants, nitric acid solutions are treated in many components. Particularly, heating portions in the components are severely corroded in the boiling solution under heat transfer (HT) conditions. Although corrosion behavior under the conditions has been investigated by many researchers, corrosion mechanism has not been sufficiently understood. Consequently, because of its safety, we need to study dominant factors of corrosion mechanism and to develop a method to predict corrosion progress.Surface temperature, heat flux, concentration and compositions of the solution have been previously considered as important factors for corrosion rate (CR). In this paper, authors have focused on the effects of surface temperature and heat flux on CRs of stainless steels in boiling nitric acid under HT conditions. We performed experimental study in consideration of dissolvers and concentrators. Two types of cells for HT and immersion conditions were applied for corrosion tests and the effects were compared. Test solution used was 33 mol·m−3 (0.033 M) vanadium added to 3 kmol·m−3 (3 M) nitric acid solution. The solution was heated at boiling temperature under atmospheric pressure. Additionally, a boiling curve which was indicated the relation between heat flux and degree of superheating was investigated experimentally. Surface temperatures during corrosion tests were estimated from a boiling curve. The relationship among measured CRs, surface temperature and heat flux were studied.The results showed that CR did not depend on heat flux, but depended on surface temperature. Arrhenius plots on CRs indicated that CR was accelerated by solution boiling against non-boiling. The cause of the acceleration of CR under boiling nitric acid solution was discussed.Copyright