Mitigation of SCC Initiation on BWR Core Internals by Means of Hydrogen Water Chemistry During Start-Up

Hydrogen injection has been applied as a preventive measure against the stress corrosion cracking (SCC) phenomenon in many boiling water reactors. However, it can be applied only during normal plant operation since hydrogen is usually injected into the feedwater and this system is in standby mode during start-up operations. It is estimated that the core internals are subjected to the strain rate that may cause susceptibility to SCC initiation during start-up. Therefore, it is beneficial to perform hydrogen injection during start-up as well in order to suppress SCC initiation.

For this purpose, we installed an additional hydrogen injection system to be used during plant start-up at the Tokai-2 power station. This trial Hydrogen water chemistry During Start-up (HDS) system was applied following the 19th refueling and maintenance outage in December 2002. By comparing results obtained during this start-up with HDS to previous start-up data using normal water chemistry, we made the following observations. First, as the reactor water temperature increased from initial conditions up to 180°C via nuclear heating, dissolved oxygen and hydrogen peroxide concentrations decreased to levels lower than previously observed. Second, during subsequent nuclear heating, up to 250°C, the dissolved oxygen concentration did not exceed 1 ppb, and the electrochemical corrosion potential was maintained in a low range near -400 mV versus the standard hydrogen electrode.