ML19093B010
| ML19093B010 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 08/25/1977 |
| From: | Stallings C Virginia Electric & Power Co (VEPCO) |
| To: | Case E, Reid R Office of Nuclear Reactor Regulation |
| References | |
| Download: ML19093B010 (20) | |
Text
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e August 25, 1977 Mr~ F.clson G *. Case;. *Ac~g* Director.*
f.;' 1e'""~-*dff)l ~"
Off ice of ~uclear Reactor. Regi:tlation. ~:f! g!L~~
Attn:* Mr~ R. W. Reid, Gh,ief..
Operating Reactbrs:Branch:No~* 4 Division of Reactor.Licensing
. U. S *. Nuclear Regulatory. Cdmnission Washington, D. * *c. *.,20555: *.
near Mr~ Case:.
. Docket Nos.**.50,..;2go. *.
- 50;..;281*
During.our.meeting* on August,24:, *1977,. we agreed.. to: perfonn.. addi-tional analytical '!/.0:tk and to provide you witlr.additional :infonmtion con-,-
cerning.our.justification for continued. operation of Surry Units* 1 and 2,.
in light of the* NOC concern relating to.riet.. positive :suction head to. the..
recirculation spray.pumps.* We stated that we.~uld furnish you,, on l'b:nday, August 29, ").97T,* ~ statement which.we*.feel:justifies*.continuecl operation~
based. on.our evaluation of analyses conducted* on.the* expectecl performance of the low head safety injection purrps. * 'IbinOrrow, by.telephone, our engineers will furnish your,staff with additional information concerning,our.evaluation of the. performance* bf.these*.pumps,.
we agreeff:further to provide you the*.following information by Sep;_
ternber* 12, 19.77:,
- 1.. Analyses of the low head sa.fety,in,jection pump
. performance* incl~g. input from.pump mmufacturer~
2 *.. The net positive: suction.head time-history for.the**.
inside and outside.recirculation.pumps a:nd*.the* low
.head safety injection pumps.for all potential nodes of operation.*
. 3.' Inspection results of. the.. tests.which were conduc-tecl on.the North: Anna.recirculation spray*pump*
with.additional information on the similarity of
. this.pump to. the.. Surry.pumpsG
- 4. *..Additional infonuation describing. our bases* for
.acceptability of operation. for these.pumps in the*.
cavita~g mode.*
.We. wi11* furnish you information within 90 days* concerning our plans.
for a more. permmenb solution including a schedule for inplei:renting any.modi-fications that may.be*.required.
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"""vrnoiNIA' ELECTRIC AND POWER COMPANY TO Mr~ Filson
- G.: case, ~~g Director Page 2 As.authorized" in.thef.meeting* on.August 24,. 1977,'-.yre have.mstalled'.
- the' flow limiting *orifices in the: discharge of the'.outside-recirculation' spray pumps of Units: t.and 2 as of ;7:-no*. a.m *. today,.*
Very-truly yours,
&JJ?. x/lrtt~+
C ~. M... Stallings.*
- Vice>President~Power*supply*
and Production Operations -*
cc:
Mr~ James*. P *. 0 1.Reilly,
- Director Office of Inspection.and.Enforcement
e VIRGINIA. ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 Mr. Edson G. Case, Acting Director
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Office of Nuclear Reactor Regulati0nt~/P!r
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Attent~on:
Mr. Robert W. Reid, C~}f r,,,,,.,
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Operating Reactors Branch 4 1-~i/.
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U.S. Nuclear Regulatory Commissi6n:-i u *.s.-NuctE hi AR REGUL
/1-ti' Washington, D. C.
20555
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Dear Mr. Case:
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August 24, 1977 Serial No:
366 PO&M/ALH Docket Nos.
50-280 50-281 Licensing Nos.
DPR-32 DPR-37 The Nuclear Regulatory Commission (NRC) was recently informed of a potential problem concerning the available net positive suction.head during LOCA for the recirculation spray pumps at the Surry Power Station *.
We have completed extensive tests and analyses in an effort to determine the severity of the problem and to devise an adequate solution.
A detailed report of these tests and analyses is attached.
We conclude from the information available that adequate NPSH would exist during LOCA to assure operation of the inside recirculation spray pumps at a reduced flow of approximately 3,000 gpm each.
Adequate NPSH would not exist during LOCA to assure operation of the o.utside recirculation spray pwnps; however, adequate safety is assured by the inside pwnps.
As a result of these findings, we propose to make a modification to the recircu-lation spray systems.
This will consist of the installation of flow~limiting orifices in the discharge of the outside recirculation spray pumps.
These orifices will be sized to assure that the outside recirculation spray pumps operate on the manufacturer's performance curve.
Analyses to support this modification are provided in the attach-ment to this letter.
This design change has been approved by our System and Station Nuclear Safety and Operating Committees.,. Materials are available on site, and station maintenance personnel are prepared to begin work immediately.
As is discussed in the attachment
e VIRGINIA ELECTRIC AND POWER COMPANY TO to this letter, new limits on containment temperature, containment air partial pressure, and service.water temperature must be imposed.
We request authorization to make these modifications immediately.
We believe that this cha.nge can be safely implemented during power operation.
Your prompt attention to this matter is requested in order to assure the availability of reliable electric service in the VEPCO system.
Attachment cc:
Mr. James P. O'Reilly, Director Region II Very truly yours, z$.m.~~t;;_
C. M. Stallings fr" Vice President-Power Supply and Production Operations
e ADEQUACY ~F RECIRCULATION SPRAY PUMP - NPSH SURRY POWER STATIONS - UNITS NOS. 1 AND 2 DOCKETS NOS. 50-280 50-281 LICENSES NOS. DPR-32 DPR-37 AUGUST 23, 1977 i
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1.0 INTRODUCTION
2.0 SOLUTION 2.1 Description 2.2 Safety Evaluation 2.2.1 Pump Tests TABLE OF CONTENTS 2.2.2 Containment NPSH Analysis 2.2.3 Operating Restrictions
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e 1.0-INTRODUCTION The containment depressurization system consists of two separate groups of subsyst<.::ms:
the quench sprar subsystems and the recirculation spray subsystems.
These subsystems are designed to depressurize the containment below one atmosphere of pressure within l hr following the Design Basis Accident and to maintain t*he containment subatmospheric thereafter.
Each of the four recirculation spray subsystems consists of a recirculation spray pump and a recirculation cooler.
Recirculation spray flow is provided through four 180 deg spray ring headers inside the containment.
Two recirculation spray pumps are located inside the containment and two are located outside; all four recirc.ulation coolers are located inside the containment.
In the event of a major loss-of-coolant accident, the vapor pressure of the water in the containment sump supplying the recirculation spray pumps may be closer to the containment pressure than previously indicated.
This is due to a number of original assumptions which have been determined to be inappropriate for analysis purposes.
This situation would.occur for only a short period of time following a loss-of-coolant accident and could result in inadequate NPSH at t_he recirculation spray pumps.
Continued operation of the plant until August 24, 1977, was permitted based on the following. information provided to the NRC by letter on August 20, 1977.
In the event of a small break in the reactor coolant system; the high system pressure would prevent the low head portion of the safety injection system from operating.
For this case, the water supplied by the high head pumps, while more than adequate for core cooling, would not be sufficient to transfer the available energy to the sump as sensible heat.
There would be a significant amount of steam and flashing water exiting from the break; the containment would not be rapidly depressurized, and the vapor pressure of the water in the containment sump would be much less than the containment pressure.
Thus, a small break would always result in adequate NPSH at the recirculation spray pumps.
The probability of a large break in the reactor coo~~nt_system during the three-day period until August 24, 1977, was calculated to be.8 x 10
, based on information in WASH-1400, "Reactor Study, 11 dated October, 1975.
Information received from the recirculation spray pump manufacturer (Bingham-Willamette) indic1?,ted that as NPSH decreased, the flow would decrease to the point where it reached 2,000 gpm at 7 ft NPSH.
When consideration is given to static head of water and pipe losses at 2,000 gpm, the minimum available NPSH for the outside spray pumps is 7.4 ft.
A review was made of the containment pressure transient, following a design basis LOCA, considering reduction in recirculation spray flow which could result from a decrease. in available NPSH to the pumps.
Based on all recirculation spray pumps operating (i.e., no single failure) and present site cooling water temperature, the results indicate a containment depressuri-zation time of less than l hr.
This report supports the commitment contained in the August 20, 1977, letter to the NRC to provide the results of analyses, and any proposed modifications, to assure adequa:te net positive suction head at the inlet of the containment recirculation spray pumps.
Table 1-1 provides a comparison between those parameters and assumption.s which differ in the original and current analyses.
e
~:ABLE 1-1 NPSH ANALYSIS - SIGNIFICANI' EFFECTS Hot Leg Break Cold Leg Break Input Parameter Old New Old New
- 1. Type of Flash for Temp Pressure Temp Pressure Break Effluent
- 2.
Spray Thermal 0.9 1.0 0.9 1.0 Effectiveness
- 3. Mixing of Steam and Yes Yes No Yes Wat er at Break
- 4. Initial Contairnnent Low/High Low/Low Low/High Low/Low Pressure and Service Water Temperature Combination
2.O SOLillION 2.l Description e
The contaimnent recirculation spray pumps are assured of adequate Net Positive Suction Head (NPSH) with a minor modification to the system and without requiring shutdovm of the plant.
Results of tests run on a pump with essentially identical hydraulic characteristics at the North Ar;ma jobsite verify the NPSH character-istics of the recirculation spray pumps and further show that the original NPSH required by the manufacturer was conservative. A description of the test procedure and test results is provided in Section 2.2.1 of this report.
The test results for the outside pumps indicate that there is adequate NPSH to provide some flow.
However, the test results indicated the probability of significant cavitation and it is deemed advisable to place a restriction orifice in the discharge to reduce the flow to a more desirable NPSH requirement. The pump tests also demonstrate that the inside pumps will perform in an acceptable mariner without modification.
The restricting orifices for the outside pump can be placed into the piping within a few hours without shutting down the plant.
As previously stated, the test results indicate that both inside and outside pumps would provide flow following a LOCA.
To justify continued plant operation during the time required to complete modifications to the outside pumps, an analysis was made in
e e
Section 2.2.2 using the assumption that the outside pumps became unoperable.
- With only the inside pumps, assuming both trains operate, the containment is depressur~zed in less than one hour.
Assuming the failure of one train, the containment did not depressurize, however, there was more than adequate heat removal capability to ensur.e containment integrity.
Thus, the plant can continue to operate for the few hours required while the outside pumps are out-of-service (orifices being installed).
The required operating restrictions.in support of these analyses are.listed in Section 2.2.3.
The tests and analyses presented in this report show that no modifications are required for the inside containment recirculation spray pumps, and the installation of flow restricting orifices in the outside pumps guarantees safe operation of the plant.
This can be accomplished without requiring plant shutdown.
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2.2 Safety Ev.aluation 2.2.1 North Anna Recirculation Spray Pump Test Procedures On August 22, 1977, a test was conducted to determine the performance characteris-tics of the recirculation spray pumps under conditions of reduced available NPSH.
The specific objectives were as follows:
- 1.
Determine the possible effect on pump head and flow of reduced NPSH.
- 2.
Determine the performance characteristics under cavitation.
- 3.
Demonstrate that after operation at reduced NPSH, the pump would perform in a normal manner after-*full restor.ation of NPSH.
In order*to accomplish the.test objectives,. a North Anna Unit 2 recirculation spray pump was installed in a closed*lciop system.
A water tank with a capacity of approximately 28,000 gal. was used as the test stand.
Holes were cut at each end of the tank top.
The pump, supported by ctibbing and a structural steel platform, was placed in one hole, so that the suction bell was located approximately 20 in.
from the*tank bottom.
A 10 in. pipe was run from the pump discharge into the hole in the other end of the tank.
The line included a flow measuring or.ifice flange, pressure indicators,.and a.throttling valve.
Temperature indicators were installed on the tank, as was a sight glass. referenced to the first stage impel*ler center line (center line = O::ft.).
A 2 in. steam line from the auxiliary boiler. was run into a sparger extending about half way through the tank on the opposite end from the pump.
With the test setup described above, it is possible to change the available NPSH(NPSHA)' of the system by changing water level and temperature~
NPSHA = P atmoshpere + suction head - vapor pressure - friction losses.
Atmospheric pressure is recorded since the tank is large compared to a pump sucti'on can; friction losses are assumed to be zero.
The two remaining terms are used to vary NPSH.
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The test program was started by running a baseline curve of pump vs. flow at a known adequate NPSH point *. The water temperature was then increased to a point (199.7F) where, with 7 ft-11 in. of water, an NPSHA of 15.6 ft. was achieved.
Another base line curve was run at this point; temperature was then increased in steps corresponding to lowering NPSH 1 ft. with each step and another curve run at each step, until 4 ft. NPSH was reached.
In this manner, a set of curves at varying NPSH was obtained.
From these data, the cavitation characteristics and the capa-bilities of the pump at varying NPSH are determined.
To further verify the adequacy of the pump, a half hour run was made at 2,000 gpm with NPSHA held at 5 ft.
- Later, the NPSH was gradually increased by raising the water level and lowering the temperature to.assure that the pump returned* to normal flow with adequate NPSH.
At the completion of all testing, the pump was run again with adequate NPSH arld wit_h adequate NPSH and a curve made and compared to the base line curve to assure no degradation of pump performance.
Results
-Referring t~ Figure 1, total dynamic head*in feet is plotted as a function of flow at constant NPSH available (which is also a constant suction*pressure).
The Hydraulic Institute Standard states that a 3 percent drop in head is usually considered as the onset of cavitation.
It can be seen that down to 8 ft. NPSH available the pump does not cavitate until a flow of 2,600 gpm is reached.
At 6.7 ft NPSH, the pump can deliver approximately 2,000 gpm without cavitation.
j When NPSH available decreases below 6 *ft *. the data show that the pump begins to cavitate throughout the 1000-3000 gpm range.
Also shown of Figure 1 is the system resistance curve for the outside and inside recirculation spray lines.
Figure 2 ~hows the NPSH required for operation of the pumps without cavitation.
This curve has been plotted using the approximate relation~that 3 percent drop in
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- head (as determined from Figure 1) is the onset of cavitation.
In order to determine system operating *flows under the postulated accident conditions, the NPSH available must be known.
As an example (on Figure 1) if the NPSH available is 8.0 ft., the pump would d~liver a capacity at a point where the NPSH available line intersects the system resistance curve.
This curve of 8.0 ft. NPSH available represents a pump curve operating at a reduced capacity, but more than 2600 gpm because cavitatioh has set in.
The data shows that the inside recirculation pump would continuously deliver a flow in excess of 3000 gpm at a total dynamic *bead of approximately 200 ft or greater.
Because the inside recirculation pump system curve is inherently stable, that is to say, there are no parameters that would cause the system curve to oscillate, the pump will not surge.
Since both the siction and discharge of the pump are connected to *the containment pressure, changes in containment pressure have no effect on system runout.
- Thus, the system/pump curve intersection is fixed in regard to contain111~nt pressure.*
The change in static head which raises the curve has no significant effect since a 1 *ft. variation _is smalL.in relation to 200 ft.
These changes in NPSH available during the postulated*occurance are gradual.
At the beginning of the t~aasient with adequate NPSH full flow of 3300 gpm (based on the most conservative resistance curve) is assumed and when the ~SH drops below 11 ft (e.g., MIN is 8.1 ft) a flow of 3.000 gpm is used.
When NfSH recovers above 11 ft.again the value of 3300 gpm was used.
For the outside recirculation spray*pumps the NPSH varies with system resistance due to the suction line friction loss in the piping.
Since this changes the resistance curve/pump intersection, it is more desirable to prevent this by preventing any cavitation.
Therefore, by adding orifices we can change the system curve to 2000 gpm at the intersection of the pump curve so that NPSH available at the worst point is 8-8 feet and the test data show.that we can run at 2000 gpm_at 6.7 NPSH without cavitating.
Therefore, there is a 2 ft NPSH margin.
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5 f-250 200 400 FOR RECIRCULATION SPRAY PUMP TEST DATA BASED ON INSIDE RECIRCULATION SPRAY PUMPS AT NORTH ANNA UNIT 2 8/22/77 D,_= NPSH IN FT D =TEMPERATURE IN °F 1000 2000 FLOW-GPM FIGURE I 3000 TOTAL DYNAMIC HEAD vs FLOW RATE SURRY POWER STATION UN ITS 1. 2 VI RGI NIA ELECTRIC AND POWER COMPANY
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1500 2000 2500 FLOW-GPM FIGURE Z 3000 3500 MINIMUM REQUIRED NET POSITIVE SUCTION HEAD TO AVOID CAVITATION vs FLOW RATE FOR RECIRCULATION SPRAY PUMP TEST NORTH ANNA 8/22/77 e
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- 2. 2. 2 Containment NPSH Analysis
.The LOGrIC computer program is used to calculate the available net positive suction head (NPSHA) for the inside and outside.. containment recirculation pumps.
Since the most limiting assumptions for an NPSH analysis are different from those for a containment depres-surization ana_lysis, both analyses must be performed for any sys-tem modification under consideration.
The former ensures that the required net positive suction head (NPSHR) ~s available during the entire accident ti-ansient' and the latter ensures that the sub-atmospheric containment criteria of depressurizing in less than one hour and remaining depressurized therea~er are satisfied.
The assumptions used for both _arialrses,, along with parameters pertinent to the containment evaluation,* are listed in Table 1.
All NPSH values are referenced to the center line of the first stage of the pump :impeller.
The.NPSHA transients for the inside recirculation pumps show that NPSHA is less than the ll ft required_ to produce full flow between 700 and 2,100 sec.
During this time interval, the reduced flow is 3,000 gpm.
The* outside recirculation spray pumps will have* a.de_quate NPSHA for the entire transient with the proposed modification.
Net positive suction head requirements for the low head safety injection pumps are satisfied when the pumps transfer to the sump and remain above NPSHR for the duration of the transient.
The contaimnent depressurization analysis transient is Fig. l.
e TABLE 1 CONTAINMENl' EVALUATION PARAMETERS
- l. Break Type
- 2.
Single Failure
- 3. Initial Pressure, psia
- a. Total
- b. Air Partial
- 4. Initial Temperature, F
- a.
Dry Bulb
- b.
Dewpoint
- 5.
Service Water Temperature, F
- 6.
- a. Temperature, F
- b. 1 Inventory, gal
- 7.
Containment Free Volume, 106ft3
- 8.
Spray Thermal Effectiveness, Percent
- 9.
Condensing Heat Transfer Correlation
- 10.
Spray Start Time, sec
- a. Q,uench Spray
- b. Recirculation Spray Inside Outside
- 11. Type of Modification TYPE OF ANALYSIS NPSH PSDER Diesel Generator 125 70 55 45 352,000 1.753 100 Tagami 68 201 367 Depressurization PSDER Diesel Generator 10.4o 9.30 125 105 85 45 352,000 1.753 95 Tagami 68 201 367 Outside recirculation spray orifice to 2,000 gpm.
Inside recirculation spray is 3,300 gpm until 700 sec, 3,000 gpm until 2,100 sec, 3,300 gpm after 2,100 sec.
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TIME AFTER LOCA (SECONDS)
FIGURE I SURRY POWER STATION - UNITS 1 & 2 TECH SPEC CONTAINMENT ANALYSIS MIN ESF. rsoER. OUTSIDE rUMrS ORIFJCEO LOCTIC RUN NUMBER R7646/002 (1702)
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- 2. 2. 3 Operating Restrictions
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In conjunction with the proposed modifications discussed above, operation of the plant will be administratively restricted.
These restrictions will remain in force until a permanent solution has been incorporated in the plant design.
These additional restrictions will ensure that assumptions made in the safety evaluatio~ presented above are satisfied during plant operation.
These restrictions limit containment air partial pressur*e, service water temperature and containment temperature as follows:
Service Water Temperature Containment Temperature Containment Air Partial
-M.aid:;muln: :Pressure 55° F to 85° F 100° F - 125° F
- 9-3 PSIA at 85° F service water temperature and 45° F RWST temperature.
This value will vary in a manner similar to existing TS 3.8.