Nagayoshi Ichikawa

Electrochemical Considerations Regarding General Corrosion of Materials in a BWR Primary Circuit

Electrochemical calculations of 3d-transition metal (Fe, Ni, Co and Cr)-water systems at 558 K were performed to evaluated the general corrosion behavior of materials in a BWR primary circuit. Thermodynamics expressed by Pourbaix diagrams, the solubilities of oxides and the equilibrium activities of H2, O2 and H2O2 in oxide formations, and kinetics determining the corrosion potential and corrosion rate of the materials were evaluated. The results assist recognition of the following corrosion behaviors: 1. Good corrosion resistance of type 304 SS under various water chemistry conditions is due to the properties of NiFe2O4 and (Fe, Ni)Cr2O4; namely wide stable regions and low diffusion coefficients of metal ions and the ability of these oxides to compensate for each others weakness. 2. A typical increase of the corrosion rate of Alloy X750 and Stellite #6 in an oxygenated aqueous environment containing H2O2 is due to the properties of NiO and CoO; namely high solubilities and high diffusion coefficients of ...

Precise evaluation of corrosion environments of structural materials under complex water flow condition, (I). Estimation of corrosion potentials in reactor pressure vessel bottom of BWRs

To estimate the corrosion potentials of structural materials under complex water flow condition, such as reactor pressure vessel (RPV) bottom region of Boiling Water Reactors (BWRs), a method was newly developed. Three-dimensional water flow analysis was performed and corrosion potentials of structural materials were calculated on the basis of the flow analysis results. Water flow analysis showed the velocities in the RPV bottom region varied from about 0.1 ms-1 to 4.5 ms-1. From the corrosion potential estimation under hydrogen water chemistry (HWC) condition, at the jet pump outlet and shroud support leg, a rather large amount of hydrogen had to be added to reduce the potential because of high flow velocity condition. Conversely, a small amount of hydrogen was sufficient in the case of the stub tube of the control rod drive (CRD) guide tubing located in the center of the bottom region.

Precise evaluation of corrosion environments of structural materials under complex water flow condition, (II): Water chemistry at downcomer region of a BWR

In order to clarify the corrosion environment of nuclear reactor structure material precisely, the water radiolysis evaluation considering a flow of reactor water was performed. This study focused on the downcomer region of a Boiling Water Reactor (BWR). Radiation intensity of the region is high and water radiolysis reaction occurs well. Therefore, the region is recognized as important for recombination of oxygen, hydrogen peroxide and hydrogen back to water under hydrogen water chemistry (HWC) condition. Precise evaluation of water chemistry in the region with consideration of water flow, however, has not been performed. In the downcomer region water flow is very complicated because the jet pumps are close together and water flows out to the recirculation system through only two nozzles. A new water radiolysis model was developed which can simulate water radiolysis under the three-dimensional water flow condition. The calculation was performed using the result of water flow analysis obtained by the computer code “a-Flow.” The water chemistry near a shroud wall and the reactor pressure vessel (RPV) wall differed greatly, and especially so under HWC condition. Also, the detailed distribution of the electrochemical corrosion potential (ECP) of structural materials was clarified by using the water chemistry analysis result.