Katsumi Ohsumi

Effects of Hydrogen Peroxide on Intergranular Stress Corrosion Cracking of Stainless Steel in High Temperature Water, (I), Effects of Hydrogen Peroxide on Electrochemical Corrosion Potential of Stainless Steel

In order to determine effects of hydrogen peroxide on stress corrosion cracking of structural materials in the primary cooling systems of boiling water reactors, a high temperature high pressure water loop with controlled hydrogen peroxide concentrations and lower possible oxygen concentrations has been fabricated. Test specimens are installed in a stainless steel autoclave which has poly tetra-fluoro-ethylene (PTFE) inner liner to prevent decomposition of hydrogen peroxide on the autoclave surfaces. Hydrogen peroxide is injected into the autoclave inlet through the injection line which also has PTFE inner liner. The concentration of hydrogen peroxide is measured at the autoclave outlet by sampling water via the PTFE-lined sampling line. More than 65% of the injected hydrogen peroxide remains at the autoclave outlet at elevated temperature (288°C). Electrochemical corrosion potential (ECP) of stainless steel is then measured in the autoclave while changing hydrogen peroxide and oxygen concentrations. From these measurements it is concluded that, at the same oxidant concentration: (1) ECP of stainless steel exposed to hydrogen peroxide is higher than that exposed to oxygen; (2) ECP is much affected by specimen surfaces; and (3) ECP shows a hysteresis pattern for on its concentration dependence. ECP of stainless steel with an oxidized surface formed under high hydrogen peroxide concentration is much higher than that with a mechanically polished surface and it is less affected by oxidant species and their concentrations.

Radiation Buildup on Stainless Steel in a Boiling Water Reactor Environment

Cobalt-60 contained in reactor water primarily causes contamination of boiling water reactor (BWR) out-of-core components. To elucidate the contamination mechanism, the deposition of radionuclides on stainless steels has been evaluated in actual reactor water at a commercially operating BWR. No significant difference was observed between Types 304 and 316L stainless steel. The deposition rate of /sup 60/Co was mainly controlled by the growth of oxide films formed on steel. The deposition kinetics of anion /sup 51/Cr was very different from that of cations /sup 60/Co and /sup 58/Co. An Arrhenius temperature dependence was established for the deposition rate of /sup 60/Co between 130 and 240/sup 0/C.

A Calculation Model for Predicting Concentrations of Radioactive Corrosion Products in the Primary Coolant of Boiling Water Reactors

A calculation model was developed to predict the shutdown dose rate around the recirculation pipes and their components in boiling water reactors (BWRs) by simulating the corrosion product transport in primary cooling water. The model is characterized by separating cobalt species in the water into soluble and insoluble materials and then calculating each concentration using the following considerations: (1) Insoluble cobalt (designated as crud cobalt is deposited directly on the fuel surface, while soluble cobalt (designated as ionic cobalt) is adsorbed on iron oxide deposits on the fuel surface. (2) Cobalt-60 activated on the fuel surface is dissolved in the water in an ionic form, and some is released with iron oxide as crud. The model can follow the reduction of /sup 60/Co in the primary cooling water caused by the control of the iron feed rate into the reactor, which decreases the iron oxide deposits on the fuel surface and then reduces the cobalt adsorption rate. The calculated results agree satisfactorily with the measurements in several BWR plants.

Application of Hydrogen Water Chemistry to Moderate Corrosive Circumstances around the Reactor Pressure Vessel Bottom of Boiling Water Reactors

Many efforts to preserve the structural integrity of major piping, components, and structures in a boiling water reactor (BWR) primary cooling system have been directed toward avoiding intergranular stress corrosion cracking (IGSCC). Application of hydrogen water chemistry (HWC) to moderate corrosive circumstances is a promising approach to preserve the structural integrity during extended lifetimes of BWRs. The benefits of HWC application are (a) avoiding the occurrence of IGSCC on structural materials around the bottom of the reactor pressure vessel (RPV) and (b) moderating the crack growth rate, even if microcracks are present on the structural materials. Several disadvantages caused by HWC (e.g., turbine dose rate increase, 60 Co radioactivity buildup, and effects on fuel cladding) are evaluated to develop suitable countermeasures prior to HWC application. The advantages and disadvantages of HWC are quantitatively evaluated based on both BWR plant data and laboratory data shown in unclassified publications. Their trade-offs are discussed, and suitable applications of HWC are described. It is concluded that an optimal amount of hydrogen injected into the feedwater can moderate corrosive circumstances, in the region to be preserved, without serious disadvantages. The conclusions have been drawn by combining experimental and theoretical results. Experiments in BWR plants - e.g., direct measurements of electrochemical corrosion potential and crack growth rate at the RPV bottom - are planned that would collect data to support the theoretical considerations

Water Quality Control at Primary Cooling System of Crud Concentration Suppressed Boiling Water Reactor, (II): Evaluation of Cobalt-58 Behavior in Iron Crud Suppressed BWR

A possible accumulation mechanism of Ni and radioactive 58Co ions in the reactor water of a low iron crud concentration plant was modeled based on a direct deposition of Ni ion on fuel cladding surfaces as a form of NiO, followed by the solid phase reaction between NiO and iron crud (α-Fe2O3) to produce NiFe2O4. The calculated results agreed satisfactorily with measurements in the first Japanese standard BWR, Fukushima 2-2 (1,100 MWe). These calculations showed that controlling the Fe/Ni corrosion products ratio in the feed water to a value larger than 2 was effective to suppress 58Co activity in the reactor water as well as Ni ion concentration.

Structural Material Anomaly Detection System Using Water Chemistry Data, (I): Basic Structure of Anomaly Detection System and Its Application for Fuel Defect Detection

A system to diagnose abnormalities on structural materials in the primary cooling systems of BWR by analyzing water chemistry data is proposed. In the system, patterns composed of more than two kinds of water chemistry data are compared with several stages of reference patterns obtained by the models which relate the water quality changes to the structural material abnormalities. Small abnormalities on the materials can be expected to be detected at very early stages and future abnormalities can be predicted by the diagnosis system. In this paper, the basic structures of the anomaly detection system using water chemistry data are explained.

An Empirical Formula Predicting Shutdown Dose Rate of the Recirculation Pipes in Boiling Water Reactors

An empirical formula is proposed to estimate the shutdown dose rates around the primary cooling system of boiling water reactors. The formula is characterized by defining the average activation time of /sup 60/Co, which is proportional to the iron feed rate into the reactor. The constants of the formula are determined by quantitatively evaluating the static balances of corrosion products in the primary coolant on the basis of measured results from the Shimane Nuclear Power Station of Chugoku Electric Power Company. The effects of four radiation reduction procedures on the shutdown dose rate are evaluated using the proposed formula. It is demonstrated that the oxygen injection method is the most effective procedure to reduce the dose rate (to approx. 1/4). Further reduction (to approx. 1/50) can be achieved by a combination of the four procedures.

Using Titanium Oxide for Cobalt Removal from High-Temperature Water

Improvement of the reactor water cleanup system is one of the most effective methods for radiation reduction in boiling water reactor plants. This necessitates development of a heat-resistant adsorbent for direct use to remove cobalt under reactor conditions (285/sup 0/C, 70 kg/cm/sup 2/). We have developed a suitable adsorbent by impregnating granular sponge titanium with titanium oxide. Adsorption of Co/sup 2 +/ on titanium oxide was evaluated as a function of temperature. Selectivity experiments showed that corrosion products, such as Co/sup 2 +/, Ni/sup 2 +/, and Cu/sup 2 +/, were preferentially adsorbed at high temperatures. The data suggested that chemical adsorption occurred with the formation of insoluble cobalt metatitanate on the TiO/sub 2/ surface. In-plant tests, carried out to evaluate the performance using actual reactor water, verified the applicability of the new adsorbent to the hightemperature reactor water cleanup system.

Application of Carboxylic Acid Cation Exchange Resin to Water Purification in Nuclear Power Plants

Weakly acidic carboxylic resin, with no sulfur atoms, was selected as an alternative to sulfonic cation exchange resin for water treatment in nuclear power plants. Pyrolysis experiments showed that no corrosive SOx gas was produced by the former, and its residual ratio was one-third that of the latter conventional resin. Spent resin treatment then becomes relatively simple for the new resin. Subsequently, filtration characteristics were examined by use of simulated condensate water, assuming that the resin was applied to a filter demineralizer. The resin lifetime was ∼1.5 times that of the conventional one, suggesting that the amount of spent resin generated from a filter demineralizer could be reduced to about two-thirds. Therefore, the carboxylic resin showed favorable features for both water purification and spent resin treatment.

Evaluation of leachable behavior from ion exchange resins effects of organic impurities on BWR water chemistry, (1)

The elution rate of leachables from ion exchange resin, which is used in condensate demineralizers and is one of several major sources of organic compounds in BWR cooling water, was measured. Properties of the leachables and elution rate depended on the kind of ion exchange resin and the years of use. The organic compounds elution rate of cation exchange resin was constant for 5 years and the molecular weight of these leachables was low. After 5 years, the elution rate increased and leachables consisted of organic compounds of high molecular weights of several thousand. The elution rate of anion exchange resin decreased yearly. The difference in the elution behavior was attributed to a dependence on oxidation degradation promoted by transition metal catalysis. The cation exchange resin included absorbed transition metal, while the anion exchange resin did not. An empirical formula showing the time dependence of the elution rate of organic compounds was derived. The formula was judged to be appropriate bas...