Satoshi Hanawa

Determination of Electrochemical Corrosion Potential Along the JMTR In-Pile Loop - I: Evaluation of ECP of Stainless Steel in High-Temperature Water as a Function of Oxidant Concentrations and Exposure Time

In-pile loop experiments are one of the key technologies that can provide an understanding of corrosion behaviors of structural materials in nuclear power plants (NPPs). The experiments should be supported not only by reliable measurement tools to confirm corrosive conditions under neutron and gamma-ray irradiations but also by theoretical models for extrapolating the measured data to predict corrosion behaviors in NPPs. The relationships among electrochemical corrosion potential (ECP), metal surface conditions, exposure time, and other environmental conditions have been determined from in situ measurements of corrosion behaviors of stainless steel specimens exposed to H2O2 and O2 in high-temperature water. Based on the relationships, a model to evaluate the ECP of stainless steel was developed by coupling an electrochemical model and a double-oxide layer model. Major conclusions obtained from the evaluation model are as follows: (a) The difference in ECP behaviors of the specimens exposed to H2O2 and O2 were mainly from the thickness and developing rate of the inner oxide layers. (b) Calculated ECP behaviors, e.g., the different responses to H2O2 and O2 and hysteresis and memory effects, agreed with the measured ones. (c) Neutron exposure might decrease ECP due to radiation-induced diffusion in the oxide layer. The ECP evaluation model will be applied to evaluation of corrosive conditions in the JMTR in-pile loop.

Determination of Electrochemical Corrosion Potential Along the JMTR In-Pile Loop - II: Validation of ECP Electrodes and the Extrapolation of Measured ECP Data

A water chemistry research program using the in-pile loop in the Japan Materials Testing Reactor (JMTR) has been launched to develop data that are applicable for model verification as well as model benchmarking. In advance of the in-pile loop experiment performed in the JMTR, reliability of in-pile electrochemical corrosion potential (ECP) measurement and applicability of the theoretical models were investigated, based on experimental data previously obtained in the in-pile loop of the LVR-15 experimental reactor at the Research Center Rez in the Czech Republic. The responses of different types of reference electrodes used for the ECP measurements were compared with each other to confirm their reliability under several different irradiation conditions corresponding to the core peripherals of boiling water reactors (BWRs). The corrosive conditions along the in-pile loop were first calculated using combined models of water radiolysis and ECP, and the calculated results were then compared with the ECP measurement data to validate the model. As a result, it was confirmed that the reference electrodes performed reliably under mixed neutron and gamma-ray irradiation conditions with minor calibration of each electrode prior to application in the loop, and that the combined models of water radiolysis and ECP can be applied for the evaluation of the corrosive conditions of the in-pile loop and BWR cores and their peripherals.

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.

An empirical model for the corrosion of stainless steel in BWR primary coolant

ABSTRACT A model to evaluate the electrochemical corrosion potential (ECP) and corrosion rate of stainless steel in neutral water was developed by coupling a static electrochemical analysis and a dynamic oxide layer growth analysis. ECP is much affected not only by water chemistry conditions but also by the oxide layers that develop on the metal surface. As an example application of the model, the effects of oxide layers on ECPs were calculated and a good agreement with the measured results was shown. Major conclusions obtained using the coupled model were as follows: The effects of H2O2 and O2 concentrations on ECP were successfully explained as the effects of oxide layer growth. Decreases in ECP due to neutron exposure were explained well by radiation-induced diffusion in the oxide layers. Hysteresis of ECP was well explained by the coupled model.

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%.

Verification of FEMAXI-7 code by using irradiation test in Halden reactor for He-pressurization effect on FGR of BWR fuels under power transient

Power ramp test for He-pressurization effect on fission gas release (FGR) of about 42GWd/tUO2 boiling water reactor (BWR) fuel rods was analyzed by the fuel performance code FEMAXI-7. The experimental data were obtained with the two rods, which were base irradiated in the Halden reactor for 12 years (IFA-409), then subjected to the power ramp tests (IFA-535) to investigate the He-pressurization effect. The FEMAXI-7 calculations were performed by inputting rod specifications and experimental conditions in both the baseand test irradiations. The results showed that the calculations reasonably followed the trends of measured cladding elongation and FGR during the power ramp test, depending on the pellet temperature and fission gas atoms diffusion rate. Based on the calculated results, the reason that no apparent He-pressurization effect was observed in the experiment was considered to be caused by insufficient gas communication during strong pellet–clad mechanical interaction (PCMI) and enhanced gap thermal conductance by the solid–solid contact due to gap closure.

Development of Water Radiolysis Code for the JMTR IASCC Test Loop

In order to evaluate the water chemistry in the irradiation field during IASCC irradiation test, a water radiolysis code for IASCC irradiation loop system was developed. In the water radiolysis code, a multiple node model was introduced since the irradiation loop system has a wide rage temperature distribution as well as the dose distribution. To investigate the applicability of developed water radiolysis code, water chemistry at the water sampling point of the irradiation loop system was measured and compared with analytical results under several water chemistry conditions. Further, water chemistry distribution in the in-pile region as well as in the out-pile region was calculated by the developed water radiolysis code.© 2006 ASME

Analytical Study on Micro-Indentation Method to Integrity Evaluation for Graphite Components in HTGR

An analytical study on micro-indentation method to integrity evaluation for graphite components was carried out. The indentation method is used as simplicity test to measure mechanical properties of materials. This method is thought to be applicable to evaluate the residual stress from the relationship between indentation load and indentation depth. In this study, in order to confirm the applicability of the micro-indentation method for lifetime evaluation of the graphite component, indentation load-depth behavior under stress/strain condition was evaluated taking account of the specified minimum ultimate strength of IG-110 graphite. Moreover, analytical investigations of indentation load-depth behavior for oxidized graphite and oxidized graphite with residual strain were also carried out. As a result, it can be said that the indentation method is potentially applicable to evaluate the integrity of graphite components. (authors)