in-process ecp monitoring at bwr utilizing olnc for...
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© 2015 Electric Power Research Institute, Inc. All rights reserved.
Susan Garcia, EPRI [email protected]
Environmental Degradation Conference August 2015
In-Process ECP Monitoring at BWR Utilizing OLNC for
Mitigation of IGSCC
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BWR IGSCC Mitigation
Hydrogen Water Chemistry (HWC) was implemented to protect BWR reactor vessel, internals and piping from intergranular stress corrosion cracking. – At moderate hydrogen injection rates (1-2 ppm FW H2), mitigation of
stress corrosion cracking is achieved when the electrochemical corrosion potential (ECP) of reactor vessel, piping and internals is <-230 mV(SHE). (1982 – current)
– Lower hydrogen injection (<0.5 ppm), combined with noble metal injection, also reduces ECP, two generations of noble metals exist: Noble Metal Chemical Addition (NMCA) – applied during shutdown
every 4-6 years (1996 – 2008) On-Line NobleChem™ (OLNC) – applied during full power
operation every 11-16 months (2005 – current)
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U.S. BWR Chemistry - History
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5
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25
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1985 1990 1995 2000 2005 2010 2015 2020
Num
ber o
f U.S
. BW
Rs
Zn Injection NMCA+HWC HWC (no NMCA) OLNC
Zn addition for shutdown dose control
Mitigation Technologies
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Monitoring of IGSCC Mitigation with OLNC
Demonstration of the effectiveness of OLNC was limited to Electrochemical Corrosion Potential (ECP) probes mounted in external piping and monitoring skids. – Chemistry conditions at these locations are not as
aggressive as in the lower vessel head region. Lower oxidant concentration as distance from the core increases. H2O2 decomposition is also known to occur.
Prior to recent Brunswick-1 experience, there had been no direct in-vessel ECP measurements during OLNC application/operation in the lower vessel head region. – At this location, oxidant concentrations are elevated
compared to recirculation loops and external monitoring skids.
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Brunswick-1 Mitigation Monitoring
Two platinum probes and a pre-oxidized stainless steel billet were installed in a modified local power range monitor (LPRM). Coolant flow holes for LPRM were shifted down to coincide
with a flow hole, allowing coolant from below the core plate to flow past the probes.
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Location of Brunswick-1 ECP Probe
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Brunswick-1 Mitigation Timeline
Moderate HWC from 1989 – August 2014 LPRM ECP modification completed during March 2014
refuel outage Hydrogen benchmark completed in May 2014 under
Moderate HWC conditions Initial OLNC application in August 2014 Hydrogen benchmark completed in October 2014 for OLNC
conditions Benchmark Testing provides valuable IGSCC mitigation data
M-HWC (25 years) Installation of LPRM
ECPs Benchmark
Test OLNC
Application Benchmark
Test
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Brunswick-1 ECP Response during HWC-M Ramp Test
Platinum to LPRM surface potential difference shown as "Pt1(2)-GND". Platinum to stainless steel electrode
potential difference shown as "Pt1(2)-SS".
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Brunswick-1 LPRM ECP Data vs Feedwater Hydrogen
Platinum to stainless steel electrode potential difference shown as "Pt1(2)-SS".
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Brunswick-1 LPRM ECP Response during OLNC Application
Platinum to LPRM surface potential difference shown as
"Pt1(2)-GND".
Current OLNC vendor recommendation is OLNC injection 60-90 days after startup
Rapid response of LPRM ECP
due to noble metal deposition on surfaces
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Nine Mile Point-1 Experience
Two Pt ECP probes in decontamination flange of reactor recirculation system (RRS) loop. One Pt ECP probe in external monitoring system (MMS -
Mitigation Monitoring System) that takes flow from reactor water cleanup (CU) loop. HWC benchmark test performed June 2013
– Feedwater hydrogen reduction from 6 scfm to 2.7 scfm – No change in MMS ECP readings while RRS ECP raw
values became more negative – All probes indicated a shift in values at 2.7 scfm when the
molar ratio of hydrogen to oxidants was less than 2:1
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Nine Mile Point-1 HWC Ramp Test Data
RGnd = Recirculation ECP
CUGnd = MMS ECP
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Hatch-1 Experience
Host site of ECP measurement study by Japanese Owners Group in late 1990s and 2000s. ECP probes in multiple locations:
– LPRM – MMS (external system) – Bottom Head Drain Line (BHDL)
Data collected pre-NMCA and post-NMCA LPRM and BHDL ECP data indicated that HWC-M and
NMCA were both effective for IGSCC mitigation LPRM ECP electrodes are no longer used at plant, but the
station continues to monitor ECP at the BHDL and MMS
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Hatch ECP Response at BHDL and LPRM Pre- and Post-NMCA
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Hatch-1 Experience
In late 2000s, a difference between “as-read” ECP measurements at the BHDL and MMS locations was noted, with a gradual increase at the BHDL starting in 2009. The increase in ECP at the BHDL location was expected
since it had been four years since an NMCA application. Not expected…MMS ECP remained low and stable several
years after NMCA and in between future OLNC applications. MMS ECP manifold was replaced and confirmed that a low
oxygen environment within the MMS was contributing to false low ECP measurements and may not be indicative of ECP measurement of reactor vessel internals. Considerations for ECP monitoring in MMS now being
evaluated, with counter measures being recommended.
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Hatch-1 ECP Data in MMS and BHDL
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Conclusions
Brunswick-1 experience demonstrated that OLNC can be effective for mitigation of IGSCC in an aggressive environment. Nine Mile Point-1 experience demonstrated that ECP can be
used to indicate when molar ratio of hydrogen to oxidant is less than 2:1. Hatch-1 LPRM and BHDL ECP demonstrated effectiveness
of HWC-M and NMCA regimes for IGSCC mitigation. In-reactor ECP monitoring can provide better trending to
show the effect of plant changes or local losses of catalytic activity. – LPRM and BHDL locations are unaffected by radiation
from the core, which drives recombination reactions. – Monitoring at these locations has great value.
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Brunswick Chemistry, Engineering, and Major Projects personnel – Steve Williams, Larry Yemma, Kurt Crytzer, Joel McLean,
John Piepmeyer, Kristi White, Pat Godsey and Terri Jones Nine Mile Point’s and Hatch Nuclear Plant’s Chemistry
Departments – Collin Custer (NMP) and Brian Colson (Hatch)
General Electric – Hitachi – Juan Varela and Bert Huie
Acknowledgements
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