ML20125D630

Safety Evaluation Re Potential for Water Hammer in Feedwater Piping at Facility

ML20125D630
Person / Time
Site:	Fort Calhoun
Issue date:	12/20/1979
From:	Office of Nuclear Reactor Regulation
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ML20125D628	List: ML20125D625 ML20125D630
References
REF-GTECI-B-06, REF-GTECI-PI, TASK-B-06, TASK-B-6, TASK-OR NUDOCS 8001140567
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SAFETY EVALUATION :EPORT v

BY THE OFFICE OF NUCLEAR REACTOR REGULATION

[Ih5. NUCLEAR REGULATORY COMMISSION, REGARDING THE POTENTIAL FOR WATER HAMMER IN FEEDWATER PIPING AT FORT CALHOUN STATION UNIT NO.1 DOCKET NO. 50-235 90018003 m

TABLE OF CONTENTS 1.0 Introduction' 2.0 Feedwater Systams 2.1 Description 2.2 General Oseration 3.0 Means to Reduca the Potential for Water Hamar 3.1 Descripti:n 3.2 Effectiveness During Transients 3.2.1

?lant Trip 3.2.2 Less of Main Feedwater Flew 3.2.3 Less of Off-Site Power

.2.4 Operator Error

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3.2.5 Steam Line Break 3.2.5 Less of Coolant Accident 4.0 C:nclusions 5.0 aeferences 90018004 e

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1.0 INTRODUCTION

Steam generator water hamer has occurred in certain nuclear power plants as a result of the rapid condensation of steam in a steam generator feedwatar line and the consequent acceleration of a slug of water wilich 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 reacter shutdown, the N'RC is concerned about these events occurring, even though an event with. potentially serious consecuences is unlikely to happen.

Because of the continuing occurrence of water hammer events, the NRC, in Se:tancer 1977, informed all PWR licensees tnat water ~

han:ner events due to the rapid concensation of steam in the feedwater lines of steam generators represented a safety concern and that further actions by licensees for Westinghouse and Combustien Engineering designed nuclear steam supply systems are warran ec 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 :cdifications, if any, which would be necessary to assure that the feedwater lines and feedrings remain filled wi water during nor nal as well as transient operating conditions.

At the same time, the NRC provided each PWR licensee with a copy of its cor.sultant's report, "An Evaluation of PWR Steam Generator Water Ha=ar," NUREG-0291. (Ref. 5.8) 90018005

2 The means e ;1oyed at the Fort Oalhoun 5:ati:n a reduce the potential f:r stec generator water hamer include:

(1) a downward turning elb w on each steam genera =r nozzle that eliminates the horizontal feedwater piping a: tne entrance to the staa::: generator and (2) a separate auxiliary feedwater nozzle in the stem senerator. hat is used wnen main feedwater is not avai able or unde.r emergency conditions when the main feedwater isolatica valve is eiesed. A staar generator water nammer has re: c:=rnd at the Fer: Calheun ita-ion and a J

review of the operating experien:e at this slan: indicates j

that it is r.c: sus:estible to staa:: genera :r water hamer.

Tne inforta:fc. for this revdew.as cotained fr:m tne references l

listed in Se::icn i of this re:c..

d 2.0 FEEDWATER SYS IMS 2.1 Jes e-i::i:n The faer aar system for the Fort Calh:un Station Unit No.

1 pr:vides an ace:;uate su; ply of wa.er for the production of stas.= uncer all nomal p an: leac =ndi:icns. Condensate is pc=ec frc: the c ncenser het wells by 2 of 3 fifty j

er

en: :a:act y, ele:trica:ly driven, ::n:ensata pumps throu;n ne hecwater heata-s :: :n se, fift/ percent caca:ity, ele::rically : riven, main fee sa ar cumps.

The feerea =

de -aen cu=ec - O;;n :ne stage f hi;n pressure feecwater nesting o ne s: s nam genera =rs.

Tne ficw o et= s aa: ;enera=r is =r.:r:llec se:arately by a I

90018006

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. main feehater controi valve.

Each main feedwater line appr: aches a steam generator at a 45' angle from vertical and ccnnects to a 45' elbow that is welded directly to the no::le of the steam genera *ar.

Inside the steam generator, the feedwater flows into a feedring that distributes the flow around the inside wall of the steam generador.

The feedring has two vent holes (175 inch Dia.) in the top surface;and the feedwater is cisenargec through holes (1.375 inches Dia.) in the bottom cf :he fendring.

A: this point the f6scwater mixes with recir:ulatory wa:ar t.nd ; asses downward around the tube bundle.

The auxiliary feedwater systec su: plies water to the stea:n generaten during shutdewn, startup and low pcwer c:erati:n.

Acequate :coling water can be supplied to both steam generat:rs by either :f tw: ourpin; systems.

The one electrically criven p::np or :he :urcine driven pump :an provide sufficient water (260 ;pm) for the removal of decay heat following a reac.or trip.

Auxiliary feedwater can be directed through two different paths t: each steam generator: one line discharges in*a the main feecwater lir.e of the steam generator upstream of the mair feecwater cor.:r:1 valve; ne other line dis:narges, directly into the steam genera:or via a secarate four iner diamete" r.:::le. This emergency feecwater noz:le is used when tne rain feedwater line is :'.csed.

- 2.2 General Oceration Norwally, 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 conditions require condenser shutdown, the auxiliary feedwater pumps are used to remove decay heat.

The emergency feedwater noz:le is used when the main feedwater line is closed. Following a containment isolation signal, the valves that permit flow to the emergency feedwater noz:les would atto&etically open fully.

in approximately seven seconds.

The auxiliary feedwater pumps would be started during a loss of feedwater transient; both auxiliary feedwater pumps would automatically ' tart when the last operating main 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 ocerator.

90018008

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. 3.0 MEANS TO REDUCE THE POTENTIAL FOR WATER HAMMER 3.1 Description The conditions most conducive to steam generator water hanner occur whan the steam generator feedrings are uncovered and steam enters the feedrings and attached horizontal feedwater piping.

5 team-water slugging and subsequent water haniner may occur when incoming cold feedwater or auxiliary fetawater m1xes with the steam in the piping and rapid condensation occurs. The condittens can be avoided by keeping the feedrings and associated piping full of water.

This can be accomplisned by: 1) keeping the water levels in the steam generators above the feedrings, 2) sup;,1ying feedwater at a higher flow rate than the rate at which l

feedwater drains through the disenarge holes on the bottom of uncovered feedrings 3) having a sufficiently short lengtn of horizontal piping such that slug fennation does not occur, or 4) refilling the steam generator through a i

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.

l At the Fog Calhoun Station Unit 1, the following features reduce the potential fer water hammer:

i) there is no nori: ental piping connected to tne steam generator feecwater no::le and 2) a separate no::le in tne steam generst:r is used to rafill the steam generator wnen the main

_p 90018009 l

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. 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 inches. The horizontal distance'from the center line of the elbow to the feedwater sparger is 29 inches. With this short i

norizontal distance, the probability of severe water hammer is low because:

1) the probability of formation 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 culd

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be trapped and thereby limits the energy of a water hamur" if it were to occur.

The separate emer;ency feedwater noz:le is used to refill the steam generator with auxiliary feedwater when the main fesowater flow path is closed.

Steam generator water ha..:ner has been experienced in other plants when cold auxiliary feedwater was directed through the horizontal feedwater l

piping that was filled with steam.

By diverting this cold water away from the main feedwater line and directly into -he steem generator via a separate no::le, the water that ever.-

tually enters the hori: ental piping as the level rises will*

come from the surface of the water in the steam generator and will nave the same temperature as the steam in the 90018010

feedwater ring and piping.

Under these cond':i:ns there is no potential for steam generator water na :mer.

3.2.

Effectiveness During Transients D}{'O

'P D 9@ng r1 d uM 3

3.2.1 Plant Trio A plant trip, f.e., rapid shutdown of the reactor and turbine generator, would result in a reduction of the volume of steam bubbles in the s:eam generators and the water level in both steam ger.erators would fall below the main feedwater ring.

Mafn feedwater would c:ntinue to flow to each steam generator but would be reduced by partial cl:sure cf each main fetcwater regulating valve.

Each main -egulating valve is programmed to close a: a rate :f aceroximately 2t of full closure per second until it saches a usition such that the valve is Et ocen.

Main feed-wa:ar c:ntinues to flow until :he steam generator wa ar level is restored.

Nemally, ne main feecwater system is used to remove decay hea-innediately after power operation when outside pcwer is available.

If outside power were not available or if operating conditions required condenser shu:::w,, then the auxiliar/ feedwater system would be used to remove

ee,ay heat.

In -his case auxiliary feecwater would be :um:ed into the main feecwa:ar line ;pstream cf ne main feecwater regulating valve.

90018011

A myiew 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 deter:.ined whether the ring drained. However, during those periods of time, there were no water ham.ar oc:urrences observed by operating personnel; and there 9as been no evidence of equipment deterioratio.. typically encoun.ered under significant flow instability

endi tions. The absence of water hamer occurrences sin:e plan: star.up in 1973, and the absence of any water ha=ers :uring tnose times when water han:ners would mst likely occur, provides assurance that fluid flow ins asilities will not occur as a result of plant trip.

3.2.2

. ss of Main Feecwater n ow j

i Tne loss of 'l:w ef main feedwater would result in a plant trip. The reactor is tripped by a low water 1evel in either steam generator.

The water level in the steam genera:cr would drop to some level belcw

-he feetring as in the :sse of a plant trip describe:

j ab:ve. However, since in this case the fetcwater

1. 1cw is ::msie ely s:c::ec, the water in the feecrin; would drain an: :e re: laced witn steam.

Tno introdu: tion

1 :sid feecwater into :he steam filled piping at nis

1:ne woulc esta:iisn the ::nditiens :ncs: conducive 90018012

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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 feedwate.r piping under these conditions and no water hammer occurred.

Three loss of feedwater events have occurred since the Fort Calhoun operating license was issued in August 1973.

The feedwater events were as follows:

Item Date Cause (1) 3-07-74 Scheduled complete loss of off-site A.C. power durin trip test; s

main feedwater pump trip.

(2) 2-21-76 Loss of 161 KV supply to house service buses followed by failure to fast transfer; main feedwater ump trip.

(3) 8-22-77 Mctentary loss of 161 KV supply to house service buses followed by failure to fast transfer; main feedwater pump trip.

Recovery of steam generator water level after these events without water 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 fetcwater flew, a plant trip and less of steam generator level as described aoove.

The steam driven 90018013

1 auxiliary feedwater pu : weuld star. and deliver-watar to the main fenc',ater piping.

The operator may start the motor driven ;umo after switching it tc the onsite emergency power source.

At least one of the events cited above followed this scenario and no water hammer. resulted.

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3.2.4 Ooerator Error It is not likely that an operator error would result in a water hanrJer.

This piant has been subjected

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to those conditions that are m:s-likely to cause waar hammer and no wa:er har=er has occurred.

Operator error :ould fr: ease the frequency with which the feedwater pi:ing is subjected to conditions conducive to water hanter but would not materially change those c:nditions.

Since plant operations have demonstratad that water han:ner does not oc:ur under these conditicr.s, operator erron can have little influence :n te Occur-ence of water hacme r.

3.2.5 Steam Line Break Tne ;ossibility of wa:er na=er oc:urring as a result of er in conjunction wi-. a steam line break is c::nsidered in Order to de.ar :ine wnether this water ha: er could cause a ru::ure that would result in

ne blowdown of more : an :ne steam generator or could -esult in :ne less Of ca:acility to su:oly i

aux m ary ree m :'"

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I In the event of a steam line break, the resulting containment isolation signal wculd initiate the flow of auxiliary feedwater directly into the steam genera.or

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thmugh the emergency feedwater nozzles. This' mode of operation reduces the probability of obtaining conditier.s that could lead to water hanr:er, i.e., even if the feed-water sparger were drained, the cold auxiliary feedwater would be directed to flow thmugh the emergency feedwater nozzles and would not flow in through the main feedwa er piping and scarger and therefore a water harn e could net be induced in the sparger.

The probability of oc:urrence and the consecuences of water hammer in the emergency feedwater lines anc auxiliarv feedwater lines have been examined by the licensee.

ven

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though there is no auxiliary feedwater sparger, ne possibility of steam-water slugging in ne emergency feedwater no::le and piping was :ensidered (Re'. 5.5); and, the licensee cencluded that it would not occur it these l ines. The emergency feedwater discharges direc-ly into the steam space in the steam generator via a t.-in:h diarteter pipe that is two feet long. This pipe is connec ed te a check valve and 3.5 feet upstream of tne check valve :nere is a control valve. A calculation of -he hea: transferred to the region :e ween the eneck valve and :ne :or:ml valve showed that the.ater in this region would nc b:11 anc cause the check valve to ocen and al".ow :ne water := crain Ou. It 90018015 1

. 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 tha; might result directly from the opening of the auxiliary feedwater c:ntrol valve was also considered.

An analysis using the PTHRUST program (Ref. 5.2) showed q

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 tne emergency or auxiliary feecwater 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.

3.2.5 Loss of Coolan: Accident (LCCA)

The possibili y of water hanner in the feecwater piping occurring in conjunction witn a LOCA is considered in order to detemine wnether the consecuences of a LOCA : night be i

increased by causing a rupture that would result in the l

clowdown of a steam generator during the LOCA. At the 1

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Fort Calhoun station, the condi:fcns recessary to pmduce a water han=er after a LOCA are avoided by the same means employed in the case of a steam line break as discussed a:cve. These means will be effective in avoiding water haarner because the condi:1:ns in the steam genera or and feedwater pi;ing that might be conduc.ive to wa.ar hammer will no-te suos antially r

influenced by enether such c:nci:1cns resuit ' rom a steam line break or a LOCA.

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4.0 CONCLUSION

S We have reviewec the o;erati:nzi charac eris-ics anc performance of the Fort Calh:un 3:a: ion :ertaining :: steam generator water hammer. The For. Calhoun plan: has been subjected to thue conditions mos itkely to ca:se stea generat:r water hansner but none has c::arred.

Tnese sa.e ::ndi-ions would be expected to result in =e fu ;re durit; n:r.a: :perati:ns and under transient and ac:ide..: c:nditi:ns. 3eref:re, since a water hammer has no: c:curned at F:r: alh:un during such conditions, none is expec ad in de futu e.

The successfu* c: era: ion of de For*. Calhoun plant without steam generat:r sa:ar ha.:mer is atWbutable t: the geometry of tne feecwater. pi;f n; and 'escring i. ne area of the staam generator noz le.

Se :::a: ".en; n :f =e wa ar sur' ace possible 1

l in :ne feecrir; :ee, =e no::le anc ce :cwnward elbow is aoparen-ly snort enougn _: :reclude si;; 3: - a-':r and 9e resulting water nanner.

90018017

. 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 monitor licensee event reports from this licensee for indications of possible water hammer.

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 hammer at these facilities.

Based on our knowledge of water hamm.er phenomena and our review of the o;erational characteristics and perfontance of the Fort Calhoun Station, we have concluded that steam generator water hammer is not likely to occur at this facility.

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.

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5.0 REFERENCES

5.1 T. E. Short, Omaha Public Power District (OPPD) letter to G. G. Lear, NRC, transmitting "Secendary System Fluid Flow Instability Report," dated July 23,1975.

5.2 T. E. Short, OPPO, letter to G. E. Lear, NRC, " Addendum to Se:endary System, Fluid Flow Instability Repert," dated Febmary 27,1976.

5.2 T. E. Shcrt, OPPD, letter to G. E. Lear, NRC, indicating no action needed regarding water hammer, dated Noveder 1, 1976.

5.4 T. E. Short, OPPD, letter to G. E. Lear, NRC, in response to Septeder 2,1977 letter from NRC, dated December 29, 1977.

5.5 T. E. Shor., OPPD, to R. W. Reid, NRC, letter dated July 19, 1978, transmitting report " Emergency Feedwater Ham:er Analysis for Fort Calhoun Station Unit 1," by Nuclear Service Corporation.

5.5 T. E. Short, OPPO, to K. V. Seyfrit, NRC:Regien IV, dated May 25, 1979, in response to IE Sulletin 79-055.

5.7 Final Safety Analysis Report, Fort Calhoun Station Unit No.1,.'!RC Occket No. 50-255.

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. 5.3 J. R. 31:ck, et al, An Evaluation of ri:. 5 aa= sanerator Water Hamer, Creare, Inc. NUREG-0291 ( tes=er 1976).

5.3 W. E. Bennett, Waterha=er in Steam Ger. era::e Feedwater Lines, Wes:inghouse Tecnical Sulletin, ?;S: ~3-75-7 (June 10,1975).

5.10 T. E. Sh:r:, OPPD to R. Reid, NRC, dated November 12, 1979, in res:ense to 'NRC *e ter dated Se::t ber 11, 1979.

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