Reduction of electrochemical corrosion potential (ECP) of Boiling Water Reactor (BWR) vessel and internals is essential for mitigating intergranular stress corrosion cracking (IGSCC). Hydrogen water chemistry in moderate concentrations (HWC-M) or at reduced injection rates with Noble Metal Chemical Application (NMCA) or On-Line NobleChem™ (OLNC) are IGSCC mitigation strategies employed by BWRs worldwide. In-vessel lower plenum ECP measurements have been obtained during operating conditions for BWRs utilizing normal water chemistry HWC-M and NMCA demonstrating the effectiveness of HWC-M and NMCA chemistry regimes for IGSCC mitigation. ECP monitoring for BWRs with OLNC has been limited to probes mounted in external piping (recirculation reactor water cleanup and bottom head drain lines) and external monitoring skids. The chemistry conditions and radiolysis effects at these locations are typically not as aggressive as in the lower vessel head region. There have been no direct in-vessel ECP measurements during an OLNC injection with ECP probes in the lower plenum at a conservative location below the core plate. With OLNC noble metal (platinum) is injected into the feedwater stream during a ten day period approximately 90 days after startup from a refuel outage and injected approximately once a year during power operation. The now obsolete NMCA was performed during shutdown for the refuel outage approximately every six years. The quantity injected with OLNC each year is a factor of 3 lower than the quantity injected when compared to the NMCA technology. In-vessel ECP measurements in the lower vessel head region during the first OLNC application at a nuclear station without a prior noble metal history will provide valuable information on the effectiveness of the OLNC process before during and after an OLNC application with respect to measured ECP values at a “conservative” location as described in a recent NRC safety evaluation report (SER) on this subject. Thus mitigating IGSCC in the more aggressive environment will be demonstrated.Brunswick Nuclear Plant (BNP) Units 1 and 2 have utilized moderate Hydrogen Water Chemistry (HWC-M) as a mitigation strategy for protection of IGSCC since 1989. To reduce operational radiation exposure associated with HWC-M the units will implement OLNC in 2014 (Unit 1) and 2015 (Unit 2). Prior to the transition on each unit two platinum ECP electrodes and a pre-oxidized stainless steel billet will be installed in a modified local power range monitor (LPRM) during the preceding refuelling outage. The coolant flow holes in the LPRM will be shifted downward to coincide with a flow hole made by EDM in the in-core housing allowing reactor coolant from well below the core plate to flow past the probes. After unit start-up from each unit’s outage the station will continue with HWC-M chemistry for approximately 90 days before the initial OLNC application is performed. ECP data will be collected during HWC-M and pre-OLNC and post-OLNC. For both HWC-M and OLNC with HWC hydrogen injection ramp tests will be conducted to determine the minimum hydrogen injection rate to achieve IGSCC mitigation of reactor recirculation piping and targeted reactor internal components and welds. For OLNC an optimum injection will be established based on the test results that provides excess hydrogen to assure IGSCC mitigation.This paper will present industry-first lower plenum in-vessel ECP measurements during the transition from HWC-M to OLNC with HWC. Reduction to < -230 mV(SHE) the NRC accepted threshold below which IGSCC will not occur in a BWR environment in the lower plenum at a “conservative” location would provide additional confirmation that sufficient catalytic material is being deposited on surfaces in regions where IGSCC mitigation is targeted. This can be demonstrated with radiolysis modelling without the need for over-conservative adjustment factors associated with ECP probes located in other areas in-vessel or ex-vessel. The station’s technical basis for reactor vessel internals IGSCC mitigation strategy and monitoring will be discussed along with ECP measurements of the two chemistry regimes and associated hydrogen ramp tests.