APPROACHES TO ENHANCING EARLY HYDROGEN WATER CHEMISTRY FOR IGSCC MITIGATION DURING BWR STARTUPS

摘要

Boiling Water Reactors (BWRs) have been injecting hydrogen into the reactor coolant via the feedwater system for the purpose of controlling primary system intergranular stress corrosion cracking (IGSCC) for over 30 years. However, plant design limitations prevent hydrogen injection until there is sufficient steam flow to support operation of the Steam Jet Air Ejector (SJAE) system, which typically occurs at greater than 5% power. The time from when the reactor coolant temperature is heated up to 200 °F (93.3 °C) until hydrogen injection starts is counted as time when IGSCC is not mitigated. Laboratory data show that crack growth rates peak at intermediate temperatures. To address this gap, Early Hydrogen Water Chemistry (EHWC) was developed by EPRI/BWRVIP for plants that use GE Hitachi NobleChem to lower the electrochemical corrosion potential (ECP) during early startup. A demonstration of EHWC performed at Peach Bottom 3 in October 2011 showed that sufficient hydrogen could be injected, while condenser vacuum was being maintained using the Mechanical Vacuum Pump (MVP), before steam flow was sufficient to place the SJAE system in service, to lower the ECP to a level indicative of IGSCC mitigation. While this demonstration successfully showed that sufficient hydrogen can be injected safely to mitigate IGSCC during early startup when temperature was between 360 °F and 460 °F (182 - 238 °C), a reactor coolant hydrogen:oxidants molar ratio >2 was not achieved during the initial heatup when the temperature reached 200 °F (93.3 °C). This paper provides the results of a BWRVIP investigation of options to mitigate IGSCC at all temperatures above 200 °F (93.3 °C) for BWRs that apply noble metals. Options investigated for plants that have applied noble metal include 1) plant capabilities to achieve conditions at which IGSCC can be mitigated from 200 °F (93.3 °C) and above through the use of hydrogen alone and 2) the use of other agents, including hydrazine or methanol as a chemical reductant, or titanium dioxide (TiO_2) as a surface treatment, followed by hydrogen injection.