The stress corrosion cracking (SCC) of structural materials used in boiling water reactors has been studied at relatively low hydrogen peroxide (H202) concentrations, around 10 ppb, which was assumed to be representative of the corrosion environment formed in hydrogen water chemistry (HWC). The 1/4T compact tension specimen was used for measurement of crack growth rates (CGRs) of sensitized type 304 stainless steel in high temperature and high purity water. Crack length was monitored by a reversing direct current potential drop method. Since H202 is easily decomposed thermally, a polytetrafluoroethylene-lined autoclave was used to minimize its decomposition on the autoclave surface. The CGR in the H20 2 environment differed from that in the 02 environment even though the electrochemical corrosion potential (ECP) for both conditions was the same. The data implied that the ECP could not be used as a common environmental deterministic parameter for SCC behavior at higher potentials for different oxidant conditions. The corrosion current density was found to play an important role as an environmental index for SCC, which was given as just the current density at the ECP at a specific oxidant concentration. The CGRs were found to be written as CGR = (3.8±0.6) X 10-3 icor+(l.5±1.6) X 10-s mm/s using the calculated corrosion current density icor below 10-4 A-cm-2 .

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Dependency of hydrogen water chemistry (H\VC) effectiveness on plant specifications and operational conditions has been studied for an empirical interpretation. The H\VC effectiveness was calculated exactly by employing a water radiolysis model requiring results of complex fiow analysis and dose distribution analysis as input, in addition to the plant specifications. It was found that decrease of oxygen concentration in the reactor water in the reactor recirculation system could be taken as an exponential function of H2 concentration added to the reactor water. Thus, the recombination factor p which was the coefficient of hydrogen concentration in the exponent could represent the recombination efficiency in the downcomer region. The p corresponded to the apparent reaction rate constant of the recombination reaction of H202 with H2, which included radical concentrations produced by irradiation, multiplied by the residence time in the downcomer. The dose rate index 'f of each plant was introduced as a measure of downcomer dose rate. Since the recombination efficiency was known to depend on the downcomer dose rate, functional dependence of p on ')' was found to be linear. The HWC effectiveness at the reactor pressure vessel bottom region could also be evaluated using p.

Hydrazine and hydrogen co-injection into reactor water is considered a new mitigation method of stress corrosion cracking in BWRs. Fundamental data such as the thermal decomposition of hydrazine, the reaction of hydrazine with oxygen and with hydrogen peroxide at temperatures ranging from 150 to 280 C are needed to evaluate suitability of this method. Reactions in bulk water were studied in a polytetrafluoroethylene pipe to separate surface reaction effects. The results were as follows. (1) The orders of the apparent reaction rate of hydrazine with oxygen were 1 and 0.5 for hydrazine and oxygen concentrations, respectively (À . Based on these data, the applicability of hydrazine and hydrogen co-injection into BWRs was considered. Hydrazine introduction to reactor water was confirmed to be accompanied by only 1% decomposition. The concentration of oxygen, which is injected to suppress the flow-assisted corrosion of carbon steel in current BWR operation, would decrease due to the reaction of hydrazine with oxygen. However oxygen concentration in feed water could be maintained at the required level if the concentration of oxygen injected in condensate water was at most doubled compared to the current operating concentration.