1 Corrosion in Pressurized Water Reactors
J. -Ph.' Berge
E l e c t r i c i t 6 d e F r a n c e , Service d e l a Producrion - a u s t e n i t i c stainless s t e e l s (piping and reactor
Thermique, Groupe d e s Laboratoires internals) ;
Carrefour Pleyel - 21, All6e Priv6e
F-93206 Saint-Denis Cedex 01 - h e a t exchanger m a t e r i a l s ( s t e a m g e n e r a t o r s in
particular) ;
- high-strength m a t e r i a l s (bolting, hardfacing
surfaces).
For e a c h group of materials, we shall present
F o r m a n y years now, in response t o t h e multiple e x a m p l e s of corrosion d a m a g e , R & D efforts and
t y p e s of corrosion-induced d a m a g e encountered resultant improvements, a n d remaining problems
in PWR components, a major R ti D e f f o r t has with respect t o plant reliability and safety.
been underway t o d e t e r m i n e t h e best materials,
fabrication processes, and operating conditions Finally, w e shall discuss in a f o u r t h part of this
with mh'ich to combat corrosion phenomena. presentation corrosion products conveyed through
t h e r e a c t o r coolant system - whose consequences
From t h e plant o p e r a t o r ' s viewpoint, corrosion is on operating and m a i n t e n a n c e personnel dose r a t e s
a s o u r c e of costly inspections, repairs, and a r e significant.
outages. Above all, i t poses a potential t h r e a t t o
r e a c t o r s a f e t y , particularly when t h e r e a c t o r
coolant pressure boundary is a f f e c t e d .
STAINLESS STEELS IN REACTOR COOLANT AND
This paper does not address fuel cladding corrosion AUXILIARY SYSTEMS AND REACTOR INTERNALS
problems, covered by Dr. Leistikow, or conventional
island components. Though corrosion d a m a g e has Austenitic stainless s t e e l s - t h e most widely used
been d e t e c t e d in condenser tubes, turbine rotors m a t e r i a l s in t h e s e systems- have experienced r wo
and nozzle rings, g e n e r a t o r binding bands, and t y p e s of problems :
s t e a m / f e e d w a t e r s y s t e m s , t h e s e cases are not
specifically "nuclear" in n a t u r e and are covered - Incidents resulting from a c c i d e n t a l contamination,
by o t h e r groups in t h e European F e d e r a t i o n of particularly by halogenated products. S t r e s s cor-
Corrosion. rosion has been observed in t h e presence of chlo-
rides produced by high- t e m p e r a t u r e decomposi tion
For t h e purposes of t h i s paper, corrosion e f f e c t s of s y n t h e t i c products. T h e solution lies in tighter
will be classified according to t h e t h r e e groups of cleanness control and stringent specifications for
materials affected : solvents, paint, g r e a s e , e t c .
1
- Concentrated boric acid, even at room temperature, water in crevice locations, or t o decomposition of
can attack stainless steel pipe in the presence of trisodium phosphate used for secondary water
oxygen. Welds that are sensitized t o intergranular treatment.
corrosion are particularly susceptible t o this form
of deterioration - Localized chemical attack from acid phosphate
residues (wastage).
A further type of corrosion observed recently and
resulting in mostly transgranular cracks is currently - Buildup of magnetite in tube-to-tube support plate
under investigation in the laboratory. This form of annuli (denting) in cases where phosphate treat-
attack occurs in the presence of concentrated boric ment has been discontinued t o resolve previous
acid at temperatures of 100 t o 300' C, even though problems, giving rise t o concentrations of unbuffered
no other impurities, such as halogenated compound, acidic chloride solutions in crevices.
are present.
- Primary side stress corrosion cracking of the most
This phenomenon has been found to affect stainless widely used material, Inconel Alloy 600 containing
steels, high-nickel alloys and high-strength alloys. 70 % nickel. This phenomenon was reported in 1959
and was a subject of debate for almost 20 years
Further research, however, will be required to before the first serious consequences -cracking in
determine the exact conditions associated with this highly stressed regions of Alloy 600 tubes- were
type of corrosion and t o understand the mechanism observed
involved A relationship, however, has already been
shown t o exist between the combined presence of Most secondary side and support plate corrosion pro-
boric acid and chlorides and t h e occurence of blems can be resolved by stringent waterchemistry
cracking. control.
STEAM GENERATORS AND OTHER HEAT
EXCHANGERS
in t e r m s of the diversity of types of damage, the
amount of applied research that it has generated,
the debate that has surrounded the results obtained
since 1959, and, more recently, its practical reper-
cussions corrosion of steam generator tubes poses
problems on a scale rarely seen in an industrial
context.
-
Alloy 601
C S
.I5O.OU
P
---
SijM
3.5 1.C
ASTM
B 163
Specifi- on aoiiaoa 0.50 1.1
cation 3
for EDF 050
However, w a t e r chemistry is generally not a
Alloy 69' .05 0.015 --- 0.50 0.1 significant factor in primary side cracking of struc-
ASMR turally susceptible Alloy 600. To eliminate this type
Code
CaSe
of degradation and determine the presence or extent
14.84.3 of damage, a number of measures have been
AlloyB)( 10 0.015 --- 1.0 1.
implemented :
ASrM
B 163 - In plants currently under construction, Alloy 600
Type has been replaced by Alloy 690 or the earlier
316 Alloy 800. Advantages and disadvantages of these
Main-
less nd 08 0.03 0.03 0.75 I materials have been discussed elsewhere.
ASTM
-
A 376
-
a Desirable minimum value
- In plants built over the past ten years, the struc-
ture of Alloy 600 was improved and residual
b - Desirable maximum value stresses from bending and straightening operations
reduced by a final vacuum heat treatment for 15
The table above shows the materials of construction hours at about 700°C.
used by various vendors. The mechanical properties,
thermal characteristics and chemical composition of - In operating plants that experienced cracking in
these materials are described elsewhere ( P h Berge - regions subject t o high levels of residual stress,
J.R. Donati - Nuclear Technology - Vol. 5 5 Oct.81). such as small-radius bends and expanded tube-to-
In response t o potential corrosion risks or t o actual tubesheet joints, the former have undergone stress
corrosion problems, new materials and fabrication relieving heat treatment, while the latter have
processes have been adopted and system operating received a prestressing treatment by shot peening.
parameters adjusted accordingly, e.g. through water
chemistry control. A t the same t i m e , new causative Although the large number of cracked tubes in a
agents have been identified single component might seem t o constitute a threat
t o plant safety, examination of tube samples and
- Secondary side stress corrosion cracking, attribu- laboratory tests have shown that cracks in the roll
table typically t o a strongly alkaline environment transition at the upper surface of the tubesheet are
resulting from boiling of slightly contaminated axially oriented I t has also been demonstrated that,
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even in the event of secondary side depressurization, cobalt-60 released into the reactor coolant system,
these cracks will result in primary-to-secondary since most hard facing alloys a r e cobalt-based
leakage before growing t o a size likely t o cause a During pickling or exposure t o borated water, cor-
significant tube failure. rosion can occur in zones depleted in chromium
Analyses are performed to determine exceptional through excessive carburizing when the hard facing
cases, where this "leak before break" criterion was deposited
might not apply because of fabrication anomalies,
and tubes are plugged accordingly as a preventive
measure. CORROSION PRODUCTS
To conclude, it seems probable that, thanks to A considerable research effort is currently being
R & D programs, f u t u r e reactors will be spared devoted t o determining optimum reactor coolant che-
the above corrosion problems, provided that mistry (injection of lithium hydroxide) when the
rigorous water chemistry control is maintained and concentration of boric acid is adjusted t o compensate
that the necessary material and design modifica- for core reactivity variations. A number of measures
tions are introduced. have been considered t o minimize the quantity of
oxides released by system components and t o avoid
As far as our understanding of the various corro- transport of corrosion products, which can become
sion mechanisms is concerned, tests in sodium activated by contact with fuel elements and accumu-
hydroxide solutions and pure w a t e r have demons- late at various locations in the system. These include
trated the significance of temperature and metallur- maintaining constant pH, selection of optimum pH,
gical structure. However, there are variations in and end-of-cycle pH adjustments. Water chemistry
behavior between the different alloys that are not specification must also cocsider the risk of damage
yet understood, and further investigation, particu- t o the fuel cladding, in the event of local boiling,
larly with respect t o grain boundary creep, will be and t o other system components.
required t o explain the effects observed and pre-
dict the behavior of other alloys. An understanding of transfer mechanisms, painsta-
king laboratory analyses, and comparison of detailed
reports from operating plants should indicate the best
BOLTING MATERIALS w a t e r chemistry specifications t o deal with this
problem. In addition, the elimination of cobalt alloys
Various types of bolting material corrosion have and a reduction of average t r a c e values of cobalt in
been observed under service conditions. structural materials should have a significant impact
on corrosion product activation problems.
- Corrosion of valve and seal assembly bolting due
t o boric acid buildup following reactor coolant
leakage.
CONCLUSION
Under these conditions, low alloyed steel bolting
is heavily corroded by boric acid, while stainless Over the 25 years since the first PWR nuclear po-
steels resist generalized corrosion But a compre- wer plants came into service, numerous corrosion
hensive study of the various materials with an problems have been encountered in various
austenitic matrix showed almost all of them t o components. Solutions t o d a t e have relied on an in-
be susceptible t o stress corrosion cracking at 300- creased understanding of the m echani s m s involved,
350' C in a 40 % solution of boric acid. This adherence t o good practice in material selection
phenomenon is very similar t o the stress corrosion and fabrication, and stringent compliance with
cracking of stainless steels in concentrated boric water chemistry specifications. Certain operators
acid discussed above. are still paying now for the failure t o observe these
principles in the past. In this domain, as in many
Here again, the corrosion mechanism is not yet others, numerous problems could have been avoided
fully understood and, in the absence of a reliable by closer collaboration between research laboratories
solution, the condition of bolting materials must and plant constructors or operators.
be monitored
- P r i m a r y fluid stress corrosion cracking o f Alloy
X 750 guide tube support pins and bolts :
Numerous failures due t o this type of corrosion
-predicted by laboratory tests in the 1960s- have
been reported The mechanisms involved appear t o
be similar t o those resulting in cracking of Alloy
600. In this case, too, the solution lay in modi-
fying the metal structure by an appropriate heat
treatment resulting in fine precipitation of chro-
mium carbide at grain boundaries t o improve
corrosion resistance in pure w a t e r and sodium
hydroxide, and in reducing stress levels by modi-
fying part geometry or tightening torque.
HARD FACING ON RUBBING SURFACES (VALVES)
Deterioration of hard facing alloys has also been
observed in some cases. This type of corrosion can
result not only in loss of leaktightness or binding
(e.g. in valves), but also in elevated dose rates from
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