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Materials and Corrosion Product
Behaviour Under CANDU-SCWR
Conditions
by
William Cook
University of New Brunswick
Presentation at the Workshop of the Canadian
National Committee of IAPWS
Toronto, May 12, 2009
• The supercritical water reactor is a logical evolution
of nuclear reactor technology.
• The benefits of operating a reactor with a
supercritical cycle include:
• Improved efficiency (up to ~ 48%)
• Simpler cycle:
•
•
•
•
More compact core;
Smaller plant footprint;
Less fuel/used-fuel inventory;
More cost effective.
• Potential for co-production of hydrogen.
CANDU-SCWR Design Parameters
25 MPa
• Material selection will be the limiting factor in the design
and operation of the SCWR.
• To date, current reactor materials all pose problems for
use in a SCWR.
• Zr cannot be used due to excessive oxidation under
supercritical conditions. May be used as the pressure
tube, with a suitable insulator (ZrO2).
• Alternate core materials (fuel cladding) will
undoubtedly be required.
• Austenitic stainless steels and nickel alloys are candidates
but would suffer from SCC, IASCC and He-embrittlement (in
the case of Ni-alloys).
• F/M alloys are candidates but suffer from high oxidation
rates.
• The current knowledge about materials’ behaviour in
SCW typically indicate the following generic
statements:
• “materials that do not corrode excessively, crack.”
• “materials that do not crack, corrode excessively.”
• New alloys or modified surfaces of currently available
alloys are potential solutions:
• F/M steels exhibit good SCC, creep and neutron
activation characteristics.
• Possibility of modifying F/M alloy surface to
improve corrosion resistance:
• Surface alloying (increasing Cr content)
• Corrosion-resistant coatings (ZrO2 ?)
• Oxide dispersion strengthened (ODS) alloys show
particular promise for use in-core and elsewhere.
• Based on a F/M steel substrate;
• Yttria (Y2O3) particles dispersed through metal
matrix through mechanical alloying. Increases
high-temperature strength and lowers hightemperature creep.
• MA956 chosen as base material for tests in SCW
• Alloys with various chromium concentrations (9,
12, 14 & 25%) have been cast at CANMET-MTL;
• Exposure in SCW in UNB’s test loop is under way.
• Regardless of material chosen, corrosion products
will inevitably be injected into the SCW coolant.
• Poses a much greater risk for material or activity
transport throughout the entire SCWR system due to:
• No boiling in the core – corrosion products carried
with bulk fluid;
• Potential for large crud deposits in core and
throughout the power cycle – drastic changes in
fluid properties, particularly density and dielectric
constant.
• Estimation of the CP transport and deposits expected
through the SCWR cycle require knowledge of many
factors including:
• Rate of corrosion product input to the coolant
(essentially a function of the corrosion rate of the
materials of construction);
• Rate of dissolution or deposition of a surface
deposit – a function of the bulk coolant corrosion
product concentration, the CP’s solubility and a
kinetic rate constant.
• Mass balances for the corrosion release, transport
and deposition can be formulated as follows:
• Corrosion rate (dmsteel/dt): evaluated from experimental data
of the alloy’s corrosion under given conditions.
• Corrosion film growth: evaluated assuming oxide film
replaces the metal corroded (q is the porosity of the
corrosion film).
din
dmsteel
 0.65
1 q 
dt
dt
• Corrosion
 release: evaluated from material not retained in
the primary corrosion film.
dmsteel
outFesteel  0.22
3.5  q 

dt
10
• A mass balance at the oxide-solution interface controls input to
the bulk coolant and growth of the deposit (Kd = deposition
kinetic constant; hm = mass transfer coefficient; Cbulk,Co/s,Csat =
the bulk coolant and oxide-solution interfacial CP concentration
and solubility):
Co / s 


0.22dmsteel dt 3.5    Kd Csat  hmCbulk
Kd  hm
• Once the corrosion-product concentration in the oxide-solution

interface
is know, deposit growth (o) is calculated as:
do
 Kd Co / s  Csat
dt


11

• Obviously, many of the parameters required are
unknown or have very limited data.
• The solubility of the primary corrosion product,
(magnetite – Fe3O4) will be a key in predicting the
overall deposition rates.
• Estimations of its solubility in SCW can be made
through limited experimental data or through
thermodynamic calculations.
• For example, consider the dissolution of magnetite to
produce ferrous hydroxide:
 
Fe3O4  2H2O  H2  3Fe OH
2 aq 
• Through the reaction’s equilibrium constant, we can
estimate the activity (concentration) of the dissolved

species:
Grxn
 ln Keq
RT
 
or

aFe (OH) 2
( aq)
a3

Fe (OH) 2 ( aq)

 ln 2
 a a

w
H2


 3 Keq aw2 aH 2
• The Gibbs energy of reaction is simply calculated as the energy
of formations of the reaction’s products minus reactants.
• The Gf’s are evaluated at the desired temperature using the
Helgeson, Kirkham and Flowers (HKF) extrapolation model.
• As a starting point, the standard state thermodynamic data
used was as presented in Beverskog’s iron Pourbaix work
(1996).
• The calculation was carried out for magnetite dissolution
accounting for:
• Ferrous and ferric hydrolysis species;
• LiOH or NH3 used for pH adjustment – for the latter, no
account has been taken for possible iron-ammonia
complexes.
• Ensuring electroneutrality of the solution is maintained.
• It’s interesting to note that on ascent to and passing
the critical temperature, all ionic species drop off to
very low concentrations.
• As expected, iron or magnetite solubility in the critical
region will be controlled by the uncharged hydrolysis
species, Fe(OH)2(aq) and Fe(OH)3(aq). Based on this
preliminary thermodynamic assessment, Fe(OH)3(aq)
is predicted to dominate above the critical point.
• Once, all speciation has been accounted for, total
solubility is calculated …
• As seen in the speciation plots, the solubility drops
precipitously at the pseudo-critical point and is
dominated by the uncharged species.
• A rough calculation, assuming the coolant is 1.5 x
oversaturated in corrosion products as it enters the
core yields fuel loadings of up to 19 mg/cm2 after one
year of operation (CR = 1mm/yr; Kd= 0.1 cm/s).
Core position (m)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Temperautre (oC)
350
367
383
400
417
433
450
467
483
500
517
533
550
Csat (g/cm3)
6.30E-09 4.58E-10 2.69E-11 9.07E-14 2.87E-14 2.27E-14 1.89E-14 1.62E-14 1.75E-14 1.77E-14 1.98E-14 2.13E-14 2.30E-14
Cbulk (g/cm3)
9.45E-09 9.31E-09 8.87E-09 8.11E-09 7.95E-09 7.80E-09 7.66E-09 7.53E-09 7.40E-09 7.28E-09 7.18E-09 7.07E-09 6.98E-09
Co/s (g/cm3)
7.97E-09 5.11E-09 5.02E-09 6.04E-09 5.99E-09 5.94E-09 5.90E-09 5.86E-09 5.83E-09 5.81E-09 5.79E-09 5.78E-09 5.77E-09
deposit rate (g/cm3s)
1.67E-10 4.66E-10 4.99E-10 6.04E-10 5.99E-10 5.94E-10 5.90E-10 5.86E-10 5.83E-10 5.81E-10 5.79E-10 5.78E-10 5.77E-10
1 year deposit (mg/cm2)
5.3
14.7
15.7
19.1
18.9
18.7
18.6
18.5
18.4
18.3
18.3
18.2
18.2
• It is important to note that the deposition kinetics will
control bulk coolant concentration and the coolant will only
“drop out” CP’s depending on the kinetics thus allowing
oversaturation to develop around the circuit.
• Significant deposition can also be expected in the steam
lines and create problems in the turbines.
• One possible solution is to use a two-loop configuration as
in conventional CANDUs.
• This has been modelled over 5 years of operation and
shows significant decrease in efficiency as deposits build
up in the steam generator.
•
SG deposit thickness and overall two-loop SCWR efficiency with Kd = 10-2 cm/s
24
Summary
• The CANDU-SCWR will provide a significant
improvement in plant efficiency and overall
economics.
• Materials must be developed that minimize corrosion
rate and corrosion product input to the reactor
coolant.
• Corrosion product transport (and activity transport)
and deposition will be significant issues in a SCWR
cycle.
• Chemistry dosing practices must be evaluated to
minimize corrosion and corrosion product deposition.
Acknowledgements
•
•
•
•
R. Olive (PhD student);
W. Fatoux (MSc student);
NSERC for financial support;
NRCan and AECL for motivation to pursue this work.