ML19282B224
| ML19282B224 | |
| Person / Time | |
|---|---|
| Site: | Surry, Calvert Cliffs  |
| Issue date: | 06/07/1978 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19282B225 | List: |
| References | |
| NUDOCS 7903090418 | |
| Download: ML19282B224 (10) | |
Text
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4 SAFETY EVALUATIC'N RE? ORT BY TH1 0FFICE OF NUCLEAR REACTOR REGULATION U. S. NUCLEAR REGULATORY CCMMISSION REGARDING WATER HAhMER IN FEEDWATER PIPING AT SURRY POWER STATION, UNITS 1 AND 2 00CXETS NO. 50-280 AND 50-251 rg903090
Table of Contents 1.
Introduction........................
1 2.
General Consi derations...................
2 2.1 Feecwater Piping Geometry................[2((
2.2 Waternammer Experience.................
3 3.
Means for Maintaining Feedwater Lines Filled with Water.
.4 3.1 Systems Description..................
4 3.2 Effectiveness During Transient.
.5 3.2.1 Pl ant Tri p...................
5 3.2.2 Feedwater Pump Failure.............
6 3.2.3 Loss of Offsite Pcwer....
.6 3.2.4 Ope ra tor Error..............
.7 3.2.5 S team Line Brea k................
7 3.2.6 Loss of Coolant Accident.
.8 4.
Conclusion......
.8
1.0 INTRODUCTION
The NRC Staff continually reviews experience from operating reactors to assure that an adequate level of safety is maintained at each individual nuclear plant and for the total population of nuclear plants. As new technical information and operating experience becomes available, the NRC evaluates whether such infonnation could significantly alter the previously determined levels of safety.
In this regard, we have noted that there have been about 50 reported water hamer events in light water reactor'., some of which resulted in structural damage to safety related systems. Of these, approxi-mately 20 water hamer events have occurred due to the rapid conden-sation of steam in feedwater lines of plants that use Westinghouse and Combustion Engineering designed steam generators. While for the most part darage from these events has been limited to piping supports, one event in the feedwater line of a pressurized water reactor did result in a significant piping failure. None of the events to date has resulted in an adverse impact on the health and safety of uhe public.
It is possible, however, that water hamer events could lead to more severe consequences; although it is expected that the probability of such events is very low.
Because of the continuing occurrence of water hamer events, the NRC staff in May 1975 initiated action toward the evaluation of the potential for water hamer events in the feedwater lines of all pressurized water reactors (PWRs). Letters were sent to all pWR licensees that requested them to (1) descriLe operational events that could lead to water han:ner events; (2) describe their piping systems; (3) describe any water ha mer events that may have occurred in the feedwater system; (4) describe all dynamic analyses of the feedwater and auxiliary feedwater systems; (5) discuss the possib. ity for water hamer events follcwing a loss-of-cociant accident; and (6) describe existing plans for modifications to preclude flow insta-bilities.
The variety of the approaches taken by licensees to solve this problem and some of their attempted quantitative analyses demon-strated that a better understanding of the steam-water slugging pher.omenon was needed before an optimal solution could be achieved.
Tcward this end, the NRC in June 1975 engaged a consulting firm to insestigate the phenomenological effects involved and attempt to quantify the hydraulic forces that might be generated by steam-water slugging. This effort did not result in a method of analysis that could accurately predict the onset of slug formaticn or the probable
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magnitude of the water hamer forces.
However, the magnitude of the forces that were pessi,le, as shown by upper bound calculaticns, and the damage that resulted from scme of the water hamer experiences indicatec that action snould be taken to eliminate those conditions that could lead to slug formation in the feedwater piping.
Cne could either prevent steam from entering the feedwater pipe or prevent subccoled water from entering it while steam was in the pipe.
, In Septemoer 1977, the NRC informed all PWR licensees that water hammer 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 Westingneuse and' Combustion Engineer-ing designed nuclear steam supply systems are warranted to assure that an acceptably low risk to public safety due to such events is maintained. The staff concluceo that such assurance would be pro-vided if it could be demonstrated that the feedwater lines and feed-water spargers would remain sufficiently filled with water to preclude water ham.er events under normal and transient operating conditions.
Accordingly, these licensees were requested to submit proposed plant design and/or procedural modifications, if any, which would be neces-sary to assure that the feedwater lines and spargers remain filled with water during normal as well as transient o;.arating conditions. At the same time the NRC pr:vided each PWR licensee with a copy of its con-sul tant's reocrt, "An Evaluation of PWR Steam Generator Water Hamer,"
We have reviewed the licensee's submittals of May 10,1976 and Decemoer 2,1977 regarding water hammer events in the feedwater systems of the Surry Power Station, Units 1 and 2.
This report sumarizes our evaluation of the means provided by the licensee for preventing water harm:er in the feedwater piping at the Surry Power Station and presents our conclusion with regard to their acceptability.
2.0 GENERAL CONSDERATIONS 2.1 Feedwater Piping Geometry The feedwater piping at the Surry Power Station was modified to reduce the likelihood of water hamer.
The present feecwater piping from the steam generator turns upward, then downward and back to a horizontal pipe at the same elevation at its point of attacthment to the steam generator. This inverted U-bend or loop seal was installed to reduce the length of piping that could be filled with steam if tne piping were to drain into the steam generator.
By reducing this length of piping, the potential for water hamar was also reduced.
H0 wever, it is not certain that these particular seals would by themselves prevent water hamar under all conditions; therefore, the feedrings were converted from bott:m discharge to t p discnarge by plugging the holes in the bottom of the rings, drilling holes in the too and adding J-tubes to direct tne ficw dcwnward. With this arrangement, the feedring will not drain rapidly wnen the water level in the steam generator is below the ring.
Mcwever, it will drain slcwly via the clearance between the feedring no::le and the steam generator noz:le into which it is fitted.
It is not expected that significant drainage would occur for at laast five minutes.
4 5.. _
The replacement steam generators to be installed at Surry Power Station during 1978-1979 will incorporate two additional design features for water han=er pre-vention.
The proposed steam generator modifications in-clude the use of a replacement feedwater nozzle which incorporates a full penetration weld between the nozzle
.and feedring rather than the present thermal sleeve.
This will prevent the small amount of leakage of water from the feedring which now occurs when the steam generator water level is below the feed ring.
In addition, the proposed new feedrings will be offset approximately 2-1/2 inches in elevation above tne center line of tne feed nozzle.
This offs,et would further delay the draining of the feed piping.
2.2 Water Han:ner Experience Surry Pcwer Station, Unit 1 experienced a severe water hammer event on October 1,1972, during hot shutdown conditions.
The unit was being maintained in a hot shut-down condition from the auxiliary control panel when a loud noise occurred followed by automatic initiation of the Safety Injection System.
The containment building pressure rose to 16.2 PSIA from its normal range of 9 PSIA to 11 PSIA and the reactor coolan_t temperature decreased 530 F in 20 minutes which is greater than the 1000
~
F per hour all'w'e'd by the technical specifications T ~ ~
o The pressurization of the building was the result of the blowdown of steam generator A through a damaged check valve in the feedwater line.
The following mechanical effects were reported:
(1) The 14 inch feedwater line for steam generator A was displaced approximately 7 to 10 inches in a clockwise direction around the containment building wall and radially inward toward the containment centerline approximately 4 to 5 inches.
(2) All seven spring hanger assemblies used tc succort the feecwater line were displaced to varying degrees.
There was no damage to any embed.ents.
(3) All seven hydraulic shock suppressors installed for seismic restraint of the feedwater line failed, either in shear or tension, from forces imparted by the water harrt:er.
(4) The bolting flange on the body of the check valve was distorted and a 1800 segment of the metal-sheathed asbestos gasket was missing. There was a crack approximately 1 1/4 inches in length in the valve body and damage to the seating surfaces of the disc and the seat.
The corrective action taken to prevent recurrence of this type of water hamer event was to install the inverted U-loops in all steam generators for Units 1 and 2.
Since then no further water hamer event of thi~s type has been experienced. However, in order to provide further assurance that this type of water harmer event would not recur, J-tubes were installed in 1975 and 1976 cn all of the feec rings in both units. The operating exoerience at tne Surry Power Station indicates that the measures taken to prevent water harrer in the feed-water piping have been successful.
3.0 MEANS FCR MAINTAINING FEEDWATER PIPING FILLED WITH WATER
3.1 System Description
Since the feedrings now installed at Surry are equipped witn J-tubes, only a small amount of feedwater flow (on the order of 10 gpm or less) is sufficient to keeo the feec rings full of water.
The auxiliary feedwater systam can easily supply the necessary flow and it operates automatically to maintain the water level above the feed-ring in the steam generator.
If the water level should drop below the feedring, the auxiliary feedwater flow is sufficient to make up for the leakage from the ring and keep it full of water.
The auxiliary feedwater system consists principally of a full-size turbine driven auxiliary feedwater pumo rated for 700 gpm, two half-size motor driven auxiliary feed-water pumos rated for 750 gpm, a 100,000 gallon storage tank anc associated pi;. ng, valves and controls. Operatice i
. of the system is automatically initiated by any one or more of the following conditions:
(1) a icw water level indication for the steam generators; (2) a loss of off-site power; (3) opening of the main feed pump breakers; or (4).any safety injection signal after a 50 second delay.
The two electric driven auxiliary feedwater pumps are automatically started in the event of a low water level condition in any one of the three steam generators, or the loss of off-site power or the opening of both main feedwater puma breakers, or by a safety injection signal after a 50 second celay.
These electric pumps are not dependent en offsite power. The steam driven
- pump starts autcmatically in the event of a loss of offsite power or in the event of low water level in any two of the steam generators.
Thus there are two diverse pumoing systems that operate automatically to supply auxiliary feecwater to the steam generators.
3.2 Effectiveness During Transients 3.2.1 Plant Trip Curing normal operaticn the water level in the steam generator is maintained above the feedring nd therefere steam cannot enter the feedring to react with cold feedwater.
Mcwever, in the event of a plant trip sucn as a turbine trip with reacter trip, the main feedwater ficw is stopped and the water level in the steam generator drops below the level of the feedring and the feedring begins to drain.
Because the Surry steam generators are equipped with J-tubes on the feed rings, the cnly leakage from a ring will be through the clearance betweer. the feed-ring noz:le and the steam generator ncz:le.
Significar:
leakage from the feedring would not be expected to occur for at least 5 minutes.
Mcwever, within 1 to 2 minutes after the plant trip, auxiliary feedwater ficw would be automatically initiated by either the ccening of the main feed pumo breakers er a low water level concition in any one of the three steam generat:rs.
The auxiliary feedwater #10w is more than sufficient to keep the feedring full of water as it ficws througn the feedring to fill the steam generator to a level above the feedring.
The feedring and the feec-
/
water piping are kept full of water during the transient resulting from a plant trip and steam water slugging is thereby prevented.
3.2.2 Feedwater Pump Failure In the event of failure of both main feedwater pumas, auxiliary feedwater ficw would be automatically initiated by the opening of the main feedwater pump breakers or the resultin.g low water level in the steam generators.
The auxiliary feecwater ficw will keep the feedwater ring and piping full of water, thus avoiding conditions conducive to water hammer events.
3.2.3 Loss of Offsite Power loss of offsite power will not result in the d. raining of the feedwater sparger because the J-tubes will prevent appreciable leakage frem the sparger until auxiliary feedwater flow is automatically established to keep the sparger full. The steam driven auxiliary feecwater pump is automatically started in the event of a loss of off-site pcwer or in the event of low-Icw water level signals' frem any two of the three steam generators.
The two electric motor driven auxiliary feecwater pumos will start as a result of a low-Icw water level in any one steam generator or loss of statir.. reserve pcwer or loss of both main feedwater pumps or a safety injection signal.
The electric driven pumps operate as a back-up for the steam driven pumps in the event of a loss of off-site power.
The turbine driven auxiliary feedwater puma is not dependert en AC electrical power and can be used as long as adequate steam is available from any one of the three main steam lines upstream of the main steam isolation valves.
Thus the features of the feedwater systems that prevent water hammer events will be fully effective even in tne event of loss of off-site pcwerl
3.2.4 Operator Error The probability of water hammer events caused by operator error has been reduced by the automatic operation of auxiliary feedwater controls to prevent draining of the feedwater sparger. However, if the feedwater sparger were to be drained, special pro-cedures would be carried out for the recovery of the steam generator water level.
Operating procedures are in effect at Surry Power Station wi ~ 5 control operator action in the event of low stec.a generator water level.
The procedures limit the auxiliary feedwater flow to less than 200 gpm under conditions of start-up, steam generater isolation, or hot standby.
The automatic operation of the auxiliary feedwater control system, together with the use of conservative procedures during times when the sparger mignt be drained, provide reasonable assurance that water hammer events will not occur in the feedwater system as a result of operator error.
3.2.5 Steam Line Break The possibility of water hammer events occurring as a result of or in conjunction with a steam line break is also considered in order to determine whethe this water hammer event could occur and cause a ruc-ture that would result in the blowdown of more than one steam generator or result in the loss of capa-bility to supply auxiliary feedwater.
In the event of a steam line break, the flow of auxiliary feedwater would be automatically initiated by a low water level signal from any two steam generators or opening of the main feed pump breakers or, after a 50 second delay, by a safety injecticn signal. Thus the feedrings in the unaffected steam generators will be kept full of water by the automat-oceration of the auxiliary feedwater system.
a
-w.
.a.
The steam for the steam driven auxiliary feed-water pump is supplied from all three main steam lines upstream of the containment isolation valves.
Check valves in the steam supply lines prevent steam from flowing into a ruptured main steam line so that an adequate supply of steam will reach the turbine for the staam driven pump.
Thus the means for precluding water hammer in the unaffected steam generators would be fully effective under the canditions of a steam line break.
3.2.5 Loss of Coolant Accident (LCCA)
Since the consequences of a LCCA might be increased by the bicwdown of a steam generator during the LOCA, the possibility of a steam generator water hammer event resulting from the plant trip or safety injection operations initiated by the LCCA should be considered.
At the Surry Pcwer Station, a LCCA would result in a safety injection signal that would autcratically initiate the ficw of auxiliary feedwater.
Thus the conditions necessary to produce a water hammer event after a LOCA are avoided by the same means employed in the case of an ordinary plant trip or loss of off-site power discussed above.
These means will be effective in precluding water hammer events because the canditions in the steam generator and feedwater picing that mignt be ccnducive to water hammer will not be substantially influenced by wnether er not the plant tris is the result of a LCCA.
4.0 CCNCLUSICN We have reviewed the licensee's submittals of May 10, 1975 and Cecember 2,1977 regarding water hamrer in :ne feedwater systems of the Surry pcwer Staticn, Units 1 and 2.
Based on this review we find that the provisions for minimi:ing the likelihocd of water hammer avents due to the racid :cncensatica Of of steam in the feedwater systems at this facility are acceptacle.