ML20205C287

Row 1 & Row 2 Heat Treatment for Sequoyah Units 1 & 2

ML20205C287
Person / Time
Site:	Sequoyah
Issue date:	01/31/1987
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML19292H001	List: ML20205C247 ML20205C259 ML20205C287
References
WCAP-11379, NUDOCS 8703300151
Download: ML20205C287 (41)

Text

_ ._ - _ __

WESTINGHOUSE CLASS 3 WCAP-11379 i

I

(

R0W 1 AND ROW 2 NEAT TREATMENT FOR SEQUOYAH UNITS 1 AND 2 January, 1ggy I

WESTINGHOUSE ELECTRIC CORPORATION NUCLEAR ENERGY SYSTEMS

. P. D. 80X 355 PITTS8URGH, PA 15230-0355

[0U 000!k E500 j7 0193v 10/011287

I R0W 1 and 2 U-BEND HEAT TREATMENT LICENSING REPORT FOR SEQUOYAH UNITS 1 AND 2

1.0 INTRODUCTION

2.0 U-BEND HEAT TREATMENT OBJECTIVES AND PROCESS DESCRIPTION 2.1 Objectives 2.2 Process Description (General) 2.3 U-bend Heat Treatment Time / Temperature Range 3.0 U-BEND HEAT TREATMENT TOOLING SYSTEM DESCRIPTION AND QUALIFICATION 3.1 Tooling System Description 3.1.1 Channel Head Manipulator 3.1.2 Heater Insertion Mechanism 3.1.3 Temperature Determination System 3.1.4 Heater and Cable Assembly 3.2 Process Controls 3.3 Process Qualification Effort 3.4 Additional Qualification Test Data 3.4.1 Power Cycle Determination 3.4.2 Fiber Optic Pop Up Instrument Correlation 4.0 U-BEND HEAT TREATMENT PROCESS VERIFICATION 4.1 Process Parameter Development 4.1.1 Stress Relief Data Base 4.2 Process Parameter Verification 4.2.1 Preparation and Testing of Row 1 U-bend 4.2.2 Accelerated Test Results for Row 1 U-bend tubes 4.3 Process Parameter Selection and Definition l

l 0193v:10/012287 i

ROW 1 and 2 U-BEND HEAT TREATMENT LICENSING REPORT FOR SEQUOYAH UNITS 1 AND 2 5.0 EDDY CURRENT RESPONSE TO U-BEND HEAT TREATMENT 5.1 Description of Eddy Current Test Program .

5.2 Eddy Current Test Results 5.2.1 Laboratory Tests 5.3 Eddy Current Inspectability Summary 6.0 SAFETY EVALUATION 6.1 Introduction 6.2 Tube Bundle Integrity Evaluation 6.2.1 Steam Generator Tube Integrity Evaluation 6.2.2 Tube Support Plate Analysis 6.3 Primary Side Impact of the U-bend Heat Treatment Process 6.4 Conclusion 0193v:10/012287 ii

1.0 INTRODUCTION

Primary water stress corrosion cracking (PWSCC) of mill annealed Alloy 600 steam generator tubing has been identified as having a potential effect on the operation of steam generators. PWSCC appears to occur in areas of high residual stress of steam generator tubes, such as the U-bend region of small radius tubes and the roll and roll transition zones within the tubesheet. As part of an ongoing steam generator PWSCC preventative measure program. -

Westinghouse has developed a process that provides additional margin against inner diameter (ID) PWSCC that may occur in the U-bend region of some row 1 and row 2 steam generator tubes by reducing residual tensi.le stress at or near the inner surface of the tube through the application of a thermal stress-relief cycle.

I The concept of the U-bend heat treatment process and system design is based on observations to date indicating that PWSCC appears to develop in the U-bend i

regicn of steam generator tubes at the tangent points of the transition between the straight and U-bend sections of the tube and at or near the apex ,

of the U-bend. The latter appears to occur only in cases with substantial ovality or denting. Examination of tubes with leaks in the U-bend region that were removed from operating plants has shown that the leakage occurred at ID initiated, generally short, tight (low leakage) axially oriented cracks thrcugh the tube wall. The examinations indicated that the cracks were initiated and propagated by intergranular stress corrosion. Because no specific contaminant in the reactor coolant has been identified as the corredant, the reactor coolant water itself is assumed to be the corrodant in a manner similar to pure water.

+

The process and tooling. system for thermal stress relief has been developed and; qualified by Westinghouse for application on the Model 51 steam generators at Sequoyah Units 1 and 2. This report presents a discussion of the development and qualification of the U-bend heat treatment program and its beneficial effect of enhancing resistance to PWSCC. More importantly, a 0193v:1 o/012987 1-1

~

safety evaluation is provided where it is demonstrated that the application of the U-bend heat treating PWSCC preventive measure process to the ID of a steam generator tube does not represent a potentially unreviewed safety question as defined in 10 CFR 50.59 (a) (2).

0193v:10/012287 1-2

2.0 U-BEND HEAT TREATMENT OBJECTIVES AND Pil0 CESS DESCRIPTION 2.1 Objectives The objective of the U-bend heat treatment program at Sequoyah Units 1 and 2 is to provide a means of reducing the residual stresses that appear to contribute to PWSCC on the ID of small radius U-bends of steam generator tubing. This is achieved through the reduction of residual tensile stresses at or near the inner surface of the tube by heating the U-bend areas of the Row 1 and 2 tubing. Stress relief from the in situ thermal treatment process has been demonstrated in the laboratory to beneficially reduce primary water initiated stress corrosion cracking in the U-bend region.

One hundred percent (100%) of row 1 and row 2 active steam generator tubes will be thermally treated in the U-bend area of the Sequoyah steam E i generators. Thermal treatment will be accomplished by insertion of a

[ ]a,c.e heater from the tubesheet to the U-hend area and heating at an optimum time and temperature cycle. The heating system is to include the [

la,c.e Process optimization involves rapid tool insertion and l removal, and selection of the most efficient [ la.c.e to achieve the desired stress relief that is beneficial in reducing PWSCC.

2.2 Process Description The objective of the U-bend Heat Treatment Program is to heat a U-bend section extending ['

ja c.e 0192v:1 o/012887 2-1

by " popping up" [

ga.c.e The heat treatment process and tooling system is described in greater detail in Section 3.0 of this report.

2.3 U-Bend Heat Treatment Time / Temperature Range The current U-bend stress relief temperature and time parameters were developed under programs partially funded by the Electric Power Research Institute (EPRI). The emphasis of these programs was on exploring the minimum temperature and time that would provide additional margin of resistance to PWSCC in the steam generator tube (I

]a,c.e Initial studies were done using reverse U-bend tubing sections in 680*F water. This program has been extended to quantify additicnal resistance to PWSCC using prototypically heated and stressed U-bends that have been evaluated in 750*F superheated steam. The continuing thermal stress-relief program shows increased tube integrity of over [

]a,c.e A description and data presentation of test programs that resulted in the establishment of the reference specification parameters is included in Section 4.0 of this report.

otesv.io/ozoss7 2-2

3.0 U-BEND HEAT TREATMENT TOOLING SYSTEM DESCRIPTION AND QUALIFICATION 3.1 Tooling System Description The tooling system description for effecting the heat treatment of the Row 1 and Row 2 U-bends of the Sequoyah Units 1 and 2 Model 51 steam generators consists of a:

~

A. Channel head end effector B. Heater insertion mechanism C. Temperature detection system D. Heater and cable assembly including power supply and control system Briefly, as noted previously, the underlying concept of the U-bend heat treatment program is to provide a heater which is inserted into the Row I and 2 tube.to a known depth (

Ja,c.e This insertion depth positions the heater in a location to effect heat treatment to an area bounded (

ja,c.e i 3.1.1 Channel Head End Effector For the U-bend heat ' treatment of the Sequoyah Units 1 and 2 Row 1 and 2 steam generator tubes, a channel head end effector is [. Ja.c.e positioned under the designated tube prior to insertion of the heater. The system is illustrated in Figure 3-1. [. i l

Ja.c.e Operation of the end effector is controlled frem a point outside the channel head in the Control Area.

0193v:10/012887 3-1 u .

3.1.2 Heater Insertion Mechanism The heater cable assembly is an integrally designed unit with a suitable outer sheath to withstand the insertion loads required by the mechanism. (

ja,c.e

+

ja.c.e 3.1.3 Temperature Determination System

('

ja.c.e

( '.

ja,c.e 0193v:1D/012847 3-2

3.1.4 Heater and Flexible Insertion Conduit Assembly A( la,c e heater is.used as the heat source for the U-bend heat treatment process. [

ja.c.e

(

la,c.e is inert at temperatures reached during the heat treatment process and, most importantly, does not sinter or fuse to itself at those temperatures which would render the heater assembly inflexible and brittle after a single heating cycle.

The structural core of the heater [

ja,c.e ,

A 5/8 inch diameter (i la.c.e tubing is used as the heater insertion conduit. A corhination of braided and solid stainless steel sections at the conduit to heatt:r interface provide a standoff distance of approximately [

, ja.c,e (1

ja.c.a 3.2 Process Controls Row I and Raw 2 heat treatment process control is provided by (-

ja c.e When a tube is heated ( l located within ois3<.t o/ous.a 3-3

the tube, the' tube wall temperature starts to rise. The tube, in turn, loses heat to its surroundings by conduction, convection, and radiation heat transfer mechanisms.' At low temperature, the heat loss from the tube wall is small compared to the heat input from the heater and hence, tube wall

~

temperature continues to increase. At higher temperatures, the heat loss approaches the heat input. Finally, an equilibrium temperature is achieved where the heat input is equal to heat loss. The numerous tests conducted in the laboratory have shown that equilibrium is achieved approximately [

la,c,e after commencement of the heating cycle. ['

ja,c.e This(

]C resultant temperature distribution through the U-bend has been confirmed and defined by many lab tests employing continuous monitoring [

ja c.e l

0133v:1o/012887 3-4

[

ja.c.e 3.3 Process Qualification Effort A thorough test and qualification program was undertaken to verify the l adequacy of the Row 1 and Row 2 U-bend heat treatment process. Functional requirements were determined prior to the qualification program. These requirements were then used during qualification to demonstrate that the U-bend heat treatment process is a viable PWSCC margin enhancement technique and that the tooling system (i.e., heater assembly, insertion mechanism, channel head manipulator, etc) can fulfill its intended function by meeting or exceeding established design criteria. A brief summary of the Watts Bar Units 1 and 2 qualification program follows:

A. The tooling system (heater assembly, insertion mechanism, etc) was tested to verify acceptance of the process parameters. Initial checkout and mechanical tests were performed on a test stand and in the field to verify the adequacy of the heater assemblies in performing their designed objective.

otesticio12ss7 3-5

. ~ . . . _ - . . - -

i B. (

ja,c.e 4

C. (

ja,c.e D. [ Ja,c.e U-bend temperature control has been verified. [.

ja,c.e .

E. Full Height U-bend Insertion and Heating testing were conducted to check full system capability. Heater life tests were also conducted in this l

4 manner. ,

F. Moisture Condensation Testing was conducted as a measure of the process capability to accommodate a moisture film on the steam generator tube secondary side. A black Row 1 tube cluster was spray soaked with water and run through a nominal heating cycle. No difference was observed in comparison to a dry cluster.

G. Runaway heater test was conducted in Row I heaters to a maximum l temperature before heater failure. In a shiny U-bend 1732*F was achieved under worst case ramping conditions.

4 l

0193v:1D/012887 3-6 n ..-, - -,~.._.n.., . - - , - . . . ,w - , - - - . - . -n-,w.---. -- ,-.-.-.,,- - - - , , - - . - - ~ - - . ~ , , . - - , , , - - - , , . . - - , , , , , , . , -

=

k 9

Nf 3.4 Additional Qualification Test Data ,

< ~

3.4.1 [ }"'C Cycle Determination ' -

s .

, \, , '

A series of U-bend mockup tests were performe [ >

t s. ,

?,C,e

+ , ,

r1

\

\

s 4

4 *

\ '

ja,C,4 i

. - s. . 3 / , s

(, -

\ s.

! \- n.

\

\ 1" ,

s t l . ,,

l

( '..

)a,C,e  !,

, l. *

'i 3.4.2 ( la,C,e r .I

( 9

\

k.

\

k, ja Ces t

otes to/otasar 37 ,

L

F w e

s

[ .

ja.c.e l

6 h

t x

r

)

0193v:10/012887 3-8

i.

d TABLE 3.1 j ,

U-BEND TUBE TEMPERATURE DEVIATION FROM MEAN TEMPERATURE

- MODEL "51" R0W 1 TU8ES a,c.e t

l

(

i I

l

\ ,

% emus '

, 0193v:10/0128tr 3-9 l

._ . ._.. . _ _ _ _ _ _ _ _ . _ _ _ . . _ _ , _ _ _ _ _ _ . - , . _ _ . _ _ _ __ ____ l

l l

l f

f TABLE 3.1 (Cont) 1 i l

U-BEND TUBE TEMPERATURE DEVIATION FROM MEAN TEMPERATURE a,c.e N00EL "51" R0W 2 TUBES l

I

~

I t

i l

l r

otsav:1o/ot2ss7 3-10

(

l

TABLE 3-2 TYPICAL POP-UP TEMPERATURE CORRELATION DATA a,c.e l

m cis3vnomiass7 3-II l .

i

- I

.a.C,e l

= \

l l

l l

l l

l l

I i

=

t Figure 3-1 Systems Concept

. S Cet l

I l

l l

t l

l Figure 3-2 Heater Assembly Schematic .

,, 4,b,c.e 4

Ig Figure 3-3 Voltage vs Temperature Curve For Row 1, Autoclave

l a,b.c.e 1 Autoclave Figure 3-4 Current vs Temperature Curve For Row

~

l l

=

4 b.c.e l

l -

I l

l l

t Figure 3-5 Voltage vs Temperaturs Curve For Row 2, SHINY

. 4.b.C.e m

Figure 3-6 Current vs Temperature Curve For Row 2, SHINY

s a,b,c.e f

= .

Figure 3-7 Row 2 Test Minimum Powee

4.b.C e c

.n Figure 3-3 Row 2 Test Average Power N - - _ _ _._

4,b,C,e O

i i

I l .

l t

Figure 3-9 Row 2 Test Maximum Power 1

0 4,b,C,4 Figure 3-10 Dry Run Test using Production Heater

, 4,b.C t l

i I

l l

l e

e Figure 3-11 Comparaton of Predicted Tube Wall Temperature With Measured Data l

4.0 U-BEND HEAT TREATHENT PROCESS VERIFICATION 4.1 Process Parameter Development A range of potential process parameters of temperature and time was examined -

in accelerated stress corrosion cracking (SCC) tests conducted by Westinghouse in programs in concert with the Electric Power Research Institute (EPRI).

Selected parameters from those programs were then applied to Row 1 U-bends using field prototypical electrical resistance heaters in a separate feasibility program, also partially funded by EPRI. In an extension of the I latter program, a second set of parameters was also applied to Row 1 U-bonds, and both sets of Row 1 U-bends, representing the two sets of selected heat treatment parameters, were subjected to accelerated SCC tests. These accelerated SCC tests of the heat treated U-bends demonstrated the effectiveness of both sets of process parameters in providing additional SCC Il resistance of both Row 1 and Row 2 U-bends. Finally, metallurgical studies '

l were performed to assess the effects of the selected process parameters, and parameters outside of the process range, on the microstructure and mechanical properties of heat treated U-bends.

. This section summarizes the initial parameter selection data base, the SCC performance of heat treated U-bends, and the metallurgical studies.

4.1.1 Stress Relief Data Base The accelerated SCC tests examined a wide range of potential heat treatment parameters as defined by heat treatment temperature and time at temperature.

Highly susceptible Alloy 600 split-tube, axially strained " reverse U-bend" (RUB) specimens were used in this phase. These were bolt-loaded across the

" legs" of the bend and stress relieved in this pre-stressed condition.

[

Ja,c.e The samples, together with

( non-stress relieved " controls", were exposed in stainless steel autoclaves to I

recirculating, pressurized high purity water containing a typical dissolved 0193v:1o/012887 4-1

hydrogen content'of - 45 cc (STP)/kg water. The test temperature was 680*F (360*C). These conditions accelerate the kinetics of primary water Stress Corrosion Cracking (PWSCC) by at least an order of magnitude over those at the inlet temperature of a steam generator tube bundle; the acceleration is even greater when the U-bend temperature is considered.

All heat treatments (- Ja,b,c.e were found to prolong the integrity of these highly susceptible RUB samples compared to the non-heat treated controls. (

3a,b,c.e Subsequent tests were later initiated on RUB samples [

]a,b,c.e These later tests used RUB's made from one of the two heats that were tested as a full-size Row 1 U-bend. The tests consisted of prolonged exposures at 680*F to lithiated, borated water containing dissolved hydrogen, as a simulant to the reactor coolant environment. This test series also contained RUB's having lower temperature / longer time stress relief cycles. After extensive exposure to the simulated reactor coolant, during which no PWSCC was observed, all of the samples were transferred to an environment of 3000 psig superheated steam at 750'F (400*C) with 11 psia hydrogen. The superheated steam test exhibits PWSCC kinetics that are at least 2 orders of magnitude faster than those at the U-bend temperature. No PWSCC occurred in any stress relieved sample in over 2000 hours0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> in this very accelerated steam test. Table 4-1 summarizes these data from the simulated reactor coolant /superheated stea:n exposves.

0193v:10/012887 4-2

4.2 PROCESS PARAMETER VERIFICATION 4.2.1 Preparation and Testing of Row 1 U-bend Tubes From the preceding sets of extensive tests, a minimum temperature (

Ja,b,c.e was selected as the stress relief temperature for the tangent point regicns of Row 1 and Row 2 U-bends. This selection was based in part uoon the field implementation criterion (-

]a,b,c.e These were subjected to a post-heat treatment differentially applied strain that simulated the hot leg and cold leg difference in axial growth at steam generator U-bend operational conditions.

Two samples of each of the two heat treatment durations, together with two differentially strained non-heat treated " control" samples, were exposed to l the reference 3000 psig, 750*F superheated steam test with hydrogen present in the steam. The test was further accelerated in that the full 3000 psi internal steam pressure also constituted the differential pressure across the tube wall. This test differential pressure was accordingly about twice the l normal steam generator primary-to-secondary operational differential pressure.

In the earlier feasibility program, a heat treatment (-

Ja,b,c.e was selected and applied to Row 1 U-bends of both 7/8 in. 00 and 3/4 in. 00. The 7/8 in. OD tubes were of the same heat as that used for the [ ~]a,b,c.e heat treatment, the 3/4 in. 00 tubes were of a second heat. The heat treatment was applied with a developmental model of the flexible ID electrical resistance heaters that were finally adopted for field implementation. The initial PWSCC evaluation of this heat treatment [

la,b,c.e used as an ID test environment recirculating, pressurized, high purity water containing about 25 cc hydrogen (STP)/kg water at 680*F and 3000 psig. The 0D surfaces were exposed to 1500 psig superheated steam. For 0193v:10/012887 4-3

each U-bend s'ize, two non-stress relieved " controls" and two heat treated ,

samples were exposed with the imposed differential hot leg / cold leg strain.

The exposure to pressurized water conditions was for 4944 hours0.0572 days <br />1.373 hours <br />0.00817 weeks <br />0.00188 months <br /> during which time both non-heat treated 7/8 in. 00 U-bends developed typical axial throughwall PWSCC near the extrados of the tangent point of the irregular transitions. No other leakage events occurred. The 3/4 in. OD U-bends in the

(' Ja,b,c e stress-relief condition and in the non-stress-relief condition were then transferred to the 750*F superheated steam test.

4.2.2 Accelerated Test Results for Row 1 U-bend Tubes The aggressiveness of the superheated steam test environment as a PWSCC test medium and the effectiveness of the stress-relief cycles [

la,b,c.e are demonstrated by these observations.

o Both non-stress relieved 7/8 in. 00 bends developed typical throughwall PWSCC at the tangent point in 25-26 hours.

o No leakage occurred on any of the four ( ']a,b,c.e tested 7/8 in.

00 bends in over 1000 hours0.0116 days <br />0.278 hours <br />0.00165 weeks <br />3.805e-4 months <br /> in test, an increase in the longevity of a factor of greater than 40.

o One 3/4 in. 00 stress relieved U-bend with 4944 hours0.0572 days <br />1.373 hours <br />0.00817 weeks <br />0.00188 months <br /> exposure to 680*F water developed'throughwall PWSCC at the tangent point after 144 hours0.00167 days <br />0.04 hours <br />2.380952e-4 weeks <br />5.4792e-5 months <br /> in steam.

o Neither [ la,b,c.e 3/4 in. 0D bend developed leakage after 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> in the steam test.

Table 4-2 summarizes the PWSCC testing of heat treated and non-heat treated Row 1 U-bends.

(

Ja,b,c.e U-bends with this stress relief have displayed no PWSCC in exposures that are more than 40 times longer than those that led to FWSCC of untreated U-bends in the same test medium.

4.3 Process Parameter Selection and Definition

(~

Ja,b,c.e This upper limit is based on extensive microstructural characterization tests and hardness determinations that have been conducted specifically for the U-bend heat treatment program.

These studies of the response of mill annealed Alloy 600 U-bends to the heat treatment cycles have shown that no exaggerated grain growth occurs [.

]a,b,c.e and that the hardness of U-bends heat treated [r .]a,b,c.e remains above the hardness of the unbent straight legs, confirming that the yield strength in the bends remains acceptable. The upper limit on temperature is fixed by the observation that treatment [ l a,b,c.e produces some exaggerated grain growth and a significant reduction in hardness in the bend section. This indicaces that the yield strengths in the bend may be compromised at [ ]a,b,c.e At the specified maximum temperature of

( !a,b,c e for the U-bend heat treatment process, these microstructural studies showed that no changes occurred during short [ la,b,c.e exposures at temperature; however, the beginnings of exaggerated grain growth l

and hardness reductions were observed (in one of two test heats) after [

]a,b,c.e maximum temperature.

In summary, stress relief that is beneficial against SCC occurs at

[ la,b,c.e and above, and no significant recrystallization, grain growth or hardness changes occur below (~ ~]a,b,c.e Therefore, the .

acceptable and optimum temperature range for the field process is defined as

[ ja,b,c.e 0193v:1o/012287 4-5

i TABLE 4-1 ADDITIONAL RESULTS ON STRESS RELIEVED ALLOY 600 REVERSE U-BENDS No. RUBS SCC /No. RUBS Tested a

Heat Treatment 680'F RCS 750*FSteam5 a,b,c.e 2650 0/2, 8500 hr. -> 0/2, 2250 hr.

2650 Not in test 0/3, 2250 hr.

2650 Not in test 0/3, 2250 hr.

2650 0/2, 8500 hr. -> 0/2, 2550 hr.

2650 0/2, 8500 hr. -> 0/2, 2000 hr.

2650 0/2, 8500 hr. -> 0/2, 2250 hr.

2650 0/2, 8500 hr. -> 0/2, 1550 hr.

2650 0/2, 8500 hr. -> 0/2, 2250 hr.

2650 2/2, 500 hr. 5/5, 100 hr.c 5/5, 650 hr.d 1019 Not in test 0/3, 2250 hr.

1019 Not in test 0/3, 2250 hr.

1019 Not in test 2/3, 250 hr.

3/3, 700 hr.

a. RCS = Lithiated, borated, Reactor Coolant System chemistry with hydrogen

b. Steam at 3000 psi + H 2 at 11 psia

c. Set i

d. Set 2 (with 1/5 in 100 hr., 4/5 in 400 hr.)

0193v:10/012287 4-6 m

TABLE 4-2 PERFORMANCE OF STRESS RELIEVED, DJFFERENTIALLY STRAINED R0W 1 U-BENDS IN ACCELERATED PWSCC TEST ENVIRONMENTS Samole Number, Condition, and Exposure Time High Purity Water 3000 Psig Steam 680'F 750*F 1500 psi AP 3000 psi AP 25cc H2 /kg H 2O 3 psia H 2 7/8 in. 00 Bends a,b,c.e No. 1 = SCC, 1361 hr. No. 3 = SCC, 24 hr.

No. 2 = SCC, 2866 hr. No. 4 = SCC, 26 hr.

No. 1 = OK, 2435 hr.

--- No. 2 = OK, 2435 hr.

l No. 1 = OK, 2435 hr.

No. 1 = SCC, 1082 hr.

No. 1 = OK, 4944 hr. ---

No. 2 = OK, 4944 hr. ---

3/4 in. OD Bends

~

a,b,c.e .

~

No. 1 = OK, 4944 hr. No. 1 = SCC, 144 hr.*

No. 2 = OK, 4944 hr. No. 2 = OK, 144 hr.*

No. 1 = OK, 4944 hr. No. 1 = OK, 1641 hr.*

No. 2 = OK, 4944 hr. No. 2 = OK, 1671 hr.*

• Exposed to steam after the 4944 hour0.0572 days <br />1.373 hours <br />0.00817 weeks <br />0.00188 months <br /> water test 0193w10/012287 4-7

_&nb, Cst i

j i

I i

Figure 4-1 Short Duration Stress Relief Treatments at Higher Temperatures are Effective Against SCC O

t

5.0 EDDY CURRENT RESPONSE TO U-BEND HEAT TREATHENT Eddy current testing was conducted to determine the effect of heat treatment on the inspectability of the U-bend region of steam generator tubing. The cbjective of the testing was to determine whether the heat treatment process, designed to reduce residual stresses in inner row U-bends, introduces additional signals or alters the existing baseline signature of the steam generator tubing ( ,

]a,c.e 5.1 Description of Eddy Current Test Program Eddy current testing was conducted on two sets of tubes. Laboratory testing was performed uaing two mill-annealed 7/8 inch outer diameter, 0.05 in. wall thickness Row 1 U-bends. One U-bend sample had been heat treated whereas the other was in its as-manufactured condition. In generator eddy current testing was conducted on two Row 1 U-bends during a plant demonstration of the heat treatment process. For the field test, the tube diameter was 3/4 in. OD with a 0.043 inch wall thickness.

Laboratory eddy c0rrent tests were conducted using a ( Ja,b,c diameter probe anc a ( Ja,b,c instrument. Test frequencies of (

la,b,c were used. The in generator testing was conducted using a [

l Ja,b,c.e 5.2 Laboratory Eddy Current Test Results Two different U-bends have been examined. A comparison of the eddy current signatures for the two U-bends, one heat treated and one as manufactured, showed no discrete signals attributable to the heat treat process. [

.ja,b,c.e 0193v:10/012837 5-1 l

[:

ja,b,c.e 5.3 Eddy Current Inspectability Summary Laboratory addy current testing of as-manufactured and heat treated U-bends shows no significant difference in eddy current response. [

Ja ,b,c.e It is concluded that heat treating the U-bend region of SG tubing does not impede the performance of standard addy current inspection systems.

,a

/

l

/

/

I

/

/

0193v:10/012887 5-2 /

s I

[

8.b.C,e B

~

A

, :. os t e t .;. as s sa .' ossoc. e - a st, e ses e ca. es

?

' l C.LL%c De=

i !l

, rise a '* - esee ses I .,

t a was. - tm l

8

,smi .

l

.g . '

1

.. g..

i Mfetto . m m8 g

g me aan

> 9 < o j run.no - av i l in ==

' em im gl , esen.

tie . m me

' I  ! um._ru_asc 3 -- .

i

• 'rwe in ==

w in

.; . m. m= .-.  ; e.,.vi., . m an 1-l }

l - --

q a em. y.p, .

os>.

[e triaim m 8 . . . . . ....

3B&8FFlIm8 l,

w- -;

.... r;: ;

m.

! -t- + _ _EC . .

so Figure 5-1 Eddy Current Results f or A> Heat Treated Tube

- 31 40% Through Wall Tube ASME Std.

I

6.0 SAFETY EVALUATION

6.1 INTRODUCTION

Primary water stress corrosion cracking (PWSCC) of mill annealed Alloy 600 steam generator tubing has been identified as having a potential impact on the operation of steam generators. Regions of the steam generator tubing that may be affected are some Rcw 1 (possibly Row 2) U-bends at the tangent points (transition from curved to straight portions of the tube), at or near the apex of the U-bend, and at the roll and roll transition zones within or near the top of the tubesheet. The in situ thermal stress relief process discussed herein addresses U-bend region PWSCC only.

Examination of tubes removed from service with leaks in the U-bend region has revealed that the leakage occurred at inner-diameter-initiated through wall

~

cracks that are generally short, tight (low leakage), and axially oriented.

The examinations indicated that cracks were initiated and propagated by l

intergranular stress corrosion cracking.

Laboratory experiments have established that the factors contributing to the occurrence of PWSCC in service are: high operating temperatures, susceptible tubing microstructure, and high local stress-strain conditions. Each of these factors may be present in varying degrees in operating steam generators. An effective means for minimizing the potential for PWSCC is to reduce or modify the residual stress in the region of the tube that may have less resistance to PWSCC.

Westinghouse has developed a process to provide additional margin against inner diameter (ID) PWSCC occurring in the Row I and 2 U-bends at both tangent points and at or near the apex of the U-bend within the steam generator. This is achieved through the reduction of residual tensile stresses in the tube wall by a thermal stress-relief cycle. Procedures have been developed and qualification tests performed for the insertion of an [

ga.c.e 0193v:1D/o20387 6-1

7

, l 7

\

~

[ f. \ g Ja c.e s The following safety evaluation is provided to demonstrate'that the -

N application of the in situ U-bend heat treatment process to the Sequoyah Units 1 and 2 Row I and 2 steam generator tubes does not comtromise st9en generator tube bundle integrity and therefore does not represent a potentially i q

~

unreviewed safety question. The weight. loss from the heater (insulation material and impact on the primary system are also evaluated.

'i g 6.2 TUBE BUNDLE INTEGRITY EVALUATION In situ U-bend heat treatment process qualification was performed utdar ,

Westinghouse Quality Assurance (QA) surveillance. Frototyobl heat treaimect ' , i testing and qualification has been performed to estabilsh, temperature distributions for the tubes and top support plate during haat treatmenth In addition, tests were performed for tube support plate gecmetries both with and +.,

without cutouts along the central axis of the plate. ' h\plements of tha  ? ,

field procedures have been developed and tested to meet' field operation 's requirements. Field procedures provide direction for all crew and site ,

4t activities and serve as the documented QA verification method for field ,

implementation of the heat treatment process. ON \

/3

, ..N, l 'v <

The current U-bend stress-relief temperature and time parameters were 5 '

developed by Westinghouse in concert with EPRI partiCly funded programs.

Emphasis was placed on exploring the minimum temperature and time that would ,

provide additional margin to resistance to PWSCC in the Row 1 and 2 U-bhed ,'

tangent point area. Accelerated tests (in 680*F water) on split tube reverse v U-bend samples showed that heat treatant temperatures [. ,

la,b,c.e eliminated PW3CC over the full test duration of 16,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. Row 1 U-bends of 3/4 in. 00 tubing that were stress relieved ( la,b,c.e; and ?ubjected to superheated steam tests at 750*F and 3000 psi internal pressura nave not resulted in a

~

leakage event in approximately 6 times longer than that of a non-stress relieved sample in the same test. These 3/4 in. Row 1 U-bends had all been 6 0193v:10/012287 6-2

~

j previously exposed to accelerated tests in 680*F pressurized water with Pijdrogen for approximately 5000 hours0.0579 days <br />1.389 hours <br />0.00827 weeks <br />0.0019 months <br />. For 7/8 in. Row 1 U-bends, 2 samplas

~~~~~-

that were stress relieved [ ,

i s

Ja,b,c.e have resisted PWSCC in the

( ', abperheated steam fo'r 64 times as long as the exposure that cracked 2

\ '

non-stress relieved 7/8 Row 1 bends. One sample of 7/8 in. Row 1 U-bends with I' ( la,b,c.e stress relief developed tangent point leakage in saperheated MLiam after an exposure of 42 times that required for SCC in the non-st ess relieved samples. These observations on actual Row 1 U-bends in these Mahly aggressive accelerated test envircnments confirm that stress y' .;

relieiy v,hich is beneficial against PWSCC, occurs [

J',b,c.e Separate < studies have shown that no significant y recrystallization, exgggerated grain growth or hardness reductions occur

[ , ja,b,c.e 1.

( , -

9 i

T t.

• \

\

i 1

v.

T

'9 )

i

\

i i 7 ja,b,c.e i

s i .

0193v:1o/012887 ; 6-3

_...___u. m. . . _ _ _ _ . _ . _ _ _ _ _ _ . . _ _ . _ _ _ . _ _ _ _ . _ . - - _ - - - _ - -

v ,

s 1

qi

[ .

.. ( '

\I

/

ja,b,c.e 4.. Laboratory and field addy current test data has revealed slight effects on the lT signal responses observed using conventional bobbin coil examination practices. The heat treated tubing remained fully inspectable.

The impact of the U-bend '

heat treatment process on steam generator tube f

. integrity and the tcp tube ;upport plate in the Sequoyah Units 1 and 2' steam generators is addressed below. The evaluation of both of these components ,

utilized a combination of finite element analysis model .and conventional

~

analysis techniques. The aoplied temperature distributilons were determined '

from prototypic heat treatmen't tests or from conservative estimates 'of component temperatures when tests results were not'available. Specifically, T for the steam generator tubes, an analysis has ben completed that considered

' the potential for increasing the residual stresses in the tube away from the area at issue during initial heatup, and also evaluated the resulting stresses in terms of ovarall' fatigue. Relative to the impact the heat treatment process on thef top tube support plate, an analysis determined the maximum stresses resulting from the process and compared the results to the ASME Code guideline for maximum stress range to preclude gross deformation of the '

support plate.

6.2.1 Steam Generator Tube Integrity Evaluation The tube' integrity evaluation considered three separate loading conditions resulting free th<q application of the. process. The first loading condition assessed was the resulting heat treatment temperature differential across the

, U-bend section. The tube temperature differential (deviaticn'from the average

/ tube wall temperature) resulting from the heat treatment process was evaluated

• g to determine whether additional residual streses are introduced into the Row 1 and Row 2 tubes during initial heatup. The potential for thermal creep during the heat treat period was addressed and determined not to be a concern. The secone.2 loading condition assessed the stresses introduced into the tube as a e

0193v:10/012287 6-4

result of the axial variation in temperature which exists at the end of the heater. Cyclic (fatigue) stresses that the tubes experience as a result of the above loading conditions were also evaluated.

l Pertaining to the first loading condition, an analysis was completed to determine if the heat treatment process would result in increased residual stress elsewhere in the U-bend, particularly at the apex. [

]a,b,c.e Analysis results revealed that the maximum induced moment was less than the elastic restoring moment (which exists during the initial bending of the tube), and that the heat treatment process results in only elastic cycling of the tube; therefore, t..a residual stresses will not

) be increased. ['

l ja,b,c.e For the second loading condition, a finite element model analysis was used to evaluate the stresses in the tube. The axial variation in temperature at the end of the heater assembly was shown by model 0 test to be a ( ]a,b,c.e decrease in temperature over a 1 inch length. Applying this data for a model 51 evaluation, the maximum tube stress for this lo~ading was determined to be low, occurring at the end of the tube hot region. This stress occurs in the vicinity'of the tube support plate, where tube bending stresses resulting from the heat treatment are low.

0193v:10/012287 6-5 m

r .

In evaluating the structural response of the tubes to the above loadings, '

consideration was given to the material response as a function of time at temperature. Three response mechanisms were considered in the analysis; cyclic fatigue, thermal creep, and time to rupture. Thermal creep and time to rupture effects were shown to be negligible. In addition, the fatigue usage for the tubes was calculat. 3 to be 0.057 which is considerably less than the allowable value of 1.0.

Additionally, the potential for a third loading condition i.e., tube-to-tube _

contact during the U-bend heat treatment process was evaluated. An analysis revealed that steam generator tube-to-tube contact would not be expected to occur during the U-bend heat treatment process. It was determined that the-l minimum tube-to-tube gap which can exist between Row 1 and Row 2 tubes is l [ 1.a c.e During the U-bend heat treatment process the tube-to-tube gap may decrease to (. la,c.e when a Row 1 tube is heated, and to [ la.c.e when a Row 2 tube is heated. These calculations incorporated the expansion of the neighboring tube which is expected to reach an average temperatu*e of ( ).a,c.e 6.2.2 Tube Support Plate Analysis Durin] the heating of any single tube, the portion of the top tube support plate which is influenced by the heat treatment is quite small in comparison with .the overall plate diameter; therefore, the general stress in the heated region was approximated (. la,b,c.e The effects of the plate perforations in the [ .]a,b,c.e stresses were determined using a finite element model of a typical ligament. The analysis also accounted for the increased plate s'tiffness in regions where the cutouts along the central plate axis do not exist. The plate temperatures used for this analysis were based on test results that utilized a heat treatment temperature of 1450*F for a timo at temperature of 6 minutes with a heater displaced one inch toward the tube support plate from the nominal position.

The heater displacement results ir. maximum plate temperatures. In order to assess the plate temperatures resulting from adjacent Row 1 heatings, a heat treat cycle was applied to a row 1 tube with the heater left in place and an 0193v 1D/012987

. 6-6

additional treat cycle applied after a five minute period. This test provides conservatively hot temperatures as a result of the tube not being allowed to cool due to the presence of the heater. In addition, the five minutes between the heat treat cycles is considerably smaller than the times obtainable in the field heat treatment of two adjacent tubes. The resulting plate stresses, which includes results for a plate with or without a central cutout, are within the allowable limits for stresses, based on the ASME Code limit of 35,(3 times ASME Code allowable stress intensity for design) for the ,

maximum range of primary plus secondary stresses.

Calculations were also performed to determine if buckling of the heated region of the top tube support plate is an issue. Calculations reveal a' critical '

elastic plate stress significantly in excess of the maximum induced plate stress generated due to the U-bend heat treatment process; therefore, buckling of the plate in the heated region is not expected to occur.

I 6.3 PRIMARY SIDE IMPACT OF THE U-BEND HEAT TREATMENT PROCESS The( ja,c.e heater assembly used in the in situ heat treatment program has been both cold and hot tested to assure that residual amounts of fiber left in the Inconel 600 tubing are within acceptable limits. The insulating material ( ]a,c.e selected for the heater contains 2a,c.e high concentrations of ( This fiber material loses.some weight during use; therefore, the potential that the residual fiber could exceed the specification limits for either [ ]a,c.e in the primary water was evaluated. The specifications for these materials are stringent to limit deposition on the fuel rods.

Based on the results from model D steam generator tests, model 51 steam generator weight losses were estimated by scaling model D weight losses by a ratio based on the heater's areas'. Hence, a total weight loss of 22.1 mg is predicted for 10 cycles of a Model 51 heater assembly. Extrapolation of the estimate for 10 cycles to 15 cycles resulted in a value of 35.1 mg. Based on the later findings, the proportional amounts of (without cleanup) [

.]a,c.e that constitute the projected heater weight loss for the 0193v:10/012987 6-7

entire U-bend heat treatment program at Sequoyah Units 1 and 2 would be expected to be within the limits of the primary water chemistry specification.

6.4 CONCLUSION

The application of the in situ heat treatment process in the Row 1 ard Row 2 U-bend region of the Sequoyah Units 1 and 2 steam generators has been demonstrated to provide a significant increase in margin to PWSCC while not adversely affecting steam generator tube bundle integrity. Briefly summarizing, relative to steam generator tube' integrity, analyses have shown that: the U-bend heat treatment program does not result in the introduction of additional stresses in the tubes, fatigue usage in tubes resulting from the combined loadings of U-bend heat treatment is minimal, heat treatment of a tube at a temperature [ la,c e introduces negligible creep strains in the tube, and the effect of air formed oxides due to the. stress relief process on corrosion resistance of the U-bends is not detrimental to the long term corrosion resistance of stress relieved Inconel 600. Relative to steam generator top tube support plate integrity, plate stresses generated during the heat treatment process were found to be acceptable for a heat treatment of [.

]a,c.e (which bounds the U-bend heat treatment process parameters)

! and buckling of the plate in the heated region was determined not to be an issue. Also, the weight change due to loss of fiber insulation, measured from cycling the U-bend heater, does not deleteriously affect the water chemistry specification for bo'th [ la,c.e in the primary side of the Sequoyah Units 1 and 2 plant after heat treating all Row I and 2 U-bends without cleanup. Additionally, per recommendations in RG 1.83 " Inservice Inspection of Pressurized Water Reactor Steam Generator Tubes", the application of the U-bend heat treatment process does not interfere with periodic in-service inspection and interpretation to assess tube structural and leaktight integrity. The U-bend heat treatment process procedures and inherent' quality assurance checks further substantiate that the application of l the heat treatment process to the Sequoyah Units 1 and 2 steam generators does not represent an unreviewed safety question pursuant to 10 CFR 50.59 criteria (a) (2).

0193v:10/012987 6-8

I l

l ENCLOSURE 1 l WCAP - 11378 Row 1 and Row 2 Heat Treatment Licensing Report for Sequoyah Units 1 and 2 Proprietary Class 2 (Proprietary) prepared by Westinghouse dated January 1987 I

l t