FLOW EFFECTS ON CORROSION OF STEELS IN
REACTOR PRIMARY COOLANT –EXPERIENCE AND UNDERSTANDINGbyDerek H. Lister
Research Chair in Nuclear Engineering
Department of Chemical Engineering
University of New Brunswick,Fredericton, NB Canada E3B 5A3
Presented at a meeting of the Atomic Energy Society of Japan, Tokyo
2008 October 20th

▲CANDU reactors
1. Bruce A
4. Gentilly-1 (decom.)
Bruce B
Gentilly-2
2. Pickering A
5. Point Lepreau
Pickering B
6. NPD (decom.)
3. Darlington■Uranium mines
4. Cigar Lake
1. Cluff Lake
2. McClean Lake 5. McArthur River6. Key Lake
3. Midwest
7. Rabbit Lake

● Research reactors
1. U of Alberta
2. Saskatchewan Research Council
3. AECL Whiteshell Labs (decom.)
4. McMaster U
5. U of Toronto (decom.)
6. Royal Military College
7. AECL Chalk River Laboratories
8. MDS Nordion (decom.)
9. Ecole Polytechnique
10. Dalhousie U

2Fredericton

FREDERICTON

3

UNB

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NUCLEAR POWER IN CANADA••In 2007, nuclear ≡ 14.6% of Canada’s electricity
Installed nuclear gross capacity in 2008 (all CANDUs):Ontario
Pickering A
4×542 MW(e) (two in safe storage)
Pickering B
4×540 MW(e)Darlington
4×934 MW(e)Bruce A
4×805 MW(e) (two being refurbished)
Bruce B
1×845 MW(e)3×872 MW(e)Québec
Gentilly-2
1×675 MW(e) (to be refurbished 2011)
New Brunswick
Point Lepreau
1×680 MW(e) (being refurbished)

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PROSPECTS FOR NEW BUILD IN CANADA
Ontario
To decide on technology for two new units inearly 2009 (ACR-1000, Areva, Westinghouse)
New Brunswick
Negotiating for new unit – Point Lepreau II –ACR-1000 preferredAlberta and Saskatchewan
Interested in new build

6CANDU REACTORS ABROAD
Korea (Wolsong)

4×CANDU-6

Argentina (Embalse)

1×CANDU-6

Romania (Cernavoda) 2×CANDU-6
China (Qinshan)

2×CANDU-6

India

3×prototype

Pakistan

1×prototype

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CANDU REACTOR CIRCUITS

8CANDU PHTS CONDITIONS•D2O with 3-10 cc/kg dissolved D2 (H2 gas added);•Temperature 265-310°C;•pH (apparent) 10.2-10.8 (lithium);•

Steam quality at core outlet → 6%;•Dissolved O2 < 5 ppb (unmeasurable – although coolant
may be oxidising at core outlet).

9MAIN PHTS MATERIALS•
•
•
•
•
•

Fuel sheaths – Zircaloy 4
Pressure tubes – Zr-2½%Nb
SG tubing (CANDU 6) – Alloy 800
SG heads – carbon steel
Feeders – carbon steel
Headers – carbon steel

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CANDU REACTOR FACE11

In 1980s, some CANDUs experienced primary systemfouling (reactor inlet temperature rise); e.g. Point Lepreau
to May 1989

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FEEDER THINNING

In 1996, ultrasonic measurements of wall thickness ofcarbon-steel outlet feeders at Point Lepreau indicatedexcessive corrosion: average rates inferred from estimatesof initial thickness.

13THINNING RELATED TO COOLANT FLOW –FAC (FLOW-ACCELERATED CORROSION)

FAC α V1.5

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In 1997, an outlet feeder at Point Lepreau developed a leak– removed and inspected.
Through-wall crack, now thought to be low-temperature
creep, possibly exacerbated by hydrogen (deuterium)
permeating through metal from FAC.
Several cracked feeders found since – one or two throughwall.
Only at Point Lepreau.

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INSIDE SURFACE OF CRACKED FEEDER•cracks intergranular•

medium-to-high-flowchannels•

areas of high residual stress(many tight bends close toreactor face – “warm bent”)

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END-FITTINGS AND FEEDERS

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DETAILS OF REMOVED FEEDER INDICATE FAC
Scallops

Effects of Cr content of steel

Low-Cr steel a consequence of specifying low-Co
material to control activity transport.

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•All pre-Qinshan CANDUs display FAC: managed by stress analysis,
dispositioning, removal and replacement if necessary;•Darlington plant particularly susceptible (has thinner-wall feeders);•Following a suggestion and early work at UNB Nuclear and later
qualification studies, Darlington injected TiO2 as possible inhibitor intoone channel at Unit 3 in 2004 – observed a fall in FAC of > 25%:
promising;•Still too many uncertainties to proceed with Ti;•Qinshan expected to have low FAC rates because 0.3% Cr steel
specified.
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FAC EXPERIMENTS AT UNB
Developed probe for on-line measurement of FAC in hightemperature water loop:

Increase in electrical resistance of thinning tube reflects FAC –“resistance probe”.
Probes made from Point Lepreau steel: SA106 Grade B (0.019%Cr)

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Also developed wire probe – on-line measurement ofresistance of thinning wire reflects FAC.
Experiments performed over range of conditions:• flow rate;• pH (LiOH);• [Fe] saturation.

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Scatter of results high …..

These results and plant data suggest shear stress important (flowdependence high).
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TENTATIVE CORRELATION DERIVED:

CR = 7.65 (1 + 0.111 τ0.75) × (1 + 1.51×10-9 e1.87pH)•Predicts reducing pH from 10.6 to 10.2 will decrease FAC rateby only ~ 12%; differences among plants indicate no consistenttrend;•Predicts flow-rate dependence similar to plant.
N.B. Experiments with coolant nominally saturated in [Fe] still
produced FAC – if at a lower rate
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FAC processes at M-O produce H2
Fe + 2H2O → Fe(OH)2 + 2H ……………(1)3Fe(OH)2 → Fe3O4 + 2H2O + H2 ………..(2)Molecular hydrogen from (2) diffuses to bulk coolant.
Atomic hydrogen from (1) effuses quantitatively through metal– measurement reflects FAC.

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“HYDROGEN EFFUSION PROBE” - HePro®
Collection cup clamped to outside of pipe – evacuated periodically –increase in pressure monitored.

Diagram of HePro®
mounted on pipeTypical pressure
increase

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INDUSTRIAL APPLICATIONS

Feeder at Point
Lepreau

Water wall at
Coleson-Cove oilfired plant

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THERMALHYDRAULIC STUDY AT UNB•High velocity dependence of FAC suggests shear stress
distribution important;•Full-scale mock-up of feeder and end-fitting constructed fromtransparent acrylic;•

Mounted in high-flow water circuit, two-phase (air-water) ∆P
and phase distribution studied at ~ 25°C;•Computer simulations with cfd code FLUENT tested against
observations.

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Flexible
Hose
ID=54mm
Weir73oHandle

SpacerPlug

R= 95.25 mm
FLOW

FLOW

FLOW

FLOW

1802 mm

Air

Sump
Compressor

Hydraulic loop for testing feeder

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PLANT AND EXPERIMENTAL CONDITIONSCANDU-6

experimentcoolant

D2O

H2O

temperature (°C)

31025chemistry (pH25°C)

10.2 (Li)neutralvoidage

0~0.3 (steam)

0~0.5 (air)Reliq~4.3x1065x10529

PREDICTABLE FLOW PATTERN IN BEND

Flow

0.05 voidage, Reliq ~ 4.5×10530

UNPREDICTABLE FLOW PATTERN IN BEND

Flow

Liquid film

0.2 voidage, Reliq ~ 4.2×10531

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FLUENT FAILS TO PREDICT PHASEDISTRIBUTION IN BEND
1.0

Air

Test section mid-plane

0.0

Recirculating flows influenced by
upstream end-fitting
0.2 voidage, Reliq ~ 4.2×105
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SCALLOPING•

Effect of surface sculpting as “scallops” still unknown;•

Research at UNB Nuclear indicates scallops cannot be treated asconventional “sand-grain roughness” (i.e., influencing ∆P via
Moody relations);•Scallop shapes create different ∆P in forward and reverse flow;•Experiments under feedwater conditions (FAC at 140°C inneutral water) indicate constant “scallop Reynolds number”.

34PROCESSES TO BE MODELLED35

MODEL REFLECTS SYNERGY BETWEENFILM DISSOLUTION AND SPALLING••••••

Electrochemical processes affected by potential – ECP computed in parallel;
Diffusing Fe2+ predicted to be unchanged Fe(OH)2 – straightforward Fick’s
Law applied;
Rate constant for magnetite dissolution from literature (one reference)
SHOULD CONTROL (<<m.t.c.);Same rate constant assumed for precipitation (no reference);
Erosion treated stochastically – random-number generator used to decidesize of particle to spall from size distribution;
Time for particle to be loosened and removed;θ∝•dτ ⋅ Q i k d ΔC

When particle disappears, film thins instantaneously by d and corrosion ratejumps.
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MODEL PREDICTS (using CFD code to calculatelocal shear stress)• corrosion rates at different positions in feeders;• ECP ~ -700 to -900 mV (vs. SHE);• oxide film thicknesses (0.7 μm to 3 μm);
• growth of oxide to steady-state;• corrosion and oxide film thickness of inlet feeders(saturated in dissolved iron, lower temperature).
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PREDICTED OXIDE FILM THICKNESSES INOUTLET FEEDERS38

PREDICTED CORROSION RATE AND OXIDEFILM THICKNESS FOR INLET FEEDER:39

MODEL FITS PLANT DATA WELL

FAC α V1.540

ACKNOWLEDGEMENT
Many people at UNB Nuclear, including students passing throughand graduating, have contributed to the findings described in thistalk; my thanks are due to them.
The Centre for Nuclear Energy Research (CNER) at UNB is
thanked for ongoing collaboration.
Finally, I am grateful to the CANDU Owners Group (COG) and its
partners and the Natural Science and Engineering ResearchCouncil (NSERC) of Canada for continuing support for UNB
Nuclear.

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