The Influence of Dissolved Hydrogen on the
Solubility and Transport of Iron and Nickel in
Reactor Coolant Systems
OLI Simulation Conference
October 23-24, 2007
Paul Sherburne
Presentation Outline
► Introduction & Background
 Overview of Reactor Coolant System Design
 Basis of Reactor Coolant Chemistry Control
 Recent Industry Challenges
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Stress Corrosion Cracking of Vessel Penetration Nozzles
Axial Offset Anomalies (Crud-Induced Power Shifts)
 Industry Response
► Application of OLI Model to PWR Chemistry
 Effects of Elevated Hydrogen on Nickel and Iron
Solubility
•
•
Model Predictions and Comparison with Literature Data
Potential Impact of Elevated Hydrogen on Crud Formation
► Future Studies
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OLI Simulation Conference October 23-24, 2007
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Pressurized Water Reactor System
Animated Diagram of a Pressurized Water Reactor. From the NRC Website
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Reactor Coolant Chemistry
► Control of reactor coolant chemistry has several
objectives:
 To minimize the general corrosion of system materials
• Austenitic stainless steels (304, 316, A-286)
• High-strength austenitic alloys (X-750, 718)
• Nickel-base alloys (Alloys 600 & 690 and related weld
materials)
 To maintain the integrity of the fuel rod zirconium alloy
(Zircaloy) cladding
 To manage the exposure of personnel to out-of-core
radiation fields to ALARA levels
• By minimizing the generation and transport of corrosion
products to the core where they become irradiated and
released
 To moderate the nuclear reaction
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OLI Simulation Conference October 23-24, 2007
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Reactor Coolant Chemistry Parameters
Controlled
Parameter
Concentration
Range
Purpose
Boron
0 – ~1300 ppm
Neutron Absorption
≤ 4 ppm
Lithium
6.8 – 7.4
pH300°C
Dissolved
Hydrogen
pH Control
Minimize corrosion
25 – 50 cm3 (STP)/kg H2O Suppress radiolysis,
establish reducing
environment
Temperature Range: 270°C – 330°C
RC Pressure: 15.6 MPa
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OLI Simulation Conference October 23-24, 2007
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Typical Fuel Cycle
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Current Industry Challenges
► Primary Water Stress Corrosion Cracking (PWSCC) has
affected nickel-based alloys – steam generator tubing,
instrumentation nozzles, pressurizer heater nozzles, and
Control Rod Drive Mechanism (CRDM) penetrations since
the mid-1980’s.
 Remedial measures have included
•
•
•
•
•
Increased number and frequency of inspections
Repair and replacement of defective nozzles and sleeves
Weld overlays of defective welds
Replacement of reactor vessel heads and steam generators
Injection of zinc to reduce rate of PWSCC
 PWSCC will continue to be a factor as plants age
► Axial Offset Anomalies (AOA) have caused axial power
asymmetries in some plants having high-duty cores
 First observed in 1988
 Has also occurred in low-duty PWRs in local areas
 Becoming more important as plants upgrade to higher power
levels
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OLI Simulation Conference October 23-24, 2007
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Main Uses of Nickel-Base Alloys in PWRs
Typical
PWR Reactor
Configuration
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PWSCC in Alloys 600 and 182 of Upper Head CRDM Nozzles
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SCC Initiation and Crack Growth Rates for Alloy 600
► Both susceptibility to SCC and maximum crack growth rates
appear to occur in proximity to the Ni/NiO phase transition
Ref: Morton, et al, “Measurement of the Nickel/Nickel
Oxide Transition in Ni-Cr-Fe Alloys and Updated
Correlations to Quantify the Effect of Aqueous Hydrogen
on Primary Water SCC,” 11th International Conference on
Environmental Degradation
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Ni/NiO Phase Transition Boundary
► Morton, et al. also
conducted CER and
corrosion coupon tests in
deaerated water (pHt=7) to
define the phase
transition between Ni and
NiO as a function of
temperature and [H2]
► MSE model was used to
determine the Ni/NiO
phase boundary for
comparison with these
test results
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OLI Simulation Conference October 23-24, 2007
Ref: Morton, et al, “Measurement of the Nickel/Nickel
Oxide Transition in Ni-Cr-Fe Alloys and Updated
Correlations to Quantify the Effect of Aqueous Hydrogen
on Primary Water SCC,” 11th International Conference on
Environmental Degradation
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Industry Proposed Solution to Reduce CGR
► Based on the work by Morton, et al., it has been proposed
that reactor coolant [H2] be increased to much higher levels
to achieve slower crack propagation rates
► Potential concerns with increasing [H2] include:
 Effect on time to initiate cracking
• Studsvik testing [Molander, et al. (2007)] suggests that decreasing
[H2] delays crack initiation without significantly increasing crack
growth rates
 Low temperature crack propagation due to increased levels of
absorbed hydrogen
 Operational concerns with greater volume of H2 to manage
 Effect on Zircaloy cladding integrity
• Increased H2 pickup
possible hydriding
 Effect on corrosion product transport and deposition in highduty cores
• Any impact on potential for AOA/CIPS?
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OLI Simulation Conference October 23-24, 2007
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Ni/NiO Stability (MSE Model)
a – min H2 to suppress radiolysis
b – present operating range
c – industry proposed maximum
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Ni/NiO Phase Transition Dependence on T and [H2]
MSE Model agrees reasonably well with accepted data
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Axial Offset Anomalies
► AOA, or Crud Induced Power
Shifts (CIPS)
 Axial asymmetry in power
observed mid-cycle in 18-24
month fuel cycles
 Associated with sub-cooled
nucleate boiling (SNB) and
substantial crud buildup in the
upper part of (mostly) highduty cores
 Attributed to boron enrichment
in fuel rod deposits (crud)
• Adsorption of boron?
• LiBO2 or Ni2FeBO5
precipitation?
 Precipitation of NiO crystals
(whiskers) observed in
deposits
► Mechanism for hideout of B is
not clear
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OLI Simulation Conference October 23-24, 2007
Ref: Bennett, et al., “Demonstration of the PWR AOA
in the Halden Reactor,” Int’l Conf on Water
Chemistry in Nuclear Reactor Systems, 2006
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Application of OLI Technology
► The MSE model is being used to develop a better
understanding of the effect of elevated [H2] on the
transport of soluble nickel and iron species in the
RCS and the precipitation of these species in the
reactor core
► To achieve the best simulation of RCS chemistry,
OLI Systems provided the following assistance:
 Improved boron-lithium chemistry at high temperatures
 Added boric acid & silicic acid vapor phase parameters
 Added several new species to the MSE database
• Lithium monoborate, LiBO2
• Nickel ferrite, NiFe2O4
• Bonaccordite, Ni2FeBO5
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Approach
► Comparison of MSE model predictions of nickel
and iron equilibrium solubilities against literature
data
► Extension of model to B-Li chemistry for Ni/NiO
and Fe3O4 solubility vs. pH(t) and [H2]
► Vaporization of solutions saturated in nickel and
iron to simulate sub-cooled nucleate boiling
(SNB) and precipitation in the upper core region
► Comparison of model predictions with physical
observations
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Model Verification
Nickel Oxide
Solubility
Magnetite
Solubility
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Nickel Solubility at 0 cc/kg H2
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Nickel Solubility at 35 cc/kg H2
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Nickel Solubility at 70 cc/kg H2
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Iron Solubility at 35 cc/kg H2
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Iron Solubility at 70 cc/kg H2
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Solids Formation due to SNB for [H2] = 35 scc/kg
► Note: LiBO2 was the major precipitate beginning at a C.F. of 200.
LiBO2 has been postulated to contribute to the occurrence of AOA.
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Solids Formation due to SNB for [H2] = 70 scc/kg
► Note: precipitation of Fe3O4 and NiO is favored over NiFe2O4 at the
increased dissolved hydrogen concentration
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Effect of Steaming on Solution pH and Boiling Point Elevation
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Observations from Modeling
► Solubility-driven precipitates on the fuel rods are nickel (metal)
and magnetite. This is in general agreement with industry data.
► Increasing [H2] to 70 scc/kg from 35 scc/kg decreases the
solubility of Ni metal
 Ni metal still exhibits retrograde solubility; however,
 Less nickel is available for precipitation
► Increasing [H2] increases the solubility of iron
 More iron is available for precipitation, which may increase porous
deposit (crud) levels in the core
► At 35 scc/kg H2, steaming in porous deposits results in
 An increase in local alkalinity from pH(t) 7.25 to 8.5 due to the volatility
of boric acid
 precipitation of LiBO2, NiO, NiFe2O4, and Fe3O4
► Increasing [H2] to 70 scc/kg modifies the makeup of precipitating
species
 Precipitation of Fe3O4 and NiO is favored over NiFe2O4
 The ratio of Fe/Ni in the deposit increases
► For the cases studied, the model did not predict the formation of
Ni2FeBO5
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Ongoing Studies
► Expand analysis to cover the complete cycle,
including startup and shutdown operations
► Consider adding non-stoichiometric nickel
ferrites to MSE data base
► Add silica and zinc to the existing model
 Identify any additional species required based on
deposit analyses
 Model the effects of silica and zinc injection on core
deposits – determine RCS silica limit
► Investigate the feasibility for modeling Zircaloy
corrosion
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Future Plans
► AREVA NP is currently working with OLI Systems
to dynamically link the OLI thermodynamic
engine to MatLab® to provide chemistry input to
AREVA’s deposition model
► For a given chemistry (.dbs file), temperature and
water analysis in MATLAB®, the thermodynamic
engine determines the chemical equilibrium at the
bubble point pressure and returns the results to
MATLAB®
► The dynamic link to OLI has been successfully
completed and test cases are being run to ensure
that accuracy is maintained through the link
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OLI Simulation Conference October 23-24, 2007
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OLI Simulation Conference October 23-24, 2007
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