ML19317G831
| ML19317G831 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 03/10/1976 |
| From: | Reid R Office of Nuclear Reactor Regulation |
| To: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| References | |
| NUDOCS 8004010659 | |
| Download: ML19317G831 (6) | |
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4 UNITE D STATES g
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WASHINGTON, D. C. 20555 E
March 10, 1976
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Docket No. 50-312 Sacramento Municipal Utility District ATTN:
Mr. J. J..Mattimoe Assistant General Manager and Chief $ngineer 6201 S Street P. O. Box 15830 Sacramento, California 95813 Gentlemen:
RE:
RANCHO SECO Your facility is one of several with nuclear steam supply systems strpplied by Babcock and Wilcox (B&W) that are equipped with reactor internals vent valves to' prevent vapor lock in the reactor vessel in the event of a reactor coolant cold leg leak.
In the past, facilities having this design feature have been assessed a flow penalty to allow for the potential,of a vent valve being stuck open durf-normal operation.
l This penalty represents a reduction in core flow of about 5%.
In Ausast,.1975, B&W submitted to the NRC a generic report entitled, "S&W Operating Experience of Reactor Internals Vent Valves.". Based on our review of this report, we have concluded that sufficient evidence has been presented to assure that the reactor internals vent' valves on B&W plants'will remain closed during normal operation.
A copy of our Report Evaluation is enclosed.
This letter is being sent to you for your information in the event that you wish to eliminate the vent valve flow penalty for your facility.
In accordance eith the conclusions reached in our evaluation, an application for a license amendment to accomplish this must include proposed changes
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to the facility Technical Specifications to require surveillance testing during each refueling outage to confirm that no vent valve is stuck in an open position and that each valve exhibits complete freedom of.Tovement.
Sincerely, Robert W. Reid, Chief Operating Reactors Branch #4 Division of Operating Reactors
Enclosure:
NRC, Staff. Safety Evaluation on "B&W Op' crating Experience of Reactor Internals-Vents Valves" 8 0 04-010 $ $~7 cc: See next rage
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Sacramento F!unicipal Utility District March 10, 1976 cc: David S. KapI n, Secretary and General Cc
,el 6201 S Street Post' Office Box 15830 Sacramento, California 95813 Business and Bfunicipal Department Sacramento City-County Library 828 I Street Sacramento, California 95814 D
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. REPORT EVALUATION: "D&W OPERATING EXPERIENCE OF REACTOR INTERNALS VENT VALVES"
SUMMARY
OF 2r. PORT In accordance with a reonest from B5N, the Nuclear Regulatory Cor.aissio$
steff (the staff) has reviewed the subjectcreport which-was enclosed with the request. This report contains operativa and test data from ben's six operating units intended to demonstratu that reac. tor internals vent valves remain closed during normal operation.
In addition to the infor-matibn submitted in this report, the staff's review included information obtained from a telephone conversation with ESW on September 24, 1975.
. The staff has taken the position on recent applications which utili:e the D6W' reactor internals vant valves that the applicantd must either:
(1) take a floti penalty for a vent valve being open during operatien, (2) provide instrucentation to ensure tlhat the vent valves remain closed, or (3) provide operating data. fr'oa other. D&U, plants t hich demonstrate that the vent valves remain closed during noraal operation.
It is BSU's contention that this report resolves the staff's position
'and that this item should no longer be considered an open item on current applications. l Uhen E&W plants fi rst began co==ercial operation, tha staff's positica vas that until suf ficient operating c::perience becaca availabic, a ficw penalty would be imposed to account for the potential of a vent valve being stuch open during normal operation, thereby reducing core flew by an amount eqbivalcat to the. bypass capability of. the open v*ent valva (about 5%).
This report tras reviewed to evaluate the feasibility of removia this flow penalty.
DSU's conclusion that a flow penalty is no longer varranted is based essentially on the follotting sources of infornation:
1.
Oconce inspection results obtained during het functional testing and du, ring a recent refueling outage.
2.
Rancho Seco internals vent valve thermocouple data.
3.
Rancho Seco loose parts monitoring system results.
B&W provided a discussion of their Oconee inspection results obtained following hot functional testing.
All.eight valve body scating surfaces THIS DOCUMENT CONTAINS l
POOR QUALITT PAGES l
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. were examinc4 by BLU.
Each valve shot.cd a difference in the tone of the oxide coating inside and outside the seating surface, representing a turbulcut inlet area (outside the vent valve)' and a lover velocity arca inside. ' An area of good contact between the valve body and seat appeared as a narrow dark line appro::f-"tely at the. center of the flat body seating surfacc.
E6U indicates that all cight vent valves showed such contact area.
Enclosed is a summary of pertinent inspections conducted by ELU on their operating reactors (Tabic 1).
Rancho Seco uas utilized as the site for the installation of vent valve thermoccuples just inside cache valve.
36W stated that a vent valve vas considered to be opan if tuo'of the three installed thermo-couples on a particular valve indicated a temperature 250F below' the reactor outlet tc=perature when the power level was greater than 807c of rated pover.
This criterion was based on results of testing conducted at ELU's Alliance Research Center in 1969.
ESU concludes that a revice of the chcrmocouple data shows that at no tiac did any of the vont valve thermocouple tc=peratures (a.verage of the-three) approach a 25 F departure from _ hot leg tenperature.
Pc have reviewed tha' representative data provided by BLU and agrec uith this'conclunion.
Using the E6U criterion, the prcsonted data do not reveal any anccalous behavior of the vent valves.
BLU han indicated that the remainir.g thermocouple data (not subnitted) showed not;hing to dispute this conclunion.
Several instances of unuc6al thermocouple temperatures were attributed to individual instrument u::1-functions.
The redundant thermocoupls on pacn valvre formeri the basis br this contention.
Loose parts munitoring programs continuously nonitor for unusual noises within the reactor vessel.
ULU points out that specific investi-nations of unusual noises,at Rancho Seco, Oconce 2 and Three Mile I Innd Unit 1 have excluded vibratind vent valycs as the sourec'.
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mention was made of the vibcation progras investigation at Rancho Seco which included an instrument location in the general vicinity of a vant valve.
Although it. pacts of a vibrating vent valve vould have been ex-
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pected to be heard at other locations, it should c1carly have been heard at this vent valve location.
No impacts coming from the area of the vent valves ucre detected.
Similarly, other loose parts monitoring progracs on the r-aining operating E6U reactors have not chaun vent valves to be vibrat' 3 While not evidence that a vent valve uns not stuck in en open I
position, such inferr.ation does provide further. assurance that vent valves do not undergo vibratory motion between the disc and seat during normal operation.
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B&W points out that the inclination (50 off. vertical) of-the valve body scal face provides.a positive closing" force.
This vent valve, design feature, coupled.uith the nor:nal differential pressure across the. core and vessel internals, keeps the' valves closed.
About 1500 lbs. closing-force on each vent valve. is calculated by 'B&W to c::ist during 2-pump o pera t.fj,0 During 4-pump operation, this closing force increases to about 6000.Ts.
B6W states that their vessel model flott tests verify that a positive hydraulic closing force is e::crted on the valve disc regardless of its ant;ular position betycen 0 and 21 degrees (full open).
STAFF CONCLUSIONS Based upon the information provided by D&W, the staff believes that suf ficient assurance er.ists that reactor internals vent valves are not opening in operating reactors and that the possibility of a stuch open vent valve is acceptably lov.
To further minimize the probability for such an occurrence, the staff uill require testing to be conducted each refuelin'; outage to cofdlra that.no' vent valve is stuck in an.open position and that cach vn)ve continucu to c::hibit cosplete f reedom of novement.
In addition, loose parts monitoring programs are recognir.ed to be the sentinel for abnormal noisa behavior.
Reports to NRC shall specify any anonsly attribut:;d to a v.ihritinr, vent ve.Jve or vent vt.1te cenponents.
We therefore conclude that Be" plants which incorporate the aforcaen:ientd survcillance requirements into their Station Technical Speelfications need nac includa a vent val.c flow penalty in their da.1gn.
and safety analyses.
Significant departurns from,the vent valve design citrrently on operating reactors would,varrant a re-evaluation of these conclusions, e
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TABLE 1 BABCOCK AND UILCOX
_VE'NT VALVE INSPECTION SU:'iARY l
1.
The valves operate freelv.
The vent valves on all six of 1:5H's or rating plante were checked prior to Hot Functional Testing and found to operate freely.
There have been no reports of binding, or excessive friction, with relation to vent valve disc freedom of novement.
The Oconee 1 valvas verc checked af ter both !!ot Functional Testing and refueling and found to operate freely.
2.
The valve disc hangs closed due to its own weip,ht.
All valves were inspected before ind af ter Hot Functional Testing on all six plants and on Oconce 1 and 2 af ter power operation.
In all cases, the valves vere found to be' closed.
. 3.
The valva seal surfaces ind'icate' contact durimgneration.
" The reacter incernals become discolored during ope.rutico due to ca.idu:.ie of the methi surfucc.
Surfaces which arc clanped tor,cther, such au the plenum to core support clamping surfaces, have a different diccolo.atic.
and stirface tenture than surfaces enposed to coolant flua.
The vent valve seal surfaces ci:hibited cnidation coloring similar to other clanped surfaces.
Inspections of the scal, surfaces have consistent.ly indicated contact between the valve bndv cnd disc.
The dincolorat!on ulse is used to indicate significant lenhage through the valve.
No signi-ficant Icakage has been observed.
4.
The valve does not open durinP. reactor operational transients.
. Coolant velocity is higher than normal during Hot Functional Testing
,due to the absence'of the cor.c.
Shotild a valve open during transients such as pump. switching, it would most lihaly happe,n at that time.
Any valve opening and closing would have left impact marks on the valve seal sur. faces and possibly on the reactor vessel.
No impact marks ha.ve been
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discovered during the post Hot Functional Testing inspe.ctions of the rdx operating BSU plants or at any other time.
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