Tsung-Kuang Yeh

Modeling Water Chemistry, Electrochemical Corrosion Potential, and Crack Growth Rate in the Boiling Water Reactor Heat Transport Circuits—I: The DAMAGE-PREDICTOR Algorithm

A computer code with the capability of simultaneously estimating the concentrations of radiolysis species, the electrochemical corrosion potential, and the kinetics of growth of a reference crack in sensitized Type 304 stainless steel is developed for the heat transport circuits of boiling water reactors (BWRs). The primary objective of this code, DAMAGE-PREDICTOR, is to theoretically evaluate the effectiveness of hydrogen water chemistry (HWC) in the BWRs as a function of feedwater hydrogen concentration and reactor power level. The power level determines various important thermal-hydraulic parameters and the neutron and gamma energy deposition rate in the core and near-core regions. These input parameters are estimated using well-established algorithms, and the simulations are carried out for full-power conditions for two reactors that differ markedly in their responses to HWC. The DAMAGE-PREDICTOR code is found to successfully account for plant data from both reactors using a single set of model parameter values.

Modeling Water Chemistry, Electrochemical Corrosion Potential, and Crack Growth Rate in the Boiling Water Reactor Heat Transport Circuits—II: Simulation of Operating Reactors

The DAMAGE-PREDICTOR computer code, which has the capability of simultaneously estimating the concentrations of radiolysis species, the electrochemical corrosion potential (ECP), and the crack growth rate (CGR) of a reference crack in sensitized Type 304 stainless steel, is used to evaluate the responses of the Dresden-2 and Duane Arnold boiling water reactors (BWRs) to hydrogen water chemistry (HWC). The HWC simulations for these two BWRs are carried out for feedwater hydrogen concentrations ([H 2 ]FW) ranging from 0.0 to 2.0 parts per million (ppm). Results such as species concentrations (H 2 , O 2 , H 2 O 2 , etc.), ECP, and CGR are predicted for various components in the heat transport circuits (HTCs) of the two reactors. It is found that while 1.3 ppm of feedwater hydrogen is needed to protect part of the lower downcomer, the recirculation system, and the lower plenum in Dresden-2 from intergranular stress corrosion cracking, only 0.3 ppm is needed to achieve the same goal in Duane Arnold. However, it is also found that the ECP in many regions (core channel, core bypass, upper plenum, downcomer, etc.) in the HTCs cannot be lowered to below the critical corrosion potential of -0.23 V SHE for sensitized Type 304 stainless steels, even when [H 2 ] FW is as high as 2.0 ppm.

Intergranular Stress Corrosion Cracking of Type 304 Stainless Steels Treated with Inhibitive Chemicals in Simulated Boiling Water Reactor Environments

Electrochemical potentiodynamic polarizations, electrochemical corrosion potential (ECP) measurements and slow strain rate tensile (SSRT) tests were conducted to investigate the intergranular stress corrosion cracking (IGSCC) characteristics of Type 304 stainless steels treated with inhibitive chemicals in simulated boiling water reactor (BWR) environments. A number of thermally sensitized specimens were prepared and were pre-oxidized in a 288°C environment with the presence of 300 ppb dissolved oxygen for 360 h. Most of the specimens were then treated with various chemicals including powdered zirconium oxide (ZrO2), powdered titanium oxide (TiO2), and zirconyl nitrate [ZrO(NO3)2] via static immersion at 90°C, 150°C, and 200°C. Test environments were specifically designed in a circulation loop to create a dissolved oxygen concentration of 300 ppb. Test results showed that the corrosion current densities of all treated specimens were lower than that of the untreated, pre-oxidized specimen at ambient temperature in a solution mixed with 1 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6. The ECPs of the treated specimens could be lower or higher than that of the pre-oxidized one at 288°C, depending upon the type of treating chemical and the treating temperature. In addition, IGSCC was observed on all specimens (treated or untreated) in the same environment. However, the untreated specimen exhibited lower elongation, shorter failure time, and more secondary cracks on the side surfaces. It was therefore suggested that inhibitive chemicals such as ZrO2, TiO2, and ZrO(NO3)2 did provide a certain degree of enhancement in improving the mechanical behavior of the treated specimens and in prolonging the IGSCC initiation time.

The efficiency of noble metals in reducing the corrosion potential in the primary coolant circuits of boiling water reactors operating under hydrogen water chemistry operation

In order to promote the effectiveness of hydrogen water chemistry (HWC) and to achieve a more effective reduction in electrochemical corrosion potential (ECP) in the primary coolant circuits of boiling water reactors (BWRs), the technology of noble metal chemical addition (NMCA) was brought into practice about 10 years ago. NMCA aims at enhancing the oxidation of hydrogen on metal surfaces and lowering the concentrations of the oxidants (oxygen and hydrogen peroxide) via recombination with hydrogen on the catalyzed surfaces, and therefore reducing the corrosion potentials of the structural alloys in a BWR primary heat transport circuit. Previous research indicates that the effectiveness of NMCA in combination with a low HWC might be evaluated via model predictions of the hydrogen-to-oxidant molar ratio (MH/O) in the primary coolant circuit. If the MH/O at a certain location is calculated to be greater than 2, it is justified that the NMCA would be effective in reducing the ECP to much below the critical potential for Intergranular Stress Corrosion Cracking (IGSCC), EIGSCC, of --0.23 VSHE. However, this statement is true only when the recombination efficiency of hydrogen with oxygen and/or hydrogen peroxide at the location of interest is 100%. Otherwise, significant amounts of oxidants may still be present, even with a stoichiometric MH/O of greater than 2. With the aid of a computer model DEMACE, we explored the impact of incomplete recombination and found that the ECP might be reduced under given circumstances, but not to a great extent, and might remain well above EIGSCC. Accordingly, considerable caution should be exercised upon using the MH/O as a sole indicator for evaluating the effectiveness of NMCA with low HWC as a means of mitigating IGSCC in a BWR. An important finding of this study is that it is necessary to quantify the recombination efficiencies of hydrogen with oxygen and/or hydrogen peroxide on the noble metal treated stainless steel surfaces in order to qualify the use of MH/O as an indicator for NMCA effectiveness in the primary coolant circuit of a BWR.

Electrochemical Corrosion Potential Modeling in the Primary Heat Transport Circuit of the Chinshan Boiling Water Reactor under the Condition of Hydrogen Water Chemistry with Noble Metal Coating

The technique of noble metal treatment, such as noble metal coating (NMC) or noble metal chemical addition, accompanied by a low level hydrogen water chemistry, is being employed by a number of nuclear power plants around the world for mitigating intergranular stress corrosion cracking in the vessel internals of their boiling water reactors (BWRs). A computer model DEM ACE was expanded and employed to assess the effectiveness of NMC throughout the primary heat transport circuit (PHTC) of a BWR. The effectiveness of NMC was justified by the electrochemical corrosion potential (ECP) and crack growth rate (CGR) predictions. In calculating the ECP, enhancing factors for the exchange current densities of redox reactions available from recently published data, were employed. The Chinshan BWR was selected as a model reactor. According to the modeling results, it was found that the effectiveness of NMC in the PHTC of a BWR could vary from region to region at different feedwater hydrogen concentrations. For the selected BWR, NMC was predicted to be of little benefit when the feedwater hydrogen concentration reached 0.9 ppm or over. In particular, the NMC technique proved to be beneficial in reducing ECP and CGR along the PHTC even if the BWR was operated under normal water chemistry.

Modeling water chemistry, electrochemical corrosion potential, and crack growth rate in the boiling water reactor heat transport circuits - III: Effect of reactor power level

The DAMAGE-PREDICTOR computer code, which has the capability of simultaneously estimating the concentrations of radiolysis species, the electrochemical corrosion potential (ECP), and the crack growth rate (CGR) of a reference crack in sensitized Type 304 stainless steel, is used to evaluate the responses of the Dresden-2 and Duane Arnold boiling water reactors (BWRs) to hydrogen water chemistry (HWC) at different power levels. The HWC simulations for these two BWRs are carried out for feedwater hydrogen concentration ([H{sub 2}]{sub FW}) ranging from 0.0 to 2.0 parts per million and for power levels at 100, 90, 80, and 70%. Variations in the oxygen, hydrogen peroxide, and hydrogen concentrations; ECP, and CGR for four specific areas (the side of the core shroud head, the base of the core shroud, the recirculation system outlet, and the bottom of the lower plenum) as a function of the feedwater hydrogen concentration and power level are analyzed. It is found that lower power levels alleviate the amount of hydrogen injected into the feedwater that is required to protect the reactor components from intergranular stress corrosion cracking. HWC is particularly effective in protecting the base of the core shroud and the recirculation system outlet but is only moderatelymorexa0» effective in protecting the bottom of the lower plenum. On the other hand, the ECP and the CGR at the side of the core shroud head seem to be indifferent to both the operating power level and the feedwater hydrogen concentration.«xa0less

The Influence of ZrO2 Treatment on the Electrochemical Behavior of Oxygen and Hydrogen on Type 304 Stainless Steels in High Temperature Water

For enhancing the effectiveness of hydrogen water chemistry (HWC) in boiling water reactors (BWRs) in the aspects of lower hydrogen consumption and of a more effective reduction in electrochemical corrosion potential (ECP), the technique of inhibitive protective coating on structural materials was brought into consideration. The application of inhibitive treatment is aimed at deterring the reduction reactions of oxidizing species occurring on metal surfaces and the oxidation reaction of metals. In the current study, electrochemical polarization analyses at 288°C were conducted to characterize the electrochemical properties of ZrO2 treated and untreated 304 stainless steel specimens in pure water with dissolved oxygen or hydrogen. The polarization results showed that the treated specimens exhibited lower corrosion potentials, corrosion current densities, exchange current densities, and cathodic current densities than the untreated one in high temperature pure water with dissolved oxygen. For the environment with dissolved hydrogen only, reductions in anodic current density and exchange current density were observed, indicating that the ZrO2 treatment also deterred the oxidation reaction of hydrogen. However, in comparison with the data obtained, the ZrO2 treatment seemed to be relatively more effective in inhibiting the oxygen reduction reaction than inhibiting the hydrogen oxidation reaction. One additional beneficial outcome was that the anodic current density of the metal was also decreased, leading to a much lower overall corrosion current density of the ZrO2 treated specimen.

Impact of Power Uprate on the Water Chemistry in the Primary Coolant Circuit of a Boiling Water Reactor Operating under a Fixed Core Flow Rate

The approach of power uprate has been adopted by the utilities of light water reactors over the past few decades in order to increase the power generation efficiency of a nuclear reactor. Upon a power uprate, the power density of a nuclear reactor would change immediately, followed by water chemistry variations due to the enhanced radiolysis of water in the core and near-core regions. For commercial boiling water reactors (BWRs), it is currently a common practice to adopt hydrogen water chemistry (HWC) for corrosion mitigation. The optimal hydrogen injection rate may require a proper adjustment after a power uprate is practiced in a BWR. A DEMACE computer code was used in the current study to investigate the impact of various power uprate levels on major radiolytic species concentrations and the electrochemical corrosion potential (ECP) behavior of components in the primary coolant circuit of a domestic BWR operating under either normal water chemistry or HWC. The results of our analysis indicated that the chemical species concentrations and ECP did not vary monotonically with increases in reactor power level at a fixed feedwater hydrogen concentration. In particular, the upper plenum and upper downcomer regions exhibited uniquely higher ECPs at a 102% power level than at the other evaluated power levels. The impact of power uprate on the water chemistry in the primary coolant circuit of a BWR is expected to vary from location to location and eventually from plant to plant due to different degrees of radiolysis and physical dimensions.

The Influence of Dissolved Hydrogen on the Corrosion of Type 304 Stainless Steels Treated with Inhibitive Chemicals in High Temperature Pure Water

In order to assess the influence of dissolved hydrogen on the intergranular stress corrosion cracking (IGSCC) characteristics of Type 304 stainless steels treated with inhibitive chemicals, electrochemical corrosion potential (ECP) measurements and slow strain rate tensile (SSRT) tests were conducted in high temperature pure water. A number of thermally sensitized specimens were prepared and then pre-oxidized in a 288°C pure water environment with the presence of 300ppb dissolved oxygen for 360h. Most of the specimens were then separately treated with various inhibitive chemicals including powdered zirconium oxide (ZrO2), powdered titanium oxide (TiO2), and zirconyl nitrate [ZrO(NO3)2] via hydrothermal deposition at 150°C. Test environments with a dissolved oxygen concentration of 300ppb and various dissolved hydrogen concentrations at 288°C were created. Test results showed that the ECPs of the treated specimens were lower than that of the untreated one no matter what the dissolved hydrogen concentration was. In addition, IGSCC was observed on all specimens (treated or untreated) in all tested environments. However, the untreated specimen exhibited lower elongation, shorter failure time, and more secondary cracks on the lateral surfaces. It was therefore suggested that inhibitive chemicals such as ZrO2, TiO2, and ZrO(NO2)2 did provide a certain degree of enhancement in improving the mechanical behavior of the treated specimens and in prolonging IGSCC initiation times.

Intergranular Stress Corrosion Cracking of Platinum Treated Type 304 Stainless Steels in High Temperature Water

Electrochemical polarization analyses, slow strain rate tensile (SSRT) tests and crack growth tests were conducted to investigate the intergranular stress corrosion cracking (IGSCC) characteristics of platinum treated Type 304 stainless steels in 288°C pure water. All specimens were thermally sensitized and pre-oxidized before the tests, and some of them were additionally treated with platinum via hydrothermal deposition. Test environments were specifically designed in a circulation loop to create water chemistry conditions of hydrogen to oxygen molar ratios (MH/O)of 0, 0.5, and 2.7 in the coolant. Test results showed that the corrosion current densities of the specimens treated with platinum were higher than those of the pre-oxidized ones under oxygenated conditions, but the corrosion potentials of the treated specimens could be either lower or higher than those of the untreated ones. In addition, IGSCC was not observed on all specimens (treated or untreated) when the MH/O in the test coolant was 0.5 or 2.7. However, the platinum treated specimens actually showed severer IGSCC after the SSRT tests conducted under the coolant condition of MH/O=0. Crack growth test results indicate a similar but worse trend to that in the SSRT tests. It was therefore suggested that one must exercise extra caution to use corrosion potential as a sole indicator in evaluating the influence of noble metal treatment on the IGSCC susceptibility of Type 304 stainless steels, especially when the MH/O in the coolant is relatively low or the dissolved oxygen concentration remains comparatively high.