Fumiyoshi Ueno

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.

Hydrogen Peroxide Production by Gamma Radiolysis of Sodium Chloride Solutions Containing a Small Amount of Bromide Ion

Abstract Gamma radiolysis experiments on solutions of a mixture of sodium chloride (NaCl) and sodium bromide (NaBr) were conducted to confirm the validity of radiolysis calculations for simulated seawater solutions and to determine the importance of bromide anion (Br−) in the production of hydrogen peroxide (H2O2) via water radiolysis. The H2O2 concentration in each solution was measured after irradiation and compared with that obtained from radiolysis calculations. It was found that the calculated and experimental results were in good agreement. The concentration of H2O2 in a 0.6 M NaCl solution increased approximately three times on the addition of 1 mM NaBr. The result showed that Br− plays an important role in the production of H2O2 by water radiolysis, presumably through the reactions of Br− with hydroxyl radical (•OH). For 1 mM NaCl solutions, there is a minimum production rate of H2O2 at pH 8, which increases when the pH changes to either lower or higher values. It was considered that the hydrated electron also plays an important role in H2O2 production under these acidic and alkaline conditions.

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.

Simulation for radiolytic products of seawater: effects of seawater constituents, dilution rate, and dose rate

Radiolysis calculations of simulated seawater were conducted using reported data on chemical yields and chemical reaction sets to predict the effects of seawater constituents on water radiolysis. Hydrogen, oxygen, and hydrogen peroxide were continuously produced from simulated seawater during γ-ray irradiation. The concentration of H2 exceeded its saturation concentration before it reached the steady-state concentration. The production behavior of these molecules was significantly promoted by the addition of bromide ions (Br−) because of the high reactivity of Br− with the hydroxyl radical, an effective hydrogen scavenger. It is also shown that the concentrations of these molecules were effectively suppressed by diluting seawater constituents by less than 1%.

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.

Hydrogen Absorption Behavior of Titanium Alloys by Cathodic Polarization

Titanium and Ti-5mass%Ta alloy has been utilized in nuclear fuel reprocessing plant material because of its superior corrosion resistance in nitric acid solutions. However, Ti alloy have been known to high susceptibility of hydrogen embrittlement. To evaluate properties of hydrogen absorption and hydrogen embrittlement of Ti alloys, cathodic polarization tests and slow strain rate tests (SSRT) under cathodic polarization were carried out. Results show titanium hydrides covered on the surface of metals and hydrides thickness were within 10μm. But hydride did not observed at inner part of metals. Ti and Ti-5%Ta did not show hydrogen embrittlement by SSRT under cathodic charging. These results suggested that Ti and Ti-5%Ta could absorb hydrogen. But hydrogen did not penetrate inner portion of the metals more than 10μm in depth because titanium hydrides act as barrier of hydrogen diffusion. It is considered that retardation of hydrogen diffusion hindered hydrogen embrittlement of Ti and Ti-5%Ta alloys.Copyright

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