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531

Water Chemistry Measures to Improve

Steam Generator Performance

S. Odar

Framatome-ANP GmbH, GermanyE-mail: [email protected]

Abstract:

World wide the majority of unscheduled plant shut-downs induced by steam generator failures are

caused by corrosion problems. Such steam generator degradation problems arise from the continuous

ingress of non-volatile contaminants, i.e. corrosion products and salt impurities accumulated in the

steam generators. These impurities have their origin in the secondary side systems. The corrosion

products generally accumulate in the steam generators and form deposits not only in the flow restricted

areas, such as on top of tube sheet and tube support plates, but also build scales on the steam generator

heating tubes. The concentration of salt impurities within these deposits at top of tube sheet and in the

tube to tube-support-plate crevices can induce a variety of corrosion mechanisms on steam generator

tubes. In addition, the tube scales in general affects the steam generator thermal performance, which

ultimately cause a reduction of power output. The most effective ways of counteracting all these

degradation problems, and thus of improving the steam generator performance is to keep them in clean

conditions.

This paper presents the water chemistry issues to improve the steam generator corrosion and thermal

performance. In particular the subjects of operating water chemistry, cleanliness criteria for steam

generators and steam generator cleaning results with this respect will be discussed.

Introduction:

Steam generators (SG) of Pressurized Water

Reactors (PWR) are key components. Their

reliability affects greatly the overall plant

performance and availability. World-wide

experience shows that significant number of

operating PWR’s have now corrosion or

mechanical degradation problems in their SG’s

[1]. These SG problems often force unscheduled

or extended outages for preventive and corrective

maintenance, which are costly in terms of repair

work, loss of power and personnel radiationexposure.

In addition to SG corrosion problems, which

are experienced mainly on the secondary side,

deterioration of SG thermal performance is also

experienced in most of the plants, which is

caused by growing SG tube scales affecting the

primary to secondary side heat transfer. Based on

field experience, corrosion product fouling in

SGs has been identified as a major cause of heat

transfer degradation in PWR plants, with the

power output at some plants being decreased to

as low as 80% of full power. The origin of these

tube scales is again the corrosion products

generated in the secondary side of the plant and

transported into SGs.

All these secondary side SG problems can be

minimized or even be prevented by adopting

suitable design and secondary side water

chemistry operation concepts. The main

prerequisite is the careful adaptation of the water

chemistry to the existing the SG design and the

structural materials used. In the following

chapters the water chemistry measures on thesecondary side of the PWR plants to achieve this

goal will be described and the experienced

results at field with respect to improvement of

the SG performance will be presented and

discussed.

Secondary side SG tube corrosion:

Secondary side SG tube corrosion is caused in

all cases due to the presence of corrosion

product deposits adherent on the SG tubes and

of the concentrated impurities within these

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deposits. Non-volatile impurities, which are

always present in trace quantities in the

feedwater, enter into the SGs with the feedwater

and concentrate within the deposits on the tubesheet, in the tube support crevices and on the

tube surfaces. The concentration mechanism is

illustrated in Figure 1.

Figure 1: Impurity concentration mechanism

The outer surface and upper part of these

deposits are wetted by SG water containing trace

amount of impurities. Accordingly impurities

penetrate in those areas. The lower part of the

deposits is usually dry due to heating by primary

side. Between wet and dry zones exists an

alternate wetting and drying zone, where the

water starts to evaporate and release this area as

steam. In those alternate wetting and drying

areas salt concentrations can reach corrosive

levels and compositions. Depending on the

prevailing beneath deposit local environmental

conditions, i.e. pH and redox potential, various

corrosion mechanisms, like Secondary side

Stress Corrosion Cracking (OD-SCC), Inter

Granular Attack (IGA), denting, pitting andwastage can occur (see Figure 2).

Water chemistry measures:

These corrosion problems can be counteracted

by keeping the SGs impurity clean (especially

the crevice areas at top of tube sheet and in tube

supports). For these purpose world wide effort

was done to implement improved water

treatment systems to achieve so far technical

possible impurity free condensate, feed and

make-up water in the plants. The impurity

control was applied by stringent water chemistry

specifications. In addition, world wide different

water chemistry modifications or changes were

performed: For example, In USA and Japan,water chemistry modifications like molar ration

control and boric acid treatment

Figure 2: SG tube corrosion as a function of

pH and redox potential [2,3]

were developed and implemented in the field tomitigate the SG tubing corrosion [3]. The aim of

these SG chemistry modifications is to control

the crevice chemistry at a neutral pH range for a

satisfactory environmental conditions with

respect to tube corrosion (see Figure 2).

Unfortunately, even based on some promising

laboratory data these modifications were claimed

to be helpful, they were not sufficient to stop or

to remarkably mitigate the SG tube corrosion

and still numerous SG tubes had to be repaired

(plugging and/or sleeving). For example, loss of

capacity factor attributed to SG corrosion

problems in US PWRs was with decreasingtendency in the range of 2-8 % for the years

1980 through 2000 causing billion of dollars

cost for the nuclear utility industry [4].

In contrast to USA and Japanese approach

another water chemistry strategy was selected in

Germany, which aims to prevent or suppress the

impurity concentration mechanism beneath SG

deposits by minimizing the corrosion product

deposition in the SGs. Those corrosion products

have their origin in the secondary side of the

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plant, where they are produced mainly in two

phase flow steam systems by erosion corrosion of

the carbon steels. To counteract the erosion

corrosion “High AVT” chemistry (All VolatileTreatment with high pH values ) for the entire

secondary side including the SGs was developed

and implemented at the beginning of the 1980s in

the plants. This water chemistry considers the

injection of only hydrazine to maintain the

reducing conditions in SGs, whereas the high pH

values, necessary to counteract the erosion

corrosion is adjusted by ammonia (see Figure 3),

which is produced by thermal decomposition of

hydrazine [5 - 10]. This new water chemistry was

Figure 3: Carbon steel metal release by erosion

corrosion as a function of pH values

very successful to prevent SG tube corrosion. In

all old German PWRs operating under low pH

conditions, where end of seventies with PO4

chemistry wastage corrosion was experienced intheir SGs, the water chemistry is converted to this

new chemistry after replacing copper bearing

materials in their condensers. The conversion to

high pH resulted in a drastically reduction of feed

water iron concentrations. Within a short time of

operation feed water iron concentrations in the

range of ~ 1 ppb or less was achieved (see Figure

4 and Figure 5). This lead to drastically reduction

of corrosion product transport into SGs. The new

PWRs starting their commercial operation in

early of eighties and after, applied High AVT

chemistry from beginning on. All German PWRs

now operating since last 25 years with High

AVT without any corrosion problems in their

SGs. Since implementing this high pH chemistryTube Sheet Lancing (TSL) is no more performed

Figure 4: Feed water iron concentration as a

function of pH in Plant E

8,5

9

9,5

10

10,5

Jan 76 Sep 78 Jun 81 Mrz 84 Dez 86 Sep 89 Jun 92 Mrz 95 Nov 97 Aug 00 Mai 03

p H - v a l u e

0

5

10

15

20

F e [ p p b ]

pH(NH3)

Fe

Figure 5: Feed water iron concentration as a

function of pH in Plant H

9

9,2

9,4

9,6

9,8

10

10,2

Jan 79 Sep 81 Jun 84 Mrz 87 Dez 89 Sep 92 Jun 95 Mrz 98 Nov 00 Aug 03 Mai 06

p

H - v a l u e

0

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10

15

20

F e [ µ g / k g ]

pH(NH3)

Fe

every year, but in average each three years.; and

the removed amount of deposits range 3-4 kg per

SG by each TSL cleanings. There are even some

plants, didn’t see the necessity of applying TSL

after 6-7 years of operation.. Annual endoscopic

visual inspections confirm the cleanness of the

SGs.

The reduction of feed water iron concentration

improved also the SG thermal performance

significantly. During the operation period with

low feed water pH values having higher iron

transport rates, remarkable steady increase of SG

tube fouling was experienced in all plants. This

was drastically reduced to almost no fouling

growth after introducing high pH and in new

plants with high pH from beginning on, it was

insignificant (see Figure 6).

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Figure 6: Influence of feed water pH on SG tube

fouling

The German field experience with high pH

operation verified the important influence of the

secondary side on SGs and confirmed that if SG

corrosion problems need to be prevented or

arrested by control of water chemistry, this needs

to be optimized for the entire secondary side, and

not just in the SGs. Today this concept of

reducing the corrosion product transport into SGs

is worldwide well accepted water chemistry

strategy to prevent SG corrosion problems. Since

mid of nineties in other countries water chemistry

modifications are ongoing by use of so called

advanced amines (Morpholine, ETA, MPA,

DMA, 5-AP) with the aim to reduce the feed

water iron concentrations [11]. Published field

experience indicating promising results [12, 13].

Steam Generator Cleaning Criteria:

Since the generation of corrosion products in

the secondary side and as a consequence of this

feed water corrosion product transport into SGs

cannot be completely eliminated, with the plant

operating time the amount of deposits in SG will

increase. This leads later to the necessity of

corrosion product removal from SG for damage

prevention. i.e. for improvement of SG corrosion

and thermal performance. With this regard many

plant operators are asking if they need the

cleaning and when they need it. To answer thesequestions, several operating chemistry parameters

can be used. The change in SG performance

usually results in a change of some SG operating

chemistry parameters, which are indicative of the

increased SG deposits:

One of the main chemistry parameters which

can be used for this purpose is the change of SG

and feed water hydrazine concentrations. The

hydrazine concentration in SG is usually higher

than in feed water due to concentration increase

by evaporation; but is limited because of

volatility and thermal decomposition (SG

temperature is higher than feed water

temperature). Since the thermal decomposition

of hydrazine is catalyzed by deposits, thedecomposition rate will increase with increasing

deposit amount. In other words, the hydrazine

concentration ratio of SG to feed water, is

expected to be decreasing with operating time of

the plant. This is exactly what is experienced in

many PWRs. In Figure 7 the behavior of

hydrazine concentration ratio is given as a

function of 12 years of operating time for one

PWRs as a typical example. The purpose of

using hydrazine is to maintain reducing

conditions in SGs. Due to the increase of thermal

decomposition of hydrazine in SG and limitation

of feed water hydrazine concentration (becauseof environmental requirements), SG hydrazine

concentration decreases with a operating time. In

general, this, can lead to a situation where SG

hydrazine concentration needed to maintain the

reducing conditions in SGs can no more

ensured; and accordingly a removal of the

deposits may be necessary. For prediction this

cleaning time the slope of SG/FW hydrazine

concentration ratio decrease can be used. This

easy evaluation using routine operating

chemistry data, can help plant operators to

establish SG cleaning strategy in advance and to

perform performance oriented SG maintenance.Further examples of the thermal decomposition

of hydrazine in SGs is given in Figure 8.

Figure 7: Typical evolution of hydrazine

concentration ratio SG/FW in a PWR

The use of hydrazine decomposition behavior

for decisions regarding SG cleaning can be

supported by evaluation of the SG hide out

behavior during plant shut downs or during out

of specification conditions, for example, due to

condenser leaks:

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.Figure 8: Hydrazine decomposition examples

0

0,5

1

1,5

2

2,5

3

Jan 80 Dez 81 Dez 83 Dez 85 Dez 87 Dez 89 Dez 91 Dez 93 Dez 95 Dez 97 Dez 99 Dez 01 Dez 03

N 2 H 4 C o n c e n t r a t i o n R a t i o

Plant G

Plant I

Plant K

Plant M

Plant P

In the SGs the concentration of impurities are

controlled by feed water impurity concentration,

SG blow down rate, steam carryover and finally

hide-out (absorption of impurities in the SGdeposits). The contribution of steam carry over is

very small and can usually be neglected. If the

impurity concentration is controlled mainly by

SG blow down rate, i.e. impurity input is same or

similar to impurity out-put, there is no need for

concern with respect to SG corrosion problems.

Because the impurities are not remaining or

concentrating in SGs. This can well be checked

by impurity mass balance calculations during

condenser leaks as indicated in an example given

in Figure 9.

Figure 9: SG Na concentration evolution during

a condenser leak in one PWR

Hide-out is the only mechanism in SGs, whichmay lead to impurity concentration up to

corrosive levels; and therefore needs to be

monitored and evaluated carefully. Increase of

deposit levels increases the SG hide-out behavior.

When hide-out starts to be dominant for impurity

control in SGs, it results in a high corrosion risk.

This situation of SG hide-out behavior can be

monitored either by hide-out return tests during

plant shut downs (release of impurities hidden-

out during operation) or by hide-out tests

(injection of Na-24 tracer into feed water).

During these tests, the constant SG Na-24

concentration (corrected by steam carry over,

blow down and natural decay) is indicative for

an insignificant SG hide-out behavior. If Na-24concentration decreases remarkably, this is due

to significant hide-out. In Figure 10 example of

SG with significant hide-out is compared with

the one without remarkable hide-out behavior.

Figure 10: SGs with significant and negligible

hide-out behavior

The hide-out behavior of SGs changes also

with the operating time; i.e. hide-out increases

with increasing deposit loading in the SGs, as

demonstrated by hide-out test performed two

years later in the same SG without hide-out

shown in Figure 10 (compare Figure 11). The

hide-out increase rate can also be used for

decision making when the SG cleaning should

be scheduled.

Figure 11: SG hide-out increase with operating

time confirmed by hide-out tests

Finally, in addition to chemistry parameters,

several outage inspections results can also be

used as criteria for making a decision on SG

chemical cleaning:

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31.08.96 00:00 05.09.96 00:00 10.09.96 00:00 15.09.96 00:00 20.09.96 00:00 25.09.96 00:00

N a [ g / h ]

0

100

200

300

400

500

600

700

800

900

1000

P o w e r ( M w e )

Na input

Na output

Power

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Heavy hard deposits are the locations for

impurity concentration to corrosive levels. This

can be confirmed by visual inspections: Different

colors of deposits are direct indication for saltconcentrations (see Figure 12). If the color of the

deposit are uniform black, in such cases only

chemical analysis can help to assess their risk for

tube corrosion.

Figure 12: Colored SG deposits

Finally, increase of tube scales (fouling) may

deteriorate the primary- to secondary-side heat

transfer, resulting in a steam pressure decrease.

Monitoring of pressure decrease is the best tool

to assess the SG thermal performance.

For decision regarding establishing SG cleaningstrategy and applying performance oriented SG

maintenance, all these data evaluation, as

described above, should be used. Table 1

summarizes these SG cleanness criteria used by

Framatome-ANP in Germany for SG chemical

cleaning. Such data evaluation results and criteria

were used to make decisions for SG chemical

cleaning in German NPPs.

Table 1: “Yes and No” decision criteria for SG

cleanness

Parameter Yes No

N2H4 ratio << R > R

Out of Spec.Conditions

Impurity controlby HO

Impurity controlby Blow down

Hide-Out Significant Minor ?

HO-Return Acidic / Caustic Neutral

Tube Fouling Power loss Insignificant

Visual Inspection Colors Black

TSL Hard sludge No hard sludge

Tube scalethickness

High growthrates

Low growth

rates

R: N2H4 SG/FW ratio value for plant specific SG reducing

conditions

Steam generator chemical cleaning:

Since mid eighties, SG chemical cleanings are

performed numerous times for improvement of

SG performance with respect to corrosion or thermal efficiency [14]. As of today two

different main SG chemical cleaning

technologies are used. The one is low

temperature non proprietary EPRI SGOG

process applied at 95-120°C mainly in USA. The

another is the high temperature FANP

proprietary SGCC process, which is applied in

more than half of all chemical cleanings world

wide. This high temperature process is applied

under hot standby conditions at 160-175°C

during plant shut down or start-up operations.

Most of these applications were in preventive

nature but some were also active to arrest the SGcorrosion. The process description is published

numerous times in the literature some examples

are given in [15, 16]. In the following several

application results with respect to improvement

of SG performance are presented.

As discussed in previous chapter, deposits in SG

increase the thermal decomposition rate of

hydrazine. Accordingly with increasing deposits

in the SG, the hydrazine concentration ratio SG

to feed water decreases. This hydrazine ratio

value, as one of the SG cleanliness criteria, was

improved after all SG chemical cleanings exceptin one case (see Figure 13). In this case (KKS:

NPP Stade,), SG chemical cleaning was applied

in SGs only partially, excluding the U-bend area.

Figure 13: Influence of SG chemical cleaning

on N2H4 SG/Feed Water ratio

0

0,5

1

1,5

2

2,5

N 2 H 4 S G / F W R a t i o

KKU KKS KWG KBR GKN-1

N2H4 ratio (precleaning)

N2H4 ratio (postcleaning)

Almost in all applications main steam pressure

increase was reported by plant operators after

SG chemical cleanings (see Table 2). These

steam pressure increases have corresponded to

~ 0.5 to 1.6 % heat transfer tube surface gains.

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Table 2: SG thermal performance improvements

by chemical cleanings

Plants Steam pressure

increase[bar]

Other observations

Unterweser ~ 0.45 ฬ 1.7% heat transfer surface gain

Stade ~ 1.22 ฬ 4.6% heat transfer surface gain

Atucha ~ 0.85 ฬ 4.3% heat transfer surface gain

Neckarwestheim-1 ~ 1.6 Simultaneous ~ 0.5°C coolant temp.decrease

Almaraz-1 ~ 0.5 In spite of ~ 500 tubes plugged

Point Lepreau Data notavailable

3.6°C coolant temperaturedecreased

Paks-1 Data notavailable

Complete fouling recovery

Several detailed results of thermal performance

improvements are given for the application of NPP Stade (see Figure 14) and Neckarwestheim

Unit 1 cleaning (see Figure 15) as examples.

Both plants were cleaned twice chemically;

where as the first applications were in both plants

only partial cleanings. After partial cleanings

fouling improvement was not significant, this

was experienced only in Neckarwestheim Unit 1

after complete SG cleaning.

Figure 14: Pre- and post cleaning SG fouling

behavior in NPP Stade

Figure 15: Pre- and post cleaning SG fouling

behavior in NPP Neckarwestheim-1

-600

0

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1200

1800

2400

3000

3600

4200

Jan 76 Jun 81 Dez 86 Jun 92 Nov 97 Mai 03

A c c u m . I r o n I n g r e s s i n t o S G [ k g ]

-1

-0,5

0

0,5

1

1,5

2

2,5

3

F o ul i n gF a c t or [ 1 0 ^ - 5 m² * K / W ]

accum. Fe ingress

Fouling Factor

Chemical

SG Cleaning

2002

ChemicalCleaning SG10

In Neckarwestheim unit 1 cleaning case in

addition to main steam pressure increase, a

simultaneous coolant temperature decrease of

0.5°C was also experienced (see Figure 16).

Figure 16: Thermal performance improvement

by SG chemical cleaning in NPP

Neckarwestheim Unit-1

Not only the thermal but also the corrosion

performance of SGs can be improved by

chemical cleaning, as it was confirmed by field

experience several times world wide. An

example for this is given in Figure 17: Due to

sodium increase into SG by malfunction in the

condensate demineralizer system ODSCC

(secondary side stress corrosion cracking) was

experienced in one PWR in 1988, which made

necessary to plug 371 tubes [17]. SG chemical

cleaning using FANP high temperature process

could arrest ODSCC in the following operatingcycles. The number of plugged tubes was

reduced in the following outages till it was zero

in three years after the sodium incident.

Figure 17: Improvement of SG corrosion

performance by chemical cleaning

0

47

2 0

371

0

50

100

150

200

250

300

350

400

450

1987 1988 1989 1990 1991

Year

P l u g g e d T u b e s

Plugged tubes

Cumulative

The field experience with chemical cleanings

confirmed the feasibility of improving the SG

corrosion performance; but also showed that if

the corrosion is wide progressed the efficiency

of the process will be limited (i.e. deep cracks

can’t be completely cleaned). Therefore it is

-500

0

500

1000

1500

2000

00 02 04 06 08 10 12 14 16 18 20 22 24 26

t[a]

m F e ; S G

[ k g ]

-2

0

2

4

6

8

R f [ 1 0

- 5 m² * K / W ]

ƁChemical Cleaning

(Partially)

ƁChemical Cleaning

mFe,SG

Rf

Phosphate treatmentwith low pH

AVTpH < 9.5

AVTpH = 9.6

56

57

58

59

60

61

62

11:00 19:00 03:00 11:00 19:00 03:00 11:00 19:00 03:00 11:00

S t e a m P

r e s s u r e ( b a r )

302

303

304

305

306

307

308

C o o l a n t t e m p e r a t u r e ( ° C )

Steam Pressure (bar) Coolant temperature (° C)

23.07.200221.03.2002

BEFORE

SG Chemical

Cleaning

AFTER

SG Chemical

Cleaning

SGCC

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538

recommended not to delay the cleanings once the

corrosion starts; better to perform cleaning as

preventive using plant specific SG cleanliness

criteria, before corrosion starts.

Conclusions:

Secondary side chemistry control programs and

measures are essential for good SG performance:

x Adequate chemistry program helps to reduce

feed water iron concentration significantly,

which leads to improvement of SG thermal

and corrosion performance.

x Operating chemistry data are indispensable to

evaluate SG performance and can support to

schedule SG performance oriented

maintenance activities.x Finally poor SG performance can be

improved significantly by chemical cleanings.

Reference:

[1] EPRI Steam Generator Progress Report,

Rev 15

[2] Source: EPRI IGA/SCC Work shops

[3] “SG degradation: Current mitigation

strategies for controlling corrosion”,

P. Millet, CNRA/CSNI Workshop on SG

tube integrity in nuclear power plants, Oct

30 – Nov. 02, 1995, Argon NationalLaboratory, USA

[4] EPRI Steam Generator Progress Report

Rev. 15, 2000

[5] “Stages of Development of Secondary

Water Chemistry in Pressurized Water

Reactors”,

A. Dörr, S. Odar, P. Schub, VGB

Kraftwerkstechnik 66, No. 11 Nov. 1986

[6] “The secondary side approach to preventing

corrosion”,

R. Riess, S. Odar, Nuclear Engineering

International, January 1991, page 33-38

[7] 2Strategies for High Steam Generator

Performance”,

R. Bouecke, S. Odar, International Power

Generation Conference, October 6 - 10,

1991. -San Diego, Ca.

[8] “Secondary Side Chemistry Operational

Experience at Trillo Unit 1”,

K. Streit, F. Yague, SNE Conference,

October 28-30, 1992, Jerez de la Frontera,

Spain, P. 108-110.

[9] “10 Years of Field Experience with High

AVT Water Chemistry”,

K. Streit, S. Odar, Steam Generator and

Heat Exchanger Conference, Toronto 1994[10] “Experience gained with the steam

generator tubing material Incoloy 800”,

S. Odar, G. Hoch, R. Kilian, Conference on

Interaction of Non-Iron-Based Materials

With Water and Steam, Piacenza, June

1996

[11] “The use of advanced amines in US

PWRs”, P.J. Millett, G.D. Bruns, G.E.

Brobst, Proceedings of International

Conference on Chemistry in Water

Reactors: Operating Experience and New

Developments, Nice, France, SFEN, April1994

[12] “Ethanolamine experience at Koeberg

Nuclear Power Station, South Africa”, K.J.

Galt, N.B. Caris, International Conference

on Water Chemistry in Nuclear Reactors

systems, FNES, April 2002, Avignon,

France

[13] “Strategic elements of steam cycle

chemistry control practices at TXU’s

Comanche Peak Steam Electric Station”, B.

Fellers, J. Stevens, G. Nichols,

International Conference on Water Chemistry in Nuclear Reactors systems,

FNES, April 2002, Avignon, France

[14] “Utility experience with steam generator

chemical cleaning”, EPRI TR-104553,

Project S523-03, Final report, Dec. 1994

[15] “KWU’s High-Temperature Chemical

Cleaning Process Application in German

PWRs to Improve Steam Generator

Performance”, G. Jacobi, B. Markgraf,

H.R. Sauer, K. Seidelmann, S. Odar, Water

Chemistry of Nuclear Reactor Systems, 8th

BNES, London 2000.

[16] “Chinon B1 SG Chemical Cleaning -

Application of Framatome ANP Inhibitor-

Free High Temperature Chemical Cleaning

Process”. To be published in International

Conference Water Chemistry of Nuclear

Reactor systems, EPRI, San Francisco,

USA, Oct. 2004

[17] EPRI Steam Generator Progress Report

Rev. 13, EPRI TR-106365-R13, Oct. 1997

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