Motohiro Aizawa

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors, (I) Temperature Dependence of Hydrazine Reactions

Hydrazine and hydrogen co-injection into reactor water is considered a new mitigation method of stress corrosion cracking in BWRs. Fundamental data such as the thermal decomposition of hydrazine, the reaction of hydrazine with oxygen and with hydrogen peroxide at temperatures ranging from 150 to 280°C are needed to evaluate suitability of this method. Reactions in bulk water were studied in a polytetrafluoroethylene pipe to separate surface reaction effects. The results were as follows. (1) The orders of the apparent reaction rate of hydrazine with oxygen were 1 and 0.5 for hydrazine and oxygen concentrations, respectively . Arrhenius parameters were k 0=69.0 s−1.μM−0.5 and (2) The orders of apparent reaction rate of hydrazine with hydrogen peroxide were each 0.5 for hydrazine and hydrogen peroxide concentrations kC 0.5 N2H4 C 0.5 H2O2 . Arrhenius parameters were k 0=1.42x 106 s−1, Ea =78.8 kJ.mol−1. Based on these data, the applicability of hydrazine and hydrogen co-injection into BWRs was considered. Hydrazine introduction to reactor water was confirmed to be accompanied by only 1% decomposition. The concentration of oxygen, which is injected to suppress the flow-assisted corrosion of carbon steel in current BWR operation, would decrease due to the reaction of hydrazine with oxygen. However oxygen concentration in feed water could be maintained at the required level if the concentration of oxygen injected in condensate water was at most doubled compared to the current operating concentration.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (IV) : Reaction Mechanism and Plant Feasibility Analysis

A calculation model has been developed in order to evaluate effectiveness of hydrazine and hydrogen co-injection (HHC) into reactor water for mitigation of intergranular stress corrosion cracking of structural materials used in boiling water reactors (BWRs). The HHC uses the strong reducing power of hydrazine radical, which is produced in the downcomer region under irradiation by γ-rays and neutrons. Some reactions and their reaction rate constants were determined based on experiments which were carried out in aerated water, hydrogenated water, and deaerated water. The calculated results were in good agreement with experimental data by a factor of two. The model was applied to a BWR and it was found that the HHC cut oxygen and hydrogen peroxide amounts dissolved in reactor water more effectively than hydrogen water chemistry alone. Thus, the required amount of hydrogen for hydrazine injection was much lower than that for hydrogen water chemistry. Consequently, electrochemical corrosion potential of structural materials could be lowered below–0:1V vs. SHE without any increase of MS line dose rate, which has been a limitation of the conventional hydrogen water chemistry. The HHC was predicted to decrease crack growth rate of structural materials by a factor of 10.

Study of Polarization Curve Measurement Method for Type 304 Stainless Steel in BWR High Temperature-High Purity Water

In order to improve the accuracy of electrochemical corrosion potential (ECP) calculations, information about the effects of the flow velocity, the type of oxidization species, the oxide film thickness, and the oxide film classification on the polarization curve is needed. Polarization curve measurements were studied here with that information goal in mind. A wire-shaped working electrode was employed to remove the IR drop caused by high solution resistance, which makes it difficult to apply a proper potential to the working electrode. In order to measure the static state current, the potential scanning condition was optimized.It was confirmed that a steady-state current can be measured by establishing a potential scanning rate for applying a potential not greater than 0.01mV.s−1 with a step-shaped waveform. This method was employed to measure the anodic polarization curve of type 304 stainless steel in deaerated 553K high-purity water. The passivation behavior was clearly observed. Also, the cathodic polarization curves in water containing dissolved oxygen of less than 1000 ppb were measured. The oxygen concentration dependence of ECP was calculated from measured polarization curves. The ECPs calculated using the measured polarization curves were in good agreement with those measured with an electrometer, confirming the validity of the measured polarization curves.

Hydrazine and hydrogen Co-injection to mitigate stress corrosion cracking of structural materials in boiling water reactors, (II) : Reactivity of hydrazine with oxidant in high temperature water under gamma-irradiation

Hydrogen and hydrazine co-injection into a boiling water reactor was considered as a new mitigation method of stress corrosion cracking. To confirm decrease of electrochemical corrosion potential by the reduction of oxygen and hydrogen peroxide in bulk water using reducing agent, reaction of hydrazine with oxygen or hydrogen peroxide under simulated downcomer conditions (temperature: 280°C, duration: 4.2 s and gamma-irradiation) was examined. All the oxygen was consumed above the equivalent concentration for the reaction with oxygen within this short time (gamma-irradiation case) and almost all the hydrogen peroxide was consumed. Reaction rates were accelerated more than five times by gamma-irradiation in each case. Reaction rates with oxygen were compared with other reducing agents such as hydrogen, methanol and ammonia. From the viewpoint of reaction rate and formation of by-product, hydrazine was the most suitable agent. Using a simple model based on the experimental results, water chemistry for the bottom region was calculated for the case of no hydrogen injection and for the case of 0.2 mmol-kg−1 hydrogen injection into feed water. For both cases, concentrations of dissolved oxygen and hydrogen peroxide were estimated to decrease enough to mitigate SCC, with concentration of ammonia suppressed below the management criteria for reactor water chemistry.

Hydrazine and Hydrogen Coinjection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (VII)—Effects of Bulk Water Chemistry on ECP Distribution inside a Crack

Water chemistry in a simulated crack (crack) has been studied to understand the mechanisms of stress corrosion cracking in a boiling water reactor environment. Electrochemical corrosion potential (ECP) in a crack made in an austenite type 304 stainless steel specimen was measured. The ECP distribution along the simulated crack was strongly affected by bulk water chemistry and bulk flow. When oxygen concentration was high in the bulk water, the potential difference between the crack tip and the outside of the crack (ΔE), which must be one motive force for crack growth, was about 0.3V under a stagnant condition. When oxygen was removed from the bulk water, ECP inside and outside the crack became low and uniform and ΔE became small. The outside ECP was also lowered by depositing platinum on the steel specimen surface and adding stoichiometrically excess hydrogen to oxygen to lower ΔE. This was effective only when bulk water did not flow. Under the bulk water flow condition, water-borne oxygen caused an increase in ECP on the untreated surface inside the crack. This also caused a large ΔE. The ΔE effect was confirmed by crack growth rate measurements with a catalyst-treated specimen. Therefore, lowering the bulk oxidant concentration by such measures as hydrazine hydrogen coinjection, which is currently under development, is important for suppressing the crack growth.

Low Corrosive Chemical Decontamination Method Using pH Control, (II) Decomposition of Reducing Agent by Using Catalyst with Hydrogen Peroxide

In the development of a new chemical decontamination method which provides a high decontamination effect, less corrosion of base metal, and less radioactive waste generation, we developed a decomposition method for oxalic acid coexisting with hydrazine to decrease the amount of radioactive waste. Using a catalyst of 0.5 wt% Ru supported by activated carbon grains, we decomposed oxalic acid and hydrazine, simultaneously and efficiently, with a stoichiometric concentration of H2O22. The decomposition ratios were decreased by the deposition of oxides. But even if the simulated reducing agent solution with high concentrations of coexisting Fe and K ions, which negatively effect decomposition ratio, was decomposed, the decomposition ratios of oxalic acid and hydrazine were kept high during decomposition of the amount of reducing agent used in actual chemical decontamination. Additionally, we examined the deposition ratios of metal ions on the catalyst as metal oxides. These results indicated about 2% of the radioactive species which were removed by the chemical decontamination were deposited on the catalyst column. 59Fe and 51Cr were estimated to be about 90% of the total deposited amount of radioactive species and about 60% of the dose equivalent in the model calculation. But this problem should be easily dealt with by using shielding.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors, (III): Effects of Adding Hydrazine on Zircaloy-2 Corrosion

The effects of hydrazine on the corrosion of Zircaloy-2 were examined in supercritical water. Hydrazine could be used as a reducing agent to control the corrosive environment for the coolant of boiling water reactors (BWRs). Before the corrosion test, the applicability of supercritical water for corrosion testing of zirconium alloys was studied. Supercritical water was found to be a useful solvent for testing corrosion based on the following facts: (1) the weight gain of Zircaloy-2 in supercritical water followed the same cubic law with the activation energy of 133 kJ/mol as that in water and steam did, and (2) the weight gain in supercritical water at 723 K and 24.5 MPa was more than 8 times greater than that in water at 561 K and 7.8 MPa depending on immersion time. The corrosion tests in supercritical water at 723 K and 24.5 MPa under γ-irradiation for 1,000 h were conducted to study the effects of adding nitrogen and ammonia on the corrosion of Zircaloy-2. Nitrogen and ammonia are decomposed products of hydrazine. The measured weight gain, oxide film thickness, and amount of hydrogen pick-up had slight differences between cases with and without the additives. Based on these data, it was concluded adding hydrazine to the coolant has little influence on the corrosion of Zircaloy-2 used in BWR cores.

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.

Effects of water chemistry and potential distribution on electrochemical corrosion potential measurements in 553 K pure water

The effects of water chemistry distribution on the potential of a reference electrode and of the potential distribution on the measured potential should be known qualitatively to obtain accurate electrochemical corrosion potential (ECP) data in BWRs. First, the effects of oxygen on a platinum reference electrode were studied in 553 K pure water containing dissolved hydrogen (DH) concentration of 26–105 μg kg−1 (ppb). The platinum electrode worked in the same way as the theoretical hydrogen electrode under the condition that the molar ratio of DH to dissolved oxygen (DO) was more than 10 and that DO was less than 100 ppb. Second, the effects of potential distribution on the measured potential were studied by using the ECP measurement part without platinum deposition on the surfaces connected to another ECP measurement part with platinum deposition on the surfaces in 553 K pure water containing 100–130 ppb of DH or 100–130 ppb of DH plus 400 ppb of hydrogen peroxide. Measured potentials for each ECP measurement part were in good agreement with literature data for each surface condition. The lead wire connecting point did not affect the measured potential. Potential should be measured at the nearest point from the reference electrode in which case it will be not affected by either the potential distribution or the connection point of the lead wire in pure water.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors (V) Effects of Hydrazine and Dissolved Oxygen on Flow Accelerated Corrosion of Carbon Steel

Hydrazine and hydrogen co-injection into reactor water via the feed water line can mitigate stress corrosion cracking in BWRs. The effects of water quality variation due to hydrazine injection on flow accelerated corrosion (FAC) of carbon steel used in the BWR feed water line should be considered from the viewpoints of radioactive exposure, radioactive waste generation and plant integrity. Optimization of oxygen injection into feed water was considered to suppress FAC during hydrazine and hydrogen co-injection. In this report, corrosion tests of carbon steel were made in hydrazine and/or dissolved oxygen injected solutions (conditions: no pH adjustment by ammonia; temperature = 215°C; flow rate = 3.5 ms−1) to study the effects of dissolved oxygen and hydrazine on FAC. Hydrazine and oxygen co-injection suppressed the total corrosion rate by about 1/5 compared with the case of 0 μmol-kg−1 hydrazine and <0.06μmol.kg−1 oxygen injection. With hydrazine and oxygen co-injection, the inner layer, which is protective oxide, was close packed. Thickness of the inner layer also became larger compared with the case for only hydrazine injection or only oxygen injection. α-Fe2O3, which is thermodynamically more stable than Fe3O4, was formed in addition to Fe3O4 which was the single species formed for the case of only hydrazine injection. From these results, it was concluded FAC will be suppressed by keeping the oxygen concentration at the current level in BWRs (about 1 μmol-kg−1) if hydrazine and hydrogen co-injection is applied.