The Influence of Dissolved Hydrogen on the Solubility and … · 2007-11-06 · The Influence of...

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

2AREVA NP INC. OLI Simulation Conference October 23-24, 2007

Presentation Outline

► Introduction & BackgroundOverview of Reactor Coolant System DesignBasis of Reactor Coolant Chemistry ControlRecent Industry Challenges• Stress Corrosion Cracking of Vessel Penetration Nozzles• Axial Offset Anomalies (Crud-Induced Power Shifts)Industry Response

► Application of OLI Model to PWR ChemistryEffects 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


Pressurized Water Reactor System

Animated Diagram of a Pressurized Water Reactor. From the NRC Website


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) claddingTo 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


Reactor Coolant Chemistry Parameters

Suppress radiolysis, establish reducing environment

25 – 50 cm3 (STP)/kg H2ODissolved Hydrogen

Minimize corrosion6.8 – 7.4pH300°C

pH Control≤ 4 ppmLithiumNeutron Absorption0 – ~1300 ppmBoron

PurposeConcentration Range

Controlled Parameter

Temperature Range: 270°C – 330°CRC Pressure: 15.6 MPa


Typical Fuel Cycle


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 coresFirst observed in 1988Has also occurred in low-duty PWRs in local areasBecoming more important as plants upgrade to higher power levels


Main Uses of Nickel-Base Alloys in PWRs

Typical PWR Reactor Configuration


PWSCC in Alloys 600 and 182 of Upper Head CRDM Nozzles


SCC Initiation and Crack Growth Rates for Alloy 600

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

► Both susceptibility to SCC and maximum crack growth rates appear to occur in proximity to the Ni/NiO phase transition


Ni/NiO Phase Transition Boundary

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

► 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/NiOphase boundary for comparison with these test results


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 crackgrowth rates

Low temperature crack propagation due to increased levels of absorbed hydrogenOperational concerns with greater volume of H2 to manageEffect on Zircaloy cladding integrity

• Increased H2 pickup possible hydridingEffect on corrosion product transport and deposition in high-duty cores

• Any impact on potential for AOA/CIPS?


Ni/NiO Stability (MSE Model)

a – min H2 to suppress radiolysisb – present operating rangec – industry proposed maximum


Ni/NiO Phase Transition Dependence on T and [H2]

MSE Model agrees reasonably well with accepted data


Axial Offset Anomalies► AOA, or Crud Induced Power

Shifts (CIPS)Axial asymmetry in power observed mid-cycle in 18-24 month fuel cyclesAssociated with sub-cooled nucleate boiling (SNB) and substantial crud buildup in the upper part of (mostly) high-duty coresAttributed 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 Ref: Bennett, et al., “Demonstration of the PWR AOA

in the Halden Reactor,” Int’l Conf on Water Chemistry in Nuclear Reactor Systems, 2006


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 temperaturesAdded boric acid & silicic acid vapor phase parametersAdded several new species to the MSE database• Lithium monoborate, LiBO2• Nickel ferrite, NiFe2O4• Bonaccordite, Ni2FeBO5


Approach

► Comparison of MSE model predictions of nickel and iron equilibrium solubilities against literature data

► Extension of model to B-Li chemistry for Ni/NiOand 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


Model Verification

MagnetiteSolubility

Nickel OxideSolubility


Nickel Solubility at 0 cc/kg H2


Iron Solubility at 35 cc/kg H2


Iron Solubility at 70 cc/kg H2


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.


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


Effect of Steaming on Solution pH and Boiling Point Elevation


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 ironMore 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 acidprecipitation 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 NiFe2O4The ratio of Fe/Ni in the deposit increases

► For the cases studied, the model did not predict the formation of Ni2FeBO5


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 modelIdentify any additional species required based on deposit analysesModel the effects of silica and zinc injection on core deposits – determine RCS silica limit

► Investigate the feasibility for modeling Zircaloy corrosion


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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