Naoshi Usui

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].

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.

Effects of Hydrogen Peroxide on Cobalt Deposition Rate on Primary Cooling System of Boiling Water Reactors

Experimental data of radioactivity deposition rates on stainless steel specimensexposed to high-temperature water including hydrogen peroxide (H2O2) and oxygen (O2) were applied to the evaluation of Co-60 accumulation rates in oxide films on the stainless steel piping of a boiling water reactor (BWR) primary cooling system and then an empirical formula of Co-60 deposition rate coefficient was proposed as a function of exposure time. The empirical formula was applied to the evaluationof the amount of Co-60 deposition on the stainless steel piping of BWR primary cooling systems under normal water chemistry (NWC) and hydrogen water chemistry (HWC) conditions. The results are summarized as follows. 1) The Co-60 deposition rate coefficient was almost independent of O2 concentration ([O2]), while it decreased as H2O2 concentration ([H2O2]) decreased. 2) The proposed empirical formula determined on the basis of experiments under H2O2 conditions could give a much better estimation of the Co-60 deposition rate coefficient on stainless steel under NWC conditions of BWR than the previous formula based on the data under simple O2 conditions. 3) The deposition amounts of Co-60 on stainless steel piping under HWC conditions were evaluated using the formula, which suggested that those without any prefilming treatment were several times as high as those under NWC conditions. 4) The increase in Co-60 deposition under HWC conditions was caused by the less protective film in mitigating Co-60 deposition and it could be avoided by several hundred hours of operation under NWC conditions prior to HWC operation. The effect on material integrity is evaluated to be negligible.

The Effects of Oxidizing Species on Cobalt Buildup in a BWR Environment

Abstract We have studied the effects of the oxidizing species on the cobalt radioactivity buildup behavior in boiling water reactors (BWRs) using both experimental results and existing literature data. The oxidizing species used to simulate the normal water chemistry (NWC) condition of BWRs were 200 ppb dissolved oxygen or 200 ppb hydrogen peroxide accompanied by 100 ppb dissolved oxygen. We found that the amount of cobalt deposited on stainless steel specimens in the oxygen-based water chemistry (200 ppb dissolved oxygen) was larger than that in the hydrogen peroxide–based water chemistry (200 ppb hydrogen peroxide and 100 ppb dissolved oxygen). The rate of cobalt deposition in the former chemistry was more than four times larger than that in the latter chemistry. This difference in cobalt deposition behavior can be attributed to two properties of oxides: surface morphology and composition. The film formed in the oxygen-based environment was less dense than the film formed in the hydrogen peroxide–based environment. Regarding the chemical constituents of the oxides, iron chromite is considered to be a major spinel-type oxide formed in oxygen-based environments. Furthermore, some literature data suggest that in hydrogen peroxide–based conditions, hematite-rich oxides are formed instead of magnetite-rich films, which are observed in oxygen-based conditions. These are likely reasons why the stainless steel specimens incorporate more cobalt radioactivity in the oxygen-based environment than in the hydrogen peroxide–based environment. The cobalt buildup behavior after switching from NWC to hydrogen water chemistry (HWC) is also affected by the oxidizing species used to simulate NWC; exposure to hydrogen peroxide–based NWC conditions tends to suppress the cobalt radioactivity buildup after switching from NWC to HWC compared to exposure to oxygen-based NWC.

e-chem page: A Support System for Remote Diagnosis of Water Quality in Boiling Water Reactors

It is important to control and maintain water quality for nuclear power plants. Chemical engineers sample and monitor reactor water from various subsystems and analyze the chemical quality as routine operations. With regard to controlling water quality, new technologies have been developed and introduced to improve the water quality from both operation and material viewpoints. To maintain the quality, it is important to support chemical engineers in evaluating the water quality and realizing effective retrieval of stored data and documents. We have developed a remote support system using the Internet to diagnose BWR water quality, which we call e-chem page . The e-chem page integrates distributed data and information in a Web server, and makes it easy to evaluate the data on BWR water chemistry. This system is composed of four functions: data transmission, water quality evaluation, inquiry and history retrieval system, and reference to documents on BWR water chemistry. The developed system is now being evaluated in trial operations by Hitachi, Ltd. and an electric power company. In addition diagnosis technology applying independent component analysis (ICA) is being developed to improve predictive capability of the system. This paper describes the structure and function of the e-chem page and presents results of obtained with the proposed system for the prediction of chemistry conditions in reactor water.Copyright

Evaluation of Impurity Behavior in the Reactor Water of a Boiling Water Reactor

A system has been developed for evaluating the effect of impurity intrusion on reactor water as part of the chemistry diagnostic system for boiling water reactor water. Past records of anomalous changes noted in reactor water quality were first surveyed to identify the substances presenting the highest probability of intrusion into reactor water. The ions and their concentrations were derived, taking into account mutual chemical reactions among the substances and the plant operating conditions affecting the balance equation of the reactor primary system. The results predicted for the peak conductivity of reactor water during plant startup were within ±15% of the measured data. This good agreement found on a comparison of estimated results with measured data from an actual plant substantiated the validity of the present method.

Reduction of volume of BWR plant radioactive wastes by operating condensate demineralizers without regeneration.

Since chemical regeneration of condensate demineralizers produces a large quantity of radioactive liquid wastes, reducing the volume of such wastes has become a significant concern. In Japanese Improvement and Standardization BWRs, by installing a prefilter upstream from the condensate demineralizer, and improving construction materials and method of controlling water chemistry, the crud and ion load on the condensate demineralizer has been reduced. Since non-regenerated operation of the condensate demineralizer has been devised and tried on the No. 2 Unit of the Fukushima Daini Nuclear Power Station (2F-2) with variable resin measurement during every scheduled outage and each fuel cycle. The result has been a reduction of the number of drums to be disposed of to 1/5 the previous number.