Hideyuki Hosokawa

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (V) Characterization of Oxide Film with Multilateral Surface Analyses

In order to understand corrosion behavior of stainless steel in BWR reactor water conditions, characteristics of oxide films on stainless steel specimens exposed to H2O2 and O2 in high temperature water were determined by multilateral surface analyses, i.e., SEM (scanning electron microscope), LRS (laser Raman spectroscope), SIMS (secondary ion mass spectroscope) and STEM-EDX (scanning transmission electron microscope). The following points were experimentally confirmed. 1. Oxide layers were divided into inner and outer layers: Outer layers of the specimen exposed to 100ppb H2O2 consisted of larger corundum type hematite (α-Fe2O3) particles, while inner layers consisted of very fine Ni rich magnetite (Fe3O4). Outer layers of the specimen exposed to 200 ppb O2 consisted of larger magnetite mixture particles, while inner layers consisted of fine Cr rich magnetite. 2. Outer oxide layers consisted of oxide particles. The oxide particles depositing on the specimens exposed to 100ppb H2O2 were divided into two groups, i.e., a larger particle group and a smaller particle group. For other specimens, the diameter distribution of depositing particles was a single peak. Particle density and size were changed by oxidant concentration. The average diameter of the particles (that of the smaller group only for the specimen expose to 100 ppb H2O2) decreased with [O2] and [H2O2]. 3. Total oxide film thickness decreased with [H2O2] and increased with [O2]. 4. A larger dissolution rate at higher [H2O2] resulted in a thinner oxide film with smaller particles and larger hematite particles.

Investigation of Cobalt Deposition Behavior with Zinc Injection on Stainless Steel under BWR Conditions

Radioactive58Co and 65Zn were used together to investigate the deposition behavior of cobalt and zinc in the oxide film formed on the stainless steel piping of an apparatus simulating the high temperature and pressure conditions of a boiling water reactor. Using these radioactive tracers allowed the experimental setup to closely approximate the conditions found in actual plants. The accumulation of 58Co and 65Zn on the stainless steel piping was monitored using an online gamma-ray detector. The results were as follows. At higher zinc concentrations, the early period of fast deposition was shortened and the later deposition phase was slowed. Two mechanisms appeared to be responsible for the suppression of cobalt deposition by zinc injection. The first worked by decreasing the growth rate of the oxide film. This effect appeared most conspicuously in the outer oxide layer under normal water chemistry conditions. The second mechanism worked by reducing the cobalt concentration in the oxide film. This second effect was prominent in the inner oxide layer under hydrogen water chemistry conditions.

Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (IV): Determination of Oxide Film Properties with Multilateral Surface Analyses

Corrosive conditions in BWRs are determined mainly by hydrogen peroxide (H2O2). Then, a high temperature, high-pressure H2O2 water loop was fabricated to identify the effects of H2O2 on corrosion and stress corrosion cracking of stainless steel. By changing concentrations of H2O2 and O2, in situ measurements of electrochemical corrosion potential (ECP) and frequency dependent complex impedance of test specimens were carried out and then characteristics of oxide film on the specimens were determined by multilateral surface analyses, i.e., laser Raman spectroscopy and secondary ion mass spectroscopy. The following points were experimentally confirmed. 1. The hematite ratio in the oxide films of the specimens exposed to H2O2 was expressed as a logarithmic function of [H2O2]. The hematite ratio was measurable for 8 ppm O2, but negligibly small for 200 ppb O2. 2. H2O2 exposure led to thicker oxide layers than O2 exposure and Cr depletion did. 3. The oxide film thickness first increased as [H2O2] decreased from 100 to 10 ppb and then it decreased. This meant that a large dissolution rate caused a thin oxide film in spite of the large growth rate of oxide film, while a low growth rate caused a thinner oxide film at low [H2O2].

Development of a Suppression Method for Deposition of Radioactive Cobalt after Chemical Decontamination: (I) Effect of the Ferrite Film Coating on Suppression of Cobalt Deposition

In the last decade, chemical decontamination at the beginning of periodical inspection has been applied to many Japanese BWR plants in order to reduce radiation exposure. However, following the chemical decontamination, a rapid dose rate increase can be seen in some plants after just a few operation cycles. Oxide film, which easily incorporates radioactivity, might be formed after the chemical decontamination. We developed a new way to reduce the recontamination after the chemical decontamination to maintain long-term continued decontamination effects without any chemical injections or chemical controls in reactor water during operation. In our approach, a fine ferrite film is formed by the Hitachi Ferrite Coat process after oxide films formed during the plant operation are removed by the chemical decontamination process.We select Fe(HCOO)2 aqueous solution, H2O2, and N2H4 as the treatment chemicals for fine ferrite film formation for suitable BWR plant application. Our laboratory experiment results confirm a 60Co deposition reduction effect of 1/5 compared with that of nontreatment for up to 3,100 hours. The fine ferritefilm that was formed on the specimen before the 60Co deposition test remains as a film structure after the test. The corrosion amount of the specimen is suppressed to 1/4 through the effect of the fine ferrite film.

Effects of Noble Metal Deposition upon Corrosion Behavior of Structural Materials in Nuclear Power Plants, (I) : Effect of Noble Metal Deposition with an Oxide Film on Type 304 Stainless Steel under Simulated Hydrogen Water Chemistry Condition

The effects of noble metal deposition under hydrogen water chemistry (HWC) condition on the features of oxide film formed on structural components in a reactor were studied. Noble metal-deposited type 304 stainless steel specimens with an oxide film were exposed to the simulated HWC condition, including co-existing Co radioactivity. Relationships between features of the oxide film which had two layers and the accumulation and distribution of Co radioactivity in the oxide film were established. The outer layer of the oxide film which consisted of α-Fe2O3, Fe3O4 and NiFe2O4 was dissolved by noble metal deposition and exposure to the HWC condition. The reasons for this were as follows. Solubility of α-Fe2O3, Fe3O4 and NiFe2O4 increased with the decrease of electrochemical corrosion potential. Dissolution of these compounds was accelerated by the anodic reaction of hydrogen which is catalyzed by noble metal. Co radioactivity was mainly incorporated into the inner layer. This was caused by the substitution of radioactive Co ions for ferrous ions in the oxide film, based on the observation that growth of the oxide film and oxidation of base metal stopped in the HWC condition. The inner layer consisted of FeCr2O4 which is stable at low ECP and it did not dissolve. Co radioactivity was not incorporated into the outer layer because it dissolved.

Effects of Hydrogen Peroxide on Radioactive Cobalt Deposition on Stainless Steel Surface in High Temperature Water

In order to understand the effects of hydrogen peroxide on 60Co deposition under water chemistry conditions of boiling water reactors, deposition amounts of 60Co on the stainless steel specimens were measured by changing hydrogen peroxide (H2O2) and oxygen (O2) concentrations in a high temperature water loop, and correlations between the nature of oxide film on the specimen surface and its deposition were examined by separating the contributions of inner and outer oxide layers to the deposition. The results are summarized as follows. 1. The weight change of the specimens and the amount of 60Co deposition were strongly affected by the presence of H2O2. Weight gain of the specimens exposed to various H2O2 concentrations after a 200-h pre-exposure to 200 ppb H2O2 was less than 2μg/cm2 for 1,000 h; this was caused by protection effects of thin and dense oxide film. 2. The amount of 60Co deposition on the specimen exposed to 200 ppb H2O2 was much less than the amounts of those exposed to 10, 5 and 0 ppb H2O2 in the water with 50 ppb H2, but in the case of 200 h-pre-exposure to 200 ppb H2O2, the latter specimens had deposition amounts as low as that of the specimen exposed to 200 ppb H2O2. 3. The amount of 60Co deposition had a fairly linear correlation with the amount of outer oxide layer, except in the case of specimens exposed to the condition without H2O2 from the first. In the latter case, 60Co was expected to be included in a Cr-rich inner oxide layer.

Cobalt Radioactivity Behaviors in a BWR Environment and Countermeasures for Dose Rate Reduction

Abstract While under normal water chemistry without any specific metal ions in reactor coolant a high electrochemical corrosion potential caused by highly oxidizing species such as hydrogen peroxide promotes the formation of hematite film on piping surfaces with a densely packed film structure, the presence of a certain amount of nickel ions prevents the magnetite film from changing to hematite by forming a nickel ferrite. This formation of nickel ferrite instead of hematite accelerates cobalt buildup, and this is especially notable for carbon steel. The observed reduction of radioactivity concentration in reactor water by zinc injection or by nickel/iron ratio control can be explained by the role of zinc or nickel in preventing the film on the fuel rod surfaces from changing to hematite, thereby stabilizing the cobalt activity on this surface. A thermodynamic evaluation suggests that zinc ferrite is more stable than cobalt ferrite only when the ratio of cobalt to zinc divalent ions, [Co2+]/[Zn2+], is <0.011 in molar units. This ratio is consistent with the ratio of 60Co activity to zinc concentration commonly used in industry to control reactor water zinc levels for a dose rate reduction under the hydrogen water chemistry condition. Based on the present understanding of radioactivity behaviors, the actual radiation dose reduction methods are classified into the several groups and summarized from the viewpoint of the interaction between the oxide and various metal ions.

Development of a suppression method for deposition of radioactive cobalt after chemical decontamination: (III) the suppression mechanism with preoxidized ferrite film for deposition of radioactive cobalt

The Hitachi ferrite coating film process (Hi-F) has been developed to lower recontamination after chemical decontamination. In this process, the chemical decontamination process is carried out, and a fine Fe3O4 coating film is formed on the surface of stainless steel piping in an aqueous solution. In order to improve the suppression of 60Co deposition further, we combined the original Hi-F with a preoxidation step. We found the deposited amount of 60Co with preoxidized Hi-F coating film (OHi-FC) was 1/10 of that for non-coated specimens. In this study, we investigated the suppression mechanism of 60Co for the OHi-FC. The composition of OHi-FC was changed from Fe3O4 to Fe2O3 and then the crystals in the OHi-FC grew three times larger than those of the original Hi-F coating film. Consequently the corrosion amount of the stainless steel base metal was reduced by getting larger grains in the coating film. Because 60Co was incorporated into the corrosion oxide, the suppression effect of 60Co deposition by preoxidation was attributed to the suppression of the formation of the corrosion oxide by the OHi-FC.

Development of a Suppression Method for Deposition of Radioactive Cobalt after Chemical Decontamination: (II) Consideration of Fe3O4 Plating Mechanism on Stainless Steel in Aqueous Solution at 363K

Recently, chemical decontamination at the beginning of periodical inspection has been applied to many Japanese boiling water reactors in order to reduce radiation exposure. However, following the chemical decontamination, a dose rate increase can be seen in some plants after just a few operation cycles. The Hitachi ferrite coating (Hi-F Coat) process has been developed to reduce the recontamination by radioactive cobalt after the chemical decontamination. In this process, a fine Fe3O4 coating film is formed on the stainless steel base metal of the piping following the chemical decontamination in aqueous solution at 363 K. In this study, we investigated a Fe3O4 plating mechanism on the base metal in aqueous solution at 363K by measurements using a quartz crystal microbalance. We found that the Fe3O4 film grew in three steps. First, the Fe3O4 particles were produced on a stainless steel surface. Second, the Fe3O4 particles grew as dome shapes and the converged domes became filmlike. Third, the film grew and became a closely packed Fe3O4 film. Furthermore, we determined the equation of the time dependence of the Fe3O4 film amount using crystal growth theory. The equation predicted the film amount at 10,000 s within a margin of error of 5%.

A Mechanism for Corrosion Product Deposition on the Carbon Steel Piping in the Residual Heat Removal System of BWRs

The dose rate of the residual heat removal (RHR) piping has been considered to be caused by accumulation of insoluble (crud) radioactive corrosion products on carbon steel surfaces. Soft shutdown procedures (i.e., plant shutdown with moderate coolant temperature reduction rate) used to be applied to reduce crud radioactivity release from the fuel surface, but these are no longer used because of the need for shorter plant shutdown times. In order to apply other suitable countermeasures to reduce RHR dose rate, assessment of plant data, experiments on deposition of crud and ion species on carbon steel, and mass balance evaluation of radioactive corrosion products based on plant and laboratory data were carried out and the following findings were made. 1. Deposits of ion species on carbon steel surfaces of the RHR piping was much more numerous than for crud. 2. Ion species accumulation behavior on RHR piping, which is temperature dependent, can be evaluated with the calculation model used for the dehydration reaction of corrosion products generated during the wet lay-up period. 3. Deposition amounts could be reduced to 1/2.5 when the starting RHR system operation temperature was lowered from 155°C to 120°C.