ML19256F395
| ML19256F395 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 12/03/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19256F390 | List: |
| References | |
| NUDOCS 7912190077 | |
| Download: ML19256F395 (17) | |
Text
,
SAFETY EVALUATION REPORT BY THE OFFICE OF NUCLEAR REACTOR REGULATION U. S. NUCLEAR REGULATORY COMMISSION REGARDING THE POTENTIAL FOR WATER HAMMER IN FEEDWATER PIPING AT FORT CALHOUN STATION UNIT NO.1 DOCXET NO. 50-285
~
1625 105 7912100 b3h
TABLE OF CONTENTS 1.0 Introduction 2.0 Feedwater Systems 2.1 Description 2.2 General Operation 3.0 Means to Reduce the Potential for Water Hammer 3.1 Description 3.2 Effectiveness During Transients 3.2.1 Plant Trip 3.2.2 Loss of Main Feedwater Flow 3.2.3 Loss of Off-Site Power 3.2.4 Operator Error 3.2.5 Steam Line Break 3.2.6 Loss of Coolant Accident 4.0 Conclusions 5.0 References 1625 106
1.0 INTRODUCTION
Steam generator water hacrner has occurred in certain nuclear power plants as a result of the rapid condensation of steam in a steam generator feedwater line and the consequent acceleration of a slug of water which upon impact within the piping system causes undue stresses in the piping and its support system. The signi-ficance of these events varies from plant to plant. Since a total loss of feedwater could affect the ability of the plant to cool down after a reactor shutdcwn, the NRC is concerned about these events occurring, even though an event with potentially serious consequences is unlikely to happen.
Because of the continuing occurrence of water hammer events, the NRC, in September 1977, informed all PWR licensees that water harmer events due to the rapid condensation of steam in the feedwater lines of steam generators represented a safety concern and that further actions by licensees for Westinghouse and Combustion Engineering designed nuclear steam supply systems are warranted to assure that an acceptably low risk to public safety due to such events is maintained. Accordingly, these licensees were requested to submit proposed hardware and/or procedural modifications, if any, which would be necessary to assure that the feedwater lines and feedrings remain filled with water during nonnal as well as transient operating conditions.
At the same time, the NRC proviced each PWR licensee with a copy of its consultant's report, "An Evaluation of PWR Steam Generator Water Hammer," NUREG-0291. (Ref. 5.8) 1625 107
. The means employed at the Fort Calhoun Ste. tion to reduce the potential for steam generator water hamer include:
(1) a downward turning elbow on each steam generator nozzle that eliminates the horizontal feedwater piping at the entrance to the steam generator and (2) a separate auxiliary feedwater nozzle in the steam generator that is used when main feedwater is not available or under emergency conditions when the main feedwater isolation valve is closed. A steam generator water hammer has not occurred at the Fort Calhoun Station and a j
review of the operating experience at this plant indicates that it is not susceptible to steam generator water hamer.
The information for this review was obtained frem the references listed in Section 5 of this report.
2.0 FEEDWATER SYSTEMS 2.1 Descriotion The feedwater system for the Fort Calhoun Station Unit No.
1 provides an adequate supply of water for the production of steam under all normal plant load conditions.
Condensate is pumped from the condenser not wells by 2 of 3; fifty percent capacity, electrically driven, condensate pumps through the feedwater heaters to three, fifty percent capacity, electrically driven, main feedwater pumps. The feedwater is then pumoed through one stage of hign pressure feecwater heating to -t:1e two steam generators. The ficw to each steam generator is controlled separately by a 1625 108
. main feedwater control valve.
Each main feedwater line approaches a steam generator at c 45' angle from vertical and connects to a 45' elbow that is welded directly to the nozzle of the steam generator.
Inside the steam generator, the feedwater flows into a feedring that distributes the flow around the inside wall of the steam generator. The feedring has two vent holes (175 inch Dia.) in the top surface,and the feedwater is discharged through holes (1.375 inches Dia.) in the bottom of the feedring. At this point the feedwater mixes with recirculatory water and passes downward around the tube bundle.
The auxiliary feedwater system supplies water to the steam generators during shutdown, startup and low pcwer operation.
Adequate cooling water can be supplied to both steam generators by either of two pumping systems. The one electrically driven pump or the turbine driven pump can provide sufficient water (260 gpm) for the removal of decay heat following a reactor trip.
Auxiliary feedwater can be directed through two different paths to each steam generator: one line discharges into the main feedwater line of the steam generator upstream of the main feedwater control valve; the other line discharges directly into the steam generator via a separate four inch diameter nozzle. This emergency feecwater nozzle is used wnen the main feedwater line is closed.
5 109
. 2.2 General Coeration Normally, the main feedwater system is used to remove decay heat immediately after power operation when outside electric power is available. If outside power is not available, or if operating ccnditions require condenser shutdcwn, the auxiliary feedwater pumps are used to remove decay heat.
The emergency feedwater nozzle is used when the main feedwater line is closed.
Following a containment isolation signal, t.'e valves that permit flow to the emergency feedwater nozzles would automatically open fully.
in approximately seven seconds.
The auxiliary feedwater pumos would be started during a loss of feedwater transient; both auxiliary feedwater pumps would automatically start when the last operating ma n feedwater pump tripped.
The valve that admits steam to the turbine driven pump would be opened; and, if offsite power were available, electrical power would be supplied to the motor driven pump. On loss of offsite power, only the turbine driven pump would start automatically; and the motor driven pump could be started after this pump is switched to the on-site emergency power, by the operator.
I625 1Iu 3.0 MEANS TO REDUCE THE POTENTIAL FOR WATER HAMMER 3.1 Descriotion The conditions most conducive to steam generator water hamer occur when the steam generator feedrings are uncovered and steam enters the feedrings and attached horizontal feedwater piping. Steam-water slugging and subsequent water hanner may occur when incoming cold feedwater or auxiliary 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 byll) keeping the water levels in the steam generators above the feedrings, 2) supplying feedwater at a higher flow rate than the rate at which feedwater drains through the discharge holes on the bottom of uncovered feedings, 3) having a sufficiently short length of horizontal piping such that slug fonnation does not occur, or 4) refilling the steam generator through a separate nozzle such that only hot water, at the saturation temperature of the steam, enters the feedwater ring and piping when it is being refilled.
At the Fort Calhoun Station Unit 1, the following features reduce the potential for water hamer: 1) there is no nori: ental piping connected to the steam generator feedwater noz:le and 2) a separate no::le in the steam generator is used to refill the steam generator when de main 1625 111
~
feedwater path is closed.
Fort Calhoun Station has the shortest practicable horizontal run of main feedwater piping adjacent to the steam generator.
A downward 45' elbow is connected directly to the steam generator nozzle and the horizontal dimension from the nozzle to the centerline of the elbow is ten inch:s. The horizontal distance from the center line of the albow to the feedwater sparger is 29 inches. With this short horizontal distance, the probability of severe water hammer is low because: 1) the probability of fomation of a slug of water is low with this small ratio of the length of the piping to its diameter; and 2) the short horizontal length limits the volume of steam that could be trapped and thereby limits the energy of a water hammer-if it were to occur.
The separate emergency feedwater nozzle is used to refill the steam generator with auxiliary feedwater when the main feedwater flow path is closed. Steam generatt-water hammer has been experienced in other plants wnen cold auxiliary feedwater was directed through the horizontal feedwater pioing that was filled with steam.
By diverting this cold water away from the main feedwater line and directly into the steam generator via a separate nozzle, the water that even-tually enters the horizontal piping as the level rises will~
come from the surface of tae water in the steam generator and will have the same temperature as the steam in the 1625 112
. feedwater ring and piping. Under these conditions there is no potential for steam generator water hammer.
3. 2.
Effectiveness Durina Transients 3.2.1 Plant Trio A plant trip, i.e., rapid shutdown of the reactor and turbine generator, would result in a reduction of the volume of steam bubbles in the steam generators and the water level in both steam generators would fall below the main feedwater ring. Main feedwater would continue to flow to each steam generator but would be reduced by partial closure of each main feedsater regulating valve.
Each main regulating valve is programmed to close at a rate of approximately 3% of full closure per second until it reaches a position such that the valve is 57. open. Main feed-water continues to ficw until the steam generator water level is restored. Nonnally, the main feedwater system is used to remove decay heat immediately after power operation when outside power is available.
If outside power were not available or if operating conditions required condenser shutdown, then the auxiliary feedwater system would be used to re'nove decay heat.
In this case auxiliary feedwater would be pumoed into the main feedwater line upstream of the main feedwater regulating valve.
1625 113 A review of the records of steam generator water levels following a plant trip at the Fort Calhoun station showed that the steam generator water levels had dropped below the feedring after a plant trip, but it was not determined whether the ring drained. However, during those periods of time, there were no water hamme-occurrences observed by operating personnel; and there has been no evidence of equipment deterioration typically encountered under significant flow instability conditions. The absence of water hammer occurrences since plant startup in 1973, and the absence of any water ham:ers during those times when water hammers would most likely occur, provides assurance that fluid flow instabilities will not occur as a result of plant trip.
3.2.2 Loss of Main Feedwater Flow The loss of flow of main feedwater would result in a plant trip. The reactor is tripped by a low water level in either steam generator. The water level in the steam generator would drop to some level belcw the feedring as in the case of a plant trip described above. However, since in this case the feedwater flow is completely stopped, the water in the feedring would drain and be replaced with steam.. The introduction of cold feedwater into the steam filled piping at tnis time would establish the conditions 2 cst conducive 1625 114
~
_g-to steam generator water hammer. Upon tripping of the last operating main feedwater pump, both auxiliary feedwater pumps would start and deliver water into the main feedwater piping. At the Fort Calhoun station the auxiliary feedwater system has been operated to refill the steam generator through the main feedwater piping under these conditions and no water hamer occurred.
Four loss of feedwater events have occurred in the past 3 years at the Fort Calhoun station. One event resulted from a loss of offsite electrical power; two events were due to a malfunction of a main feedwater regulating valve; and one event was caused by a loss of electrical. power to the feed-water system. Recovery of steam generator water level after these events without watcr hammer indicates that the Fort Calhoun station is not susceptible to steam generator water hammer.
3.2.3 Loss of Offsite Power A loss of offsite power would result in a loss of feedwater flow, a plant trip and loss of steam generator level as described above. The steam driven auxiliary feedwater pump would start and deliver water to the main feedwater piping.
The operator may start the motor driven pumo after switching it to the onsite emergency power source. At least 1625 115 one of the events cited above followed this scenario and no water hammer resulted.
3.2.4 Ocerator Error It is not likely that an operator error would result in a water hammer.
This plant has been subjected to those conditions that are most likely to cause water hamer and no water hammer has occurred.
Operator error could increase the frequency with which the feedwater piping is subjected to conditions conducive to water hamrer but would not materially change those conditions. Since plant operations have demonstrated that water hamrer does not occur under those conditions, operator errors can have little influence en the occurrence of water h ammer.
3.2.5 Steam Line Break The possibility of water hammer occurring as a result of or in conjunction with a steam line break is considered in order to determine whether this water hammer could cause a ructure that would result in the blowdown of more than one steam generator or could result in the loss of capability to supply auxiliary feedwater.
1625 116
. In the event of a steam line break, the resulting containment isolation signal would initiate the flow of auxiliary feedwater directly into the steam generator thmugh the emergency feedwater nozzles. This made of operation reduces the probability of obtaining conditions that could lead to water hammer, i.e., even if the feed-water sparger were drained, the cold auxiliary feedwater would be directed to flow through the emergency feedwater nozzles and would not flow in through the main feedwater piping and sparger and therefore a water hamer could not be induced in the sparger.
The probability of occurrence and the consequences of water hammer in the emergency feedwater lines and auxiliary feedwater lines have been examined by the licensee.
Even though there is no auxiliary feedwater sparger, the possibility of steam-water slugging in the emergency feedwater nozzle and piping was considered (Ref. 5.5); and, the licensee concluded that it would not occur in these lines.
The emergency feedwater discharges directly into the steam space in the steam generator via a 4-inch diameter pipe that is two feet long. This pipe is connected to a check valve and 3.5 feet upstream of the check valve there is a control valve. A calculation of the heat transferred to the region between the check valve and the control valve snowed that the wa
- in this region would not boil and cause the check valve to open and allow the water to drain out.
It 1625 117
. further showed that even if there were some leakage past the check valve and this region were somehow initially filled with steam, sufficient steam would condense in approximately one half hour to refill this region with water. The pipe between the check valve and the steam gener-ator, although filled with steam, is sufficiently short so that steam generator water hamer is unlikely in this region.
Water hammer that might result directly from the opening of the auxiliary feedwater control valve was also considered.
An analysis using the PTHRUST program (Ref. 5.2) showed that the most severe hydraulic transient in the auxiliary feedwater piping would result from the opening of the emergency feedwater control valve but that the resulting forces, based on ADL program calculations, would not cause an overstress condition in any of the emergency or auxiliary feedwater piping.
Thus, the consideration of water hammer in the feedwater systems of the Fort Calhoun Station would not affect the consequences of a steam line break.
3.2.6 Loss of Coolant Accident (LOCA)
The possibility of water hamer in the feecwater piping occurring in conjunction with a LOCA is considered in order to detemine whether the consequences of a LOCA might be increased by causing a rupture that would result in the blowdown of a steam generator during the LOCA. At the 1625 118
~
. ". ort Calhoun station, the conditions necessary to produce a water hammer after a LOCA are avoided by the same means employed in the case of a steam line break as discussed above. These means will be effective in avoiding water hamer because the conditions in the steam generator and feedwater piping that might be conducive to water hammer will not be substantially influenced by whether such conditions result from a steam line break or a LOCA.
4.0 CONCLUSION
S _
We have reviewed the operational characteristics and performance of the Fort Calhoun Station pertaining to steam generator water hammer.
The Fort Calhoun plant has been subjected to those conditions most likely to cause steam generator water hammer but none has occurred.
These same conditions would be expected to result in the future during nonnal operations and under transient and accident conditions.
Therefore, since a water hammer has not occurred at Fort Calhoun during such conditions, none is expected in the future.
The successful operation of the Fort Calhoun plant without steam generator water hammer is attributable to the geometry of the feedwater piping and feedring in the area of the steam generator nozzle. The total length of the water surface possible in the feedring tee, the nozzle and the downward elbow is apparently short enough to preclude slug formation and the resulting water hammer.
1625 119
~
.. However, even though steam generator water hammer is not likely to occur, the licensee should be vigilant and monitor for water hammers that might impose significant stresses on the piping systems or their supports. We will continue to twr.itor licensee event reports from this licensee for indications of possible water hamer.
If such indications appear in the future, this matter will be reexamined and may result in additional requirements to reduce the probability of steam generator water hamer at these facilities.
Based on our knowledge of water hamer phenomena and our review of the operational characteristics and perfonnance of the Fort Calhoun Station, we have concluded that steam generator water hammer is not likely to occur at this facili ty.
We, therefore, find no undue risk to the health and safety of the public as a result of the continued operation of the Fort Calhoun Station.
1625 120
5.0 REFERENCES
5.1 T. E. Short, Omaha Public Power District (OPPD) letter to G. E. Lear, NRC, transmitting " Secondary System Fluid Flow Instability Report," dated July 23, 1975.
5.2 T. E. Short, OPPD, letter to G. E. Lear, NRC, " Addendum to Secondary T; stem Fluid Ficw Instability Report," dated February 27, 1976.
5.3 T. E. Short, OPPD, letter to G. E. Lear, NRC, indicating no action needed regarding water hammer, dated November 1, 1976.
5.4 T. E. Short, OPPD, letter to G. E. Lear, NRC, in response to September 2,1977 letter from NRC, dated December 29, 1977.
5.5 T. E. Short, OPPD, to R. W. Reid, NRC, letter dated July 19, 1978, transmitting report " Emergency Feedwater Hammer Analysis for Fort Calhoun Station Unit 1," by Nuclear Service Corporation.
5.6 T. E. Short, OPPC, to K. V. Seyfrit, NRC: Region IV, dated May 25, 1979, in response to IE Sulletin 79-06B.
5.7 Final Safety Analysis Report, Fort Calhoun Station Unit No.1, NRC Docket No. 50-285.
1625 12I
. 5.8 J. R. Block, et al, An Evaluation of PWR Steam Generator Water Hammer, Creare, Inc. NUREG-0291 (December 1976).
5.9 W. E. Bennett, Waterhammer in Steam Generator Feedwater Lines, Westinghouse Technical Bulletin, NSD-TB-75-7 (June 10, 1975).
1625 122
.