ML19294B465
| ML19294B465 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 01/31/1980 |
| From: | Dearien J EG&G, INC. |
| To: | |
| Shared Package | |
| ML19294B462 | List: |
| References | |
| TASK-05-13, TASK-5-13, TASK-RR NUDOCS 8002280442 | |
| Download: ML19294B465 (13) | |
Text
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YD P.o.somis2s 6dano FaHe. Idaho 8M01 January 9,1980 Mr. R. E. Tiller, Director Reactor Operations and Programs Division Idaho Operations Office - DOE Idaho Falls, ID 83401 YANKEE R0WE POWER STATION STEAM GENERATOR WATER HAMMER TECHNICAL EVALUATION (A6257) - JAD-7-80 Ref:
J. A.
Dearien Ltr to R. E. Tiller,
JAD-225-79, PWR Steam Generator Water Hammer Reviews (A6257),
November 8, 1979 4
Dear Mr. Tiller:
The attachment completes the assessment of the effective-ness of the existing means to reduce the potential for steam generator water hamer at the Yankee Rowe Power Station (YRPS).
We have reviewed the operating history of the YRPS pertinent to steam generator water hamer and the related operational and procedural characteristics of the feedwater system. The review has shown that condi-tions ccnducive to steam generator water hamer have occurred at the YRPS but no water hamer. events have been observed subsequent to the 1966 feedwater system modifications. The conditions have been encountered during nomal operating transients and startup and shutdown operations. Such conditions would also be ex-pected in the future during the nomal and accident operating situations addressed in the review.
Based on this review we have concluded that the potential for steam generator water hamer is sufficiently low to pemit continued operation of this facility.
0002280 Y Y 1-
R. E. Tiller January 9, 1980 JAD-7-80 Page 2 This transmittal constitutes completion of the YRPS SER, Task A6257 of the Milestone Chart in the referenced letter.
Very truly yours, w&
J.
Dearien,
Manager Code Assessment and Applications Program DDC:tn
Attachment:
As stated cc: /S. D. MacKay, NRC-DOR R. W. Kiehn, EG&G Idaho w/o attach.
STEAM GENERATOR WATER HAMMER TECHNICAL EVALUATION YANKEE R0WE POWER STATION January 1980 EG&G Idaho, Inc.
CONTENTS I.
INTRODUCTION.
1 II. WATER HAMMER EXPERIENCE.................... 2 III. MEAN5 TO REDUCE THE POTENTIAL FOR WATER HAMMER
....... 3 IV. OPERATING EXPERIENCE AND WATER HAMMER SUSCEPTIBILITY..... 5 V.
CONCLUSIONS AND RECOMMENDATIONS................
8 VI.
REFERENCES..........................
9 O
I.
INTRODUCTION An evaluation qs performed for the Yankee Rowe Power Station (YRPS) feedwater system.
The purpose of this evaluation was to assess the susceptibility of the feedwater system to water hamer during operating transients and situations that could result in conditions conducive to water hammer.
Steam-water slugging resulting in water hamer in the steam generator feedrings and adjacent feedwater piping was considered in this review. This type of watar hamer is generally referred to as steam generator water hamer and is characterized by rapid steam condensation causing water slug acceleration and impact.
The impact of a water slug in the feedwater system has the potential for overstressing system components and supports.
The information for this evaluation was obtained from:
- 1) discussions with the licensee, 2) licensee submittals to NRC of July 18, 1975,1 November 2, 1977,2 May 8, 1979,3 December 7, 1979', and December 21, 1979, 3) the " Yankee Nuclear Power 5
Station Final Safety Analysis Report",6 4) "An Evaluation of PWR Steam Generator Water Hamer", NUREG-0291, and 5) Westinghouse Technical Bulletin, NSD-TB-75-7.8 A review of the steam generator water hamer experience at the YRPS is presented in Section II. The means to reduce the potential for water hamer at this facility are presented in Section III.
Section IV presents descriptions of the feedwater system geometry and operation pertaining to the susceptibility of the system to steam generator water hamer. This section also presents a description and tabulation of operating transients and situations that could result in conditions conducive to water hamer. Finally, conclusions and recomendations are presented in Section V concerning the susceptibility of the feedwater system at this facility to steam generator water hamer.
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II. WATER HAMMER EXPERIENCE During the " S4 and 1965 refueling outages, inspections revealed damage to the steam generator internals. Damage was found in three (unspecified) of the four steam generators in each inspection.
In 1964, the damage consisted of weld failures causing displacement of the feedring inlet cover plate. The damage found in 1965 consisted of broken feedring supports and weld failures on the feedring mixing tees. A maximum feedring displacement of two inches was found in one of the steam generators.
Subsequent to each inspection, necessary repair or replacement was made to all damaged components prior to resurption of reactor operation.
In 1964, the faudring cover plates were resecured with U-bolts. New feedring supports were installed in 1965 and shoe pads were added between the feedrings and the steam generator shell contact points.
The damage found during the refueling outages was reported to be the result of steam generator water hammer. The exact dates of the damaging water hamer events are unknown; however, many events had been observed since full power operation of the YRPS began in January 1961.
During the 1966 refueling outage, modifications (described in the next section) were made to the feedwater system to reduce the potential for damaging steam generator water hamer. Steam generator water hammer has not been observed since the modifications were made nor has any damage been found.
The integrity of the feedwater system and steam generator internals has been verified by inspections made during refueling outages from 1966 to the present.
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III. MEANS TO REDUCE THE POTENTIAL FOR WATER HAMMER The following means
.e employed to reduce the potential for damaging steam generator water hamer at the YRPS:
1.
" Loop seals" were installed in the feedwater piping to reduce the effec +' c horizontal length adjacent to the steam generators.
2.
A steam line was installed for preheating main feedwater during startup and shutdown.
3.
A bypass line was installed around each main feedwater containment isolation valve to maintain constant feedwater flow during periods of low demand.
These modifications were designed and installed in September and October of 1966.
An identical " loop seal" geometry was incorporated into the feedwater piping adjacent to each steam generator. Each seal consists of a U-shaped section of piping attached to eacn steam generator nozzle via a downward turning elbow. This arrangement reduces the effective horizontal length of piping connected to each steam generator feedring that could drain through the bottom discharge holes of the feedring.
By reducing this length of piping, the maximum volume which could fill with steam during periods of feedring uncovery is also reduced. Thus, reduction of the piping volumes will proportionally reduce the energy of potential water hammers due to steam-water slugging.
A two-inch steam line was installed to supply steam to the high pressure feedwater heater from the plant auxiliary boilers via the auxiliary steam supply system.
The line is used to preheat main 3
feedwater during startup, shutdown, and low power conditions. During these situations, the main steam supply from the steam generators is insufficient or unavailable for normal extraction feedwater heating.
The purpose of the preheater line 's to reduce the temperature differential (unmeasured qua..it,1 between incoming main feedwater and the steam in the steam generators.
By reducing the temperature differential, the steam condensation ratas are lowered.
Thus, if any steam-water slugging instabilities were to exist, the magnitude of the instabilities would be reduced.
A one-inch line with a bypass regulating valve was installed around each main feedwater containment isolation valve.
The bypass lines are used during startuo, shutdown, and low power conditions when feedwater requirements are below about 15% of the flow required at full reactor power (about 1S5 K4e).
The lines were installed to permit more accurate and responsive manual flow control than would be possible with the main feedwater regulating valves during low flow operation. The use of the bypass lines allows more accurate steam generator water level control to maintain the water levels above the feedrings to preclude admission of steam and thus avoid steam generator water hamer.
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IV. OPERATING EXPERIENCE AND WATER HAMMER SUSCEPTIBILITY The conditions most iducive to steam generator water hammer occur when the steam generator feedrings are uncovered and steam enters the feedrings and attached horizontal feedwater piping.
Steam-water slugging and subsequent water hac er may occur when incoming cold feedwater mixes with the steam in the piping and rapid condensation occurs. The conditions can be avoided by keeping the feedrings and associated piping full of water.
This can be accomplished by 1) keeping the water levels in the steam generators above the feedri..gs or 2) supplying feedwater at a higher flow rate than the rate at which feedwater drains through the discharge holes on the bottom of uncovered feedrings.
Following a reactor trip, the steam generator water levels drop by a quantity approximately proportional to the reactor power prior to the trip. For example, the greatest drop would occur at full power.
The drop or " shrinkage" in water level is the result of interrupted reactor power production and increased steam generator steam pressure causing the collapse of steam voids within the secondary side of the steam generators. A similar situation would also be experienced during events such as loss of main feedwater, loss of offsite power, steam line break, and loss-of-coolant accident.
A total of 102 reactor trips were reported to have occurred at the YRPS since initial startup. About 40 trips occurred at very low power levels during initial startup and usually did not result in feedring uncovery. The remaining 62 trips occurred during normal power operation and resulted in feedring uncovery in all of the steam generators.
There have been six loss of feedwater avents since 1961 and all were reported to have resulted in feedring uncovery.
Each event was either the result of a malfunction upstream of the main feedwater 5
pumps causing loss of suction to the pumps or incorrect feedwater regulation associated with valve malfunctions. Three of these events occurred since the modifications made in 1966 and no stetm generator water hammer has occurred since.
Information was not available to determine whether or not feedring uncovery events occurred during startup and shutdown conditions.
However, feedring uncovery would be a likely occurrence since steam generator water levels are maintained only about ten inches above the feedrings and feedwater flow is being controlled manually. Although the main feedwater bypass lines facilitate the maintenance of the steam generator water levels, perturbations such as steam generator blowdown and changing steam demand cause abrupt water level fluctuations.
Similar past reviews for other facilities have revealed the difficulty in maintaining proper water levels during operating situations requiring manual feedwater control.
It is apparent from the review of the plant operating history that it is not always possible to avoid drainage of the feedringt and adjacent piping by keeping the feedrings covered with water. Although
.the drainage time (or uncovery time) varied among the uncovery events, complete drainage of the feedrings and adjacent piping has been frequent since the time required for complete drainage is less than a minute and the uncovery times were generally of longer duration.
The alternative to continuous coverage of the feedrings with water is to maintain sufficient feedwater flow through uncovered feedrings to keep the feedrings full of water.
The flow required for this at the YRPS is approximately 160 gpm per steam generator based on static pressure drainage calculations. Since a feedwater flow rate of about 4300 gpm is required for a reactor power level of about 150 MWe (power and required flow are approximately proportiontal quantities),
the feedrings and feedwater piping would be kept full only with feedwater flow rates corresponding to power levels above about 22 MWe or 12". of full power.
Thus, during startup, shutdown, and low power 6
operation, feedwater flow requirements are insufficient to keep the feedrings full of water during periods of feedring uncovery.
This situation would also be expected during recovery from events resulting in reactor trips from power operation such as loss of main feedwater, loss of offsite power, steam line break, and loss of coolant accident.
Feedwater can also be supplied to the steam generators via the auxiliary feedwater system. Auxiliary feedwater, when required, is pumped into each of the four main feedwater lines at a point downstream of the main feedwater containment isolation valve.The single turbine driven auxiliary feedwater pump has a maximum capacity of about 90 gom and is reported to have been used only for hydrostatic testing and filling of the steam generators during cold startup and shutdown conditions.
Even if the auxiliary feedwater system were used for other operating situations when conditions conducive to water hammer could exist, the flow capacity of the system is too low to keep uncovered feedrings full of water.
The review of the operating history at the YRPS indicates that conditions conducive to steam generator water hammer exist during normal operating situations. However, it was reported that damaging water hammer has not occurred since the feedwater system modifications were made.
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V.
CONCLUSIONS AND RECOMMENDATIONS We have reviewed the operating history of the YRPS pertinent to steam generator water hammer and the related operational and procedural characteristics of the feedwater system. The review has shown that conditions conducive to steam generator water hammer have occurred at the YRPS but no water hammer events have been observed subsequent to the 1966 feedwater system modifications.
The conditions have been encountered during normal operating transients and startup and shutdown operations. Such conditions would also be expected in the future during the normal and accident operating situations addressed in the review.
Based on this review we have concluded that the potential for steam generator water inammer is sufficiently low to permit continued operation of this facility.
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VI. REFERENCES 1.
J. L. French, Yankee Atomic Electric Company (YAEC), letter to R.
A. Purple, NRC, Subject
" Response to May 13, 1975 NRC letter on Steam Generator Water He,mmer", July 18, 1975.
2.
D. E. Moody, YAEC, letter to A. Schwencer, NRC, Subject
" Feed Line Water Hammer", November 2, 1977.
3.
H. A. Autio, YAEC, letter to D. L. Ziemann, NRC, Subject -
"Respense to May 4, 1979 NRC Questions on Auxiliary Feedwater",
May 8, 1979.
4.
D. E. Moody, YAEC, letter to D. L. Ziemann, NRC, Subject
" Water Hammer", December 7, 1979.
5.
D. E. Moody, YAEC, letter to D. L. Ziemann, NRC, Subject
" Water Hammer", December 21, 1979.
6.
Final Safety Analysis Reoort, Yankee Nuclear Power Station, YAEC, NRC Docket No. 50-29.
7.
J. A. Block, et al, An Evaluation of PWR Steam Generator Water Hammer, Creare, Inc., NUREG-0291 (December 1976).
8.
W. E. Bennett, Waterhammer in Steam Generator Feedwater Lines, Westinghouse Technical Bulletin, NSD-TB-75-7 (June 10, 1975).
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