ML17325B566
| ML17325B566 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 01/27/1999 |
| From: | Okeefe M, Oleary T INDIANA MICHIGAN POWER CO. |
| To: | |
| Shared Package | |
| ML17325B564 | List: |
| References | |
| NUDOCS 9904300150 | |
| Download: ML17325B566 (33) | |
Text
s s
Technical Position: Unit 1 Steam Generator Layup Chemistry out of Specification During September 24, 1997 to December 31, 1997.
~Soo sis:
\\
This technical position documents the evaluation ofthe out ofspecification parameters and the potential effect on steam generator corrosion during the time period from September 24, 1997 to December 31, 1997. During this period, the unit remained in mode 5. Revisions 6 and 7 of procedure 12 THP 6020 CHM.201, Steam Generator, governed the steam generator chemistry specifications for all modes of operation, including layup (Ref 1 and 2), during this period. The chemistry control detailed in the Steam Generator procedure was developed using the guidelines in EPM TR-102134-R4, PWR Secondary Water Chemistry Guidelines-Revision 4, November 1996 (Ref 3).
Following the unit 1 shutdown which commenced on September 8, the steam generators were not placed immediately into wet layup as the length ofthe outage was not determined.
Layup of the steam generators was subsequently started on. September 24.
Initially,only carbohydrazide was used as an oxygen scavenger.
The steam generators were drained and refilled between December 6 and 9, 1997, to adjust the pH and oxygen scavengers.
With the refill, both hydrazine and carbohydrazide were used as oxygen scavengers.
Steam generator 12 was drained and refilled December 18, 1997, due to elevated sulfate concentration.
According to the monthly reports, the appropriate amounts of chemicals were added when the steam generators were placed in wet layup in September.
Although not completely documented in the monthly reports, from the layup data it appears the appropriate amounts of chemicals were added when the steam generators were drained and refilled in December.
During January 1998, the steam generators were prepared for startup.
The startup did not occur and the steam generators were returned to layup in February 1998 where they have essentially remained.
Independent reviews ofthe layup data indicated that some parameters were at times out of specification during the period from September 24, 1997, to December 31, 1997. The parameters that were out ofspecification included boron, ethanolamine, pH, hydrazine and carbohydrazide.
During the period, the unit underwent a heatup to about 180'F to 195'F four times. While available data could not confirm, it is possible that some or all of these heatups were in preparation to enter mode 4 and unit restart. Ifthis were indeed the case, pH, hydrazine and carbohydrazide would be controlled lower than typical layup levels to prepare for operating chemistry control. Contaminants and potential corrosive species, sodium, chloride and sulfate, were maintained within specification and were generally very low during the period.
In order to determine the impact ofthe out ofspecification condition on the steam generator integrity, a thorough review ofchemistry data for the period was conducted.
The data that was evaluated was taken from data sheets 12 THP 6020 ADM.010 Attachment 1, with the exception of condensate storage tank dissolved oxygen data that was taken from CDMS. The raw data and graphed data are included in this technical position as attachments 1 and 2 respectively.
9904300i50 990422 PDR ADQCK 05000$ i5 1
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Discussion:
Boron:g Boric acid is an additive used to mitigate outside diameter stress corrosion cracking, ODSCC, of the steam generator tubes during operation.
The control parameter for boric acid is boron.
During operation, boron concentration in the steam generator is maintained at about 5 to 10 ppm. During startup, 1000 to 2000 ppm boron is used for crevice flushing and about 50 ppm boron is used for the boric acid soak.
As the environment conducive to ODSCC is not present during shutdown, boric acid is not needed during layup. Ifboric acid is present in the layup solution, it can depress the pH. The consequence of elevated boric acid levels during layup is the need to use more amounts of ethanolamine, ETA, which is used to raise the pH ofthe layup solution. ETA is used during operation and layup to adjust pH. It does not ofitself present a corrosion concern for the steam generators.
The EPRI guidelines (Ref 3) do not identify a corrosion concern with elevated levels ofboric acid during layup. The guidelines identify the consequence ofelevated boric acid levels during layup as the need to use more amounts ofETA.
The upper limitfor boron during layup is 1 ppm.
During the period from September 24, 1997 to December 31, 1997, the boron was above the 1 ppm limitas shown on the attached graph.
When pH was raised above 9.8 and the concentrations ofhydrazine and carbohydrazide were increased significantly prior to December 15, 1997, the indicated boron concentrations also increased significantly. It is unlikely that these values were accurate.
No boric acid had been added,.the steam generators had been drained twice since the shutdown and the reported levels were about ten times normal operating levels.
It is more likelythat the reported values were a result of interference with the analytical method coincident with the high pH and hydrazine levels.
Some samples were checked using ICPMS and no boron was detected when the reported boron levels were ranging from about 20 to 70 ppm.
Subsequent conversion to the ICPMS method for boron analysis in layup solutions in January 1998, resulted in the reported boron levels being less than 1 ppm. The possible interference may have'also contributed to the reported boron levels being above 1 ppm during the previous part ofthe layup.
Regardless ofwhether the values were accurate, having boron greater than 1 ppm during this period did not result in an increase in steam generator corrosion.
H~hl i
A ETA is used to raise the pH of the secondary side during operation and layup to minimize corrosion of low alloy or carbon steel.
The upper limitof 50 ppm forETA during layup is based on environmental considerations when draining the steam generators.
There is no corrosion concerns with layup relative'to the upper limitofETA. The procedure was revised effective February 6, 1998, to raise the upper ETA limitto 100 ppm. ETA does not present a corrosion concern.
The EPRI guidelines (Ref 3) do not set limits for the pH additive during layup, rather Page 2 of23
0
the level ofpH additive is determined by the desired pH level. A corrosion concern with high levels ofthe pH additive is not identified.
~ I During the period under review; ETA was above the 50 ppm limitin steam generators 11 and 12 for essentially the entire period. For steam generators 13 and 14, ETA was above the 50 ppm limitfor a short period in mid November and from December 8 to 31, 1997. The high concentrations during December were used effectively to raise the pH in the steam generators to provide corrosion protection.
An increase in steam generator corrosion would not result from the period when ETA levels were above 50 ppm as ETA is not corrosive to steam generator materials at the levels experienced.
N~iO Use ofnitrogen overpressure during layup is recommended but not required by procedure 12 THP 6020 CHM.205 Rev 1, Steam Generator Wet Lay-up (Ref 4), and the EPRI guidelines (Ref 3). The purpose is to minimize air ingress above the water level during layup. The use of nitrogen overpressure is dependent on the mode the unit is in, the length ofthe outage and the maintenance scheduled to be performed on the steam generators,-feedwater and steam piping, turbine and condensers.
The personnel safety hazard is a primary concern when determining if nitrogen willbe used.
Nitrogen overpressure is typically u'sed'for extended outages in modes 5 and 6 when,.as described above, plant conditions permit. It is not atypical that a nitrogen overpressure was not maintained during the entire layup period.
Prior to placing the steam generators in initial layup in September, they were drained with a nitrogen overpressure.
This prevented air ingress.
After adding layup chemicals and fillingthe steam generators, they were sparged with nitrogen. The sparging along with draining under nitrogen ensure an oxygen free environment in the steam generators during layup. Sparging was done repeatedly during the period under review.
Steam generators 11 and 14 were sparged at the end of October.
Steam generators 12, 13 and 14 were sparged between November 10 and 13.
Allsteam generators were sparged before and after the drains and refills in December and at the end ofDecember.
Nitrogen use during the period was acceptable.
It provided an oxygen free environment and corrosion protection.
H and Ox en Scaven er Carboh drazide and/or H drazine '~
These parameters are being addressed together as their function of corrosion protection during layup is interrelated.
Carbohydrazide and hydrazine can be used together or individually.
Page 3 of23
Unlike under operating conditions, pH is a control parameter for layup. Elevated pH in conjunction with an oxygen scavenger is used to provide corrosion protection ofthe low alloy or carboti steel'components ofthe steam generators.
These components include the tubesheet, tube support plates, wrapper, shell, and the feed ring. To prevent localized attack, such as pitting, of the alloy 600 steam generator tubes during layup, reducing conditions should be maintained or the pH should be greater than 8.5. Also, chloride and sulfate concentrations should be kept below 1 ppm (Ref 3 and 7).
In an oxygenated environment, elevated pH in conjunction with an oxygen scavenger aids in the formation of a stable iron oxide film on carbon steel components.
This filmhelps to minimize corrosion by acting as a barrier to diffusion. In the absence ofoxygen, there is no significant corrosion ofsteel in neutral or near neutral water at ambient conditions (Ref 5, 6). In the absence ofoxygen and minimum levels ofother corroding species such as chloride and sulfate, it is expected that the result would be the same at the layup temperatures of 140'F to 195'F.
Most ofthe test data contained in reference 6 were obtained using cleaned carbon steel specimens with no existing protective layer. The carbon steel surfaces in the steam generators enter an outage with a protective oxide layer as a result of the reducing, high pH environment present during operation.
Therefore, the test conditions used in reference 6 are much more severe than the conditions in the steam generators during layup.
Per procedure 12 THP 6020 CHM.201, the lower limitfor layup pH is 9.2 when using carbohydrazide as the oxygen scavenger and 9.8 when using hydrazine.
Carbohydrazide is very reactive and tends to react quickly, significantly faster than hydrazine, to scavenge oxygen and passivate the metal surfaces in'the steam generator.
A result ofthis reaction is the generation of carbon dioxide. This acts to depress the pH, which is the reason for the lower pH limitwhen using carbohy'drazide.
The lower limitgiven in the procedure for carbohydrazide and hydrazine is 10 ppm and 75 ppm respectively.
The EPRI guidelines recommend a minimum pH of 9.8 and minimum hydrazine concentration of 75 ppm to provide corrosion protection during layup. The guidelines do not specifically address carbohydrazide but allow alternatives to hydrazine.
The high minimum levels for pH and hydrazine are intended to provide protection under aerated conditions and to provide protection in the event ofair ingress.
As discussed in references 5, 6 and 7, pH and hydrazine levels need not be as high ifthe environment is deaerated and without significant levels ofother contaminants.
It is conservative and good practice to maintain the elevated levels in the event ofair ingress.
However, in a deaerated environment, corrosion protection exists without these high levels.
From the start of layup to the drain and refill on December 6 to 9, 1997, carbohydrazide was used as the oxygen scavenger.
Carbohydrazide was below the lower limitfor this period. A sufficient amount ofcarbohydrazide was added when the, steam generators were initiallyplaced in wet layup to achieve a level near the upper limitof40 ppm. However, the carbohydrazide reacted quickly to passivate the internal metal surfaces ofthe steam generators and a significant level was not maintained in the steam generators.
During this time, the pH in two ofthe steam generators, 12 and 14, was maintained above 9.2. The pH in steam generator 11 was above 9.2 Page 4 of23
I except for two weeks preceding the drain and refillwhen the pH was at about 9.0. The pH in steam generator 13 was above 9.2 except for one week preceding the drain and refillwhen the pH was 9.1.
'ollowing the drain and refill, hydrazine and carbohydrazide were used. Hydrazine and carbohydrazide were above the lower limits for steam generator 11. Hydrazine was above the lower limitfor steam generator 12. Carbohydrazide was below the lower limitpart of the time, ranging from 5.3 to 20.8 ppm. For steam generator 13, hydrazine was below the lower limitpart ofthe time, ranging from 47 to 194 ppm. Carbohydrazide was below the lower limit, ranging from 5.4 to 8.1 ppm. For steam generator 14, hydrazine was below the lower limitpart ofthe time, ranging from 55.2 to 162.8 ppm. Carbohydrazide was below the lower limitpart ofthe time, ranging from 8.5 to 13.6 ppm. The hydrazine values reported on December 15 are
~
apparently in error based on hydrazine levels before and after December 15, and the fact that another chemical addition was not made. Following the drain and refill, the pH was maintained
~
at or above 9.8.
During the period under review, dissolved oxygen in the steam generators was 0 ppb. The dissolved oxygen in the fillsource, unit 1 condensate storage tank, during the initial and subsequent fillswas low. Chloride and sulfate were within limits during the period and generally maintained low. Sulfate did spike up'uring the second halfofDecember but was maintained within limits.
The environment in the steam generators was deoxygenated.
The carbon steel surfaces in the steam generators have an existing protective oxide layer entering the outage.
An oxygen scavenger was added with the initialfillthat acted to passivate the carbon steel surfaces.
Oxygen scavengers, both carbohydrazide and hydrazine, were added during subsequent fills and a significant residual maintained.
The pH was elevated and above 9.2 for all or most ofthe time up to the December 6 to 9 period. Following this it was generally greater than or equal to 9.8.
Contaminant levels, such as chloride and sulfate, were low. As a result ofthese conditions, adequate corrosion protection was provided during the period.
Position:
Considering the environment was deoxygenated, pH was elevated and contaminant levels were low, adequate corrosion protection was provided during the period ofSeptember 24 to December 31, 1997. The conditions in the steam generators during the period of September 24 to December 31, 1997 when layup chemistry was out ofspecification did not present an increased risk of corrosion over that which would have been present had the chemistry been maintained within specification the entire time.
Perform the following actions:
1.
Revise 12 THP 6020 CHM.205, Steam Generator%et Layup, to include the lower limitfor carbohydrazide and hydrazine during layup that had been deleted from 12 THP 6020 CHM.201 in rev 8.
Page 5 of23
2.
To provide optimal corrosion protection in the event of oxygen ingress during layup, increase the minimum pH limitto 9.8 when using carbohydrazide.
3.
Perform'an internal visual inspection ofsecondary side of at least one steam generator in the area ofthe water level during layup. Although the chemistry data indicates that corrosion protection was provided, the inspection is prudent considering the length ofthe outage.
Page 6 of23
References:
1.
12'THP 6020 CHM.201, Revision 6, Steam Generator, Effective Date September 11, 1997.
2.
12 THP 6020 CHM.201, Revision 7, Steam Generator, Effective Date October 6, 1997.
3.
PWR Seconda Water Chemist Guidelines-Revision 4, Project 2493, S520 Final Report.
Palo Alto, CA.: Electric Power Research Institute, November 1996. TR-102134-R4.
4.
12 THP 6020 CHM.205, Revision 1, Steam Generator Wet Lay-up, Effective Date February 28, 1997.
5.
H. H. Uhlig and R. W. Revie. Corrosion And Corrosion Control Third Edition. New York:
J. Wiley Ec Sons, 1985 pp. 91-99.
6.
J. A. Armantano and V. P. Murphy. "Standby Protection ofHigh Pressure Boilers" Proceedings ofthe 25 Annual Water Confer'ence ofthe Engineers'ociety ofWestern Pennsylvania, Pittsburgh, PA., September 28-30, 1964, pp. 111-124.
7.
Laborato Pro ram to Examine Effects ofLa Conditions on Pittin ofAllo 600, Research Project S124-1 Final Report. Palo Alto, CA.: Electric Power Research Institute, April 1983. NP-3012.
Prepared by:
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Date:
Approved by:
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Date:
Page 7 of 23
Attachment 1
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Attachment 1
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Attachment 1
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Attachment 1
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~SG 11
~SG 12
~SG 13
~SG 14 Min pH limit CO 0
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hD hD Ql tQ CO COV CO CO CO CO Date CO tO 0)
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CP CO CO Page 13 of 23 Carbo, N2H4 ppm 160.000 SG 11 Carbo, Hydrazine, DO2 DO2 ppm 120.000 80.000 N2H4 Limit75-350ppm Carbo Limit 10-40ppm 2
4 I
SG drained 8 refilled
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Page 14 of 23
Carbo, N,H4 ppm 280.000 SG 52 Carbo, Hydrazine, DO, DO2 ppm 24p ppp N2H Limit75-350PPm 200.000 160.000 120.000 Carbo Limit 10-40ppm N2H4 12/15 value inaccurate SG drained 8
~Carbo
~ Hydrazine
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Page 15 of 23
Carbo, N,H4 ppm 200.000 160.000 120.000 80.000
- SG 13 Carbo, Hydrazine, DO2 N,H, Limit75-350ppm Carbo Limit 10-40ppm alue-inaeeuret SG drained 8 refilled DO2 ppm
~Carbo
~ Hydrazine
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hD Ca cn CO CD CD 4
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Page 16 of23 Carbo, N2H~ ppm 200.000 160.000 120.000
. 80.000 SG 14 Carbo, Hydrazine, DO2 N2H< Limit75-350ppm Carbo Limit10-40ppm N+~tM4271&rdumnaccurat SG drained 8 refilled DO> ppm
~Carbo
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~ DO2 40.000 0.000 CO (O
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(0 CD CD CD V
V h3 CD CO CO V
V Date M
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8oron ppm 80.00 70.00 60.00 Boron upper limit 1ppm SG Boron SG's drained 8 fille 50.00 40.00 30.00 20.00
~SG 11
~SG 12
~SG 13
~SG 14 10.00 0.00 CD tQ CD CD M
h3 CD tO CD CD CJl CD CD Date CD tO CD CD CD V
lO tO CD Page 18 of 23 ETA ppm 180.00 SG ETA 160.00 140.00 120.00 100.00 80.00 60.00 SG's drained 8 fille
~SG 11 SG 12
~SG 13
~SG 14 40.00 20.00 ETA upper limit50ppm 0.00 CO 0
0 0
0 co
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to e
CO
'CO CO V
V V
CO COV CO Date CO CD CD CO CD to CO CD V
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Page 19 of23 Na ppb 7.0 6.0 Na upper limit 1000ppb SG Na 5.0 4.0 3.0 2.0 rai refilled SG 11
~SG 12
~SG 13
~SG 14 1.0 0.0 C)
CO CO Ql CO O
O h3 M
CJl CO CO COV Date (n
CO 4)
O CO CO V
CO M
CO CO Page 20 of23
Cl ppb 45.0 SG CI 40.0 35.0 CI upper limit 1000ppb SG's drained 8 30.0.
25.0 20.0 15.0
~SG 11
~SG 12
~SG 13
~SG 14 10.0 5.0 0.0 D
CO CO CO D
h3 D
h3 Ql CO CO V
V CO Date CO CO CO h3 4)
CO CO CO V
Page 21 of23 SO4 ppb 1000.0 900.0 800.0 700.0 600.0 500.0 400.0 300.0 200.0 100.0 0.0 SOupper limit 1000ppb SG SO SG's drained 8 efille
~SG 11
~SG 12
~SG 13
~SG 14 CO O
h3 CO CO 0
0 co CO CO Q
O hD hJ CO CO CO V
COV CO COV Date CD CO 4)
O CO CO M
CO hD CP CO CO V
Page 22 of 23 Wb Unit1 CST DO2 180 160 140 CST cross tied with U2 120 100
~CST DO 80 60 40 20 0
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tQ M
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ATTACHMENT 2 TO AEP:NRC:1295B
SUMMARY
OF UNIT 1 STEAM GENERATOR LAYUP CHEMISTRY FROM JANUARY 1g 1999 TO FEBRUARY 18'999
ATTACHMENT 2 TO AEP:NRC:1295B
SUMMARY
OF UNIT 1 STEAM GENERATOR LAYUP CHEMISTRY FROM JANUARY 1, 1999 TO FEBRUARY 18, 1999