ML18153A349
| ML18153A349 | |
| Person / Time | |
|---|---|
| Site: | Surry, North Anna  |
| Issue date: | 12/29/1997 |
| From: | Ohanlon J VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 97-594A, GL-97-04, GL-97-4, NUDOCS 9801060160 | |
| Download: ML18153A349 (16) | |
Text
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VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 December 29, 1997 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATIONS UNITS 1 AND 2 NORTH ANNA POWER STATIONS UNITS 1 AND 2 Serial No.
NL&OS/ MWH:
Docket Nos.:
License Nos.:
RESPONSE TO NRC GENERIC LETTER 97-04, ASSURANCE OF SUFFICIENT NET POSITIVE SUCTION HEAD FOR EMERGENCY CORE COOLING AND CONTAINMENT HEAT REMOVAL 97-594A R#3 50-280, 50-281 50-338, 50-339 DPR-32, DPR-37 NPF-4, NPF-7 On October 7, 1997, the NRC issued Generic Letter 97-04, "Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps." The Generic Letter requested that licensees review the current design-basis analyses to determine the available net positive suction head (NPSH) for the emergency core cooling and containment heat removal pumps and then provide specific information regarding the design-basis NPSH analyses for these pumps to the NRC within ninety days. The purpose of this letter is to provide the requested information.
The requested information has been divided into a North Anna Power Station response which is provided in Attachment 1 and a Surry Power Station response which is provided in Attachment 2. The information contained in the attachments of this letter complies with the information requested in NRC Generic Letter 97-04.
If you have any further questions, please contact us.
Sincerely, No commitments are being made by this letter.
Attachments
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9801060160 971229 PDR ADOCK 05000280 P
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Serial No. 97-594A Response to NRC GL 97-04 Page 2 e
cc:
U.S. Nuclear Regulatory Commission Region II Atlanta Federal Center 61 Forsyth Street, S.W., Suite 23T85 Atlanta, Georgia 30303 Mr. R. A. Musser NRG Senior Resident Inspector Surry Power Station Mr. M. J. Morgan NRC Senior Resident Inspector North Anna Power Station
COMMONWEAL TH OF VIRGINIA )
)
COUNTY OF HENRICO
)
The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by M. R. Kansler, who is Vice President - Nuclear Operations, for J. P. O'Hanlon, who is Senior Vice President - Nuclear, of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.
Acknowledged before me this 2!/!!rJay of LrJl'/hnhA.2, 19..9..:l..
My Commission Expires:(~ 3/
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Notary Public (SEAL)
ATTACHMENT 1 NORTH ANNA RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 1 1.
Specify the general methodology used to calculate the head loss associated with the ECCS suction strainers.
Response
In accordance with the requirements of Regulatory Guide 1.82, (Reference 1 ), an insulation debris inventory and transport analysis was performed on the containment emergency sump to evaluate the insulation installed on the Unit 1 and 2 replacement steam generators.
The steam generator cubicles contain the largest quantity of insulation that could be exposed to a high-energy jet and/or whipping pipe. No other mechanism for insulation dislodgment has been identified. The area of influence due to a high-energy coolant jet is also the largest in the steam generator cubicles due to the;large pipe diameters present.
The debris generation and transport calculation used the following methodology:
- a.
The amount and size distribution of the debris that was generated is estimated using the guidance of NUREG-0897, Rev. 1 (Ref. 2) and NUREG/CR-2791 (Ref. 3).
- b.
The volume of water on the containment floor was calculated for various times after the postulated LOCA, including the effects of known delays in the water flow to the floor.
Two specific cases were investigated:
- i.
Hot Leg Double Ended Rupture (HLDER) with maximum ESF and one LHSI pump failure. This case provides the minimum NPSH-A for the inside and outside recirculation spray pumps.
ii.
Pump Suction Double Ended Rupture (PSDER) with minimum ESF. This case provides the minimum NPSH-A for the low head safety injection pumps after switchover to recirculation mode.
- c.
The sump screen areas were calculated and an expression was developed for relating the volume of water on the containment floor to the wetted screen area. This information, combined with the flow rate through the screens, was used to calculate the velocity through the screens.
- d.
The containment floor was partitioned into concentric circular rings extending out from the sump.
- e.
The velocity of water on the floor was calculated as it approaches the sump at several times after the start of flow through the sump screens. Flow between the floor regions was estimated using simple mass continuity and information regarding the water sources into the regions.
ATTACHMENT 1 NORTH ANNA RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 2
- f.
The initial distribution of transportable debris was determined in the regions on the floor before the start of recirculation spray. The amount and timing of debris transport to the sump screens was then estimated based on the calculated water velocities. The required velocities to initiate motion of various size debris pieces are based on NUREG-0897, Rev.
1 (Ref. 2).
- g.
The additional pressure drop resulting from the debris was calculated on the screens at several times following the LOCA. This pressure drop was based on an experimental correlation developed for the specific insulation type used at North Anna.
- 2.
e ATTACHMENT 1 NORTH ANNA RESPONSE Identify the required NPSH and the available NPSH.
Serial No. 97-594A Response to NRC GL 97-04 Page 3 The current results for North Anna Units 1 and 2 are as follows:
A. Injection Mode Pump NPSH-Available {ft)
NPSH Required (ft)
Low Head Safety Injection
[Bounded by Recirculation Mode]
High Head Safety Injection 64.8 23.7 B. Recirculation Mode Pump NPSH-Availab.le (ft)
NPSH Required (ft)
Low Head Safety Injection 13.8 13.1 Outside Recirculation Spray 12.79 11.0 Inside Recirculation Spray 10.53 9.4 High Head Safety Injection
[Bounded by Injection Mode]
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ATTACHMENT 1 NORTH ANNA RESPONSE Serial No. 97-594A Response to NRC GL 97-04 Page 4
- 3.
Specify whether the current design-basis NPSH analysis differs from the most recent analysis reviewed and approved by the NRC for which a safety evaluation was issued.
The current design-basis NPSH analyses for North Anna were implemented under the provisions of 10 CFR 50.59, and as such, have not been submitted for review and approval.
The most recent NRG-approved analyses of NPSH were submitted in Reference 4 and approved in Ref. 5. These analyses were submitted to support the current Technical Specification limits on operating containment conditions (TS Figure 3.6-1) for both Units 1 and 2.
Subsequent to the Reference 4 analyses, revised NPSH analyses were implemented under the provisions of 10 CFR 50.59 which supported design changes to replace the steam generators on both Units 1 and 2. The revised analyses included a detailed assessment of debris generation and transport from within the steam generator cubicles and quantify the effects of the d~bris on NPSH margins. (See response to Item 1, above). These analyses did not receive formal NRC approval, but were audited by the NRC staff and found to be in compliance with the requirements of 10 CFR 50.59 (Reference 6).
Additional analyses have been subsequently implemented under 10 CFR 50.59. The current analyses of record reflect the following changes to analysis parameters:
a 5% reduction in concrete heat sink surface area and mass, which conservatively accounts for the removal of certain floor plugs in containment, a reduction in casing cooling tank available volume, a reduction in casing cooling pump flow rate, and an increased casing cooling water temperature which were to be consistent with our testing program,
)
an increase in the maximum assumed recirculation mode transfer times application of more conservative uncertainties on the IRS/ORS timer delays modest reductions in assumed flow rates for the quench spray and inside and outside recirculation spray pumps as well as reduced service water flow rates through the recirculation spray heat exchangers.
UFSAR updates which reflect the revised analyses have been approved by Station SNSOC and are being incorporated into the UFSAR.
e ATTACHMENT 1 NORTH ANNA RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 5
- 4.
Specify whether containment overpressure (i.e., containment pressure above the vapor pressure of the sump or suppression pool fluid) was credited in the calculation of available NPSH. Specify the amount of overpressure needed and the minimum overpressure available.
North Anna Units 1 and 2 operate with subatmospheric containments. The limiting recirculation pump NPSH results occur during the injection phase of spray system operation. As such the NPSH analysis is performed consistent with the guidance of NUREG-0800, Section 6.2.2:
The recirculation spray system for a subatmospheric containment is designed to start about five minutes after a loss-of-coolant accident, i.e.,
during the injection phase of spray system operation. For subatmospheric containments, the guidelines of Regulatory Guide 1. 1 as defined above will apply after the inj~ction phase has terminated, which occurs about one hour after the accident. Prior to the termination of the injection phase the NPSH analyses should include conservative predictions of the containment atmosphere pressure and sump water temperature transients.
For the low head safety injection and recirculation spray pumps, the margin between available and required containment overpressure at the time of minimum available NPSH is presented below. This occurs prior to the termination of quench spray injection (i.e. prior to the termination of the spray system injection phase as discussed in Section 6.2.2 of NUREG-0800, excerpted above).
Pump Time (Sec)
NPSH Margin (ft)
Containment Pressure Margin, (psi)*
IRS 660 1.13 0.48 ORS 700 1.79 0.74 LHSI 3160 0.70 0.29
- Pressure margin is defined here as actual containment pressure minus the containment pressure required to maintain positive NPSH margin. It is calculated as follows:
Pressure margin (psi) = NPSH margin (ft) x sump fluid density(lb/ft 3
) /144
e ATTACHMENT 1 NORTH ANNA RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 6
- 5.
When containment overpressure is credited in the calculation of available NPSH, confirm that an appropriate containment pressure analysis was done to establish the minimum containment pressure.
The assumptions made for the depressurization analysis maximize the energy release to the containment atmosphere (minimize energy release to the sump) in order to overestimate the containment pressure. The assumptions made for NPSHA analyses of the recirculation spray pumps minimize the energy release to the containment atmosphere and maximize the energy release to the containment floor. Thus, the containment pressure is underestimated and the containment floor water vapor pressure is overestimated. Since containment pressure is a positive term in the NPSHA equation and the floor water vapor pressure is a negative term, a conservative calculation of NPSHA results.
These assumptions were implemented by use of Westinghouse mass and energy data in a LOCTIC analysis which employs the pressure flash break effluent modeling.
The pressure flash model assumes that the break effluent expands at constant enthalpy to the containment total pressure. The saturated vapor component goes to the containment atmosphere, and the saturated liquid component goes to the sump, unmixed with the containment atmosphere. This assumption neglects the evaporative cooling effect that the liquid component will realize. This modeling therefore assumes that the steam and liquid components of the break effluent are perfectly mixed, and that the liquid component becomes saturated at the containment pressure before falling to the containment sump.
In this manner, the energy contained in the sump water is maximized, which is conservative for NPSH calculations.
References for Attachment 1 ATTACHMENT 1 NORTH ANNA RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 7
- 1.
USNRC, Regulatory Guide 1.82, Water Sources for Long-Term Recirculation Cooling Following a Loss-of-Coolant Accident, Rev. 2, May 1996.
- 2.
NUREG-0897, Rev. 1, Containment Emergency Sump Performance, October 1985.
- 3.
NUREG/CR-2791, Methodology for Evaluation of Insulation Debris Effects, September 1982.
- 4.
Letter from W. L. Stewart (Virginia Power) to USN RC, North Anna Power Station Units 1 and 2, Proposed Technical Specification Changes, Serial No.87-385, March 2, 1988.
- 5.
Letter from Leon B. Engle (USN RC) to W. R. Cartwright (Virginia Power), North Anna Units 1 and 2, Issuance of Amendments, Re: Containment Upper Limit Temperature (TAC Nos. 67535 and 67536), December 14, 1988.
- 6.
Letter from Leon B. Engle (USN RC) to W. L. Stewart (Vepco), Final Report, Steam Generator Replacement Program (SGRP) 50.59 Audit/Review: North Anna Power Station, Unit No. 1 (NA-1), February 24, 1993.
ATTACHMENT 2 SURRY RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page 1
- 1.
Specify the general methodology used to calculate the head loss associated with the ECCS suction strainers.
Head losses in the ECCS system suction strainers are negligible for Surry. Section 6.3 of the Surry UFSAR presents the basis for concluding that the potential for fibrous material installed or stored in containment to detrimentally affect the functional capability of the Emergency Core Cooling System (ECCS) due to the clogging of suction strainers is negligible. This information was also provided in our response to NRC Bulletin 93-02 (References 1-2).
Furthermore, if the first-stage or second-stage screens of one of the suction points did become clogged so that no water could be supplied to a pump suction, the cross-connecting line would supply water to that pump from the section of the sump supplying the redundant pump through the other 12-inch suction line. Thus, both pumps would remain operational because adequate flow is available for both pumps through the cross-connecting line.
The screen assembly is designed to prevent large debris from clogging the spray nozzles during the recirculation mode. The screen assembly for the pump suctions is divided into two stages. The first stage is a trash rack and roughing screen arrangement completely surrounding the sump. The second stage consists of cylindrical screens of fine mesh over each suction point. The trash rack, screening and screen supports are designed to the Seismic Class I requirements.
The first stage of the screen assembly consists of inclined grating which acts as a trash screen to prevent large pieces of debris from reaching the sump. Inside the grating, there are two layers of screening, the first consisting of a roughing mesh and the second of a final mesh with an opening approximating the size of the smallest nozzle orifice in the recirculation spray header. The first-stage screening is divided at the centerline of the sump by a screening partition so that the physical failure of either half of the first stage will have little or no effect on the operation of the other half.
Each half of the first stage has an area of approximately 65 ft 2 per section, for a total first-stage screen area of approximately 130 ft 2
- The second stage of the screen assembly consists of cylindrical screens, each with an area of approximately 47 ft 2
, surrounding the pumps suction points. The cylindrical screens extend from the containment liner in the sump to the pump suction point, which protects the suction point in the event of failure of the first stage screen assembly.
The probability of screen clogging is remote, and sufficient screen area is provided to ensure that system operation during incident conditions is not impaired. Furthermore, entrance flow velocities are low enough to prevent entrainment of most small particles.
- 2.
ATTACHMENT 2 SURRY RESPONSE Identify the required NPSH and the available NPSH.
e The current results for Surry Units 1 and 2 are as follows:
A. Injection Mode Pump NPSH-Available (ft)
Low Head Safety Injection 70.5 High Head Safety Injection 53.6 B. Recirculation Mode Pump NPSH-Available (ft)
Low Head Safety Injection 16.87 Outside Recirculation Spray 9.93 Inside Recirculation Spray 12.93 Serial No. 97-594A Response to NRC GL 97-04 Page2 NPSH Required (ft) 15.0 24.0 NPSH Required (ft) 15.8 9.1 10.2
e ATTACHMENT 2 SURRY RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page3
- 3.
Specify whether the current design-basis NPSH analysis differs from the most recent analysis reviewed and approved by the NRG for which a safety evaluation was issued.
The Injection Mode results above were calculated in 1989. Virginia Power re-evaluated the Refueling Water Storage Tank (RWST) level instrument uncertainties, and determined that an adjustment to the recirculation mode transfer level setpoint was appropriate. Since the setpoint was lowered, the injection mode NPSH analyses for the safety injection pumps were reperformed. The results for the injection mode are nonlimiting with respect to the recirculation mode results. The analysis was implemented under the provisions of 10 CFR 50.59 and the results were subsequently incorporated into the UFSAR (see Table 6.2.13).
The R~circulation Mode results above are among the analyses submitted with the Surry core power uprating request (Reference 3) and are currently reflected in Tables 6.2-13 and 6.2-14 of the Surry UFSAR for the safety injection and recirculation spray pumps, respectively. A recent assessment was performed for changes which involved removal of concrete heat sinks and relaxation of the recalibration/recertification schedules for certain containment RTDs used in monitoring key parameter initial conditions. These changes modified the reported NPSH results from the previously submitted uprating analysis. This revised analysis was implemented under the provisions of 10 CFR 50.59.. The UFSAR updates which reflect the revised analyses have been approved by Station SNSOC and are being incorporated into the UFSAR.
1 I
I ATTACHMENT 2 SURRY RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page4
- 4.
Specify whether containment overpressure (i.e., containment pressure above the vapor pressure of the sump or suppression pool fluid) was credited in the calculation of available NPSH. Specify the amount of overpressure needed and the minimum overpressure available.
Surry Units 1 and 2 operate with subatmospheric containments. The limiting recirculation pump NPSH results occur during the injection phase of spray system operation. As such the NPSH analysis is performed consistent with the guidance of NUREG-0800, Section 6.2.2:
The recirculation spray system for a subatmospheric containment is designed to start about five minutes affer a loss-of-coolant accident, i.e.,
during the injection phase of spray system operation. For subatmospheric containments, the guidelines of Regulatory Guide 1. 1 as defined above will apply affer the injection phase has terminated, which occurs about one hour affer the accident. Prior to the termination of the injection phase the NPSH analyses should include conservative predictions of the containment atmosphere pressure and sump water temperature transients.
For the low head safety injection and recirculation spray pumps, the margin between available and required containment overpressure at the time of minimum available NPSH is presented below. This occurs prior to the termination of containment spray injection (i.e.
prior to the termination of the spray system injection phase as discussed in Section 6.2.2 of
- NUREG-0800, excerpted above).
Pump Time (Sec)
NPSH Margin (ft)
Containment Pressure Margin, (psi)*
IRS 700 2.73 1.1 ORS 700 0.83 0.35 LHSI 3250 1.07 0.45
- Pressure margin is defined here as actual containment pressure minus the containment pressure required to maintain positive NPSH margin. It is calculated as follows:
Pressure margin (psi) = NPSH margin (ft) x sump fluid density(lb/ft 3
) /144
I I
I I
ATTACHMENT 2 SURRY RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Pages
- 5.
When containment overpressure is credited in the calculation of available NPSH, confirm that an appropriate containment pressure analysis was done to establish the minimum containment pressure.
The assumptions made for the depressurization analysis maximize the energy release to the containment atmosphere (minimize energy release to the sump) in order to overestimate the containment pressure. The assumptions made for NPSHA analyses of the recirculation spray pumps minimize the energy release to the containment atmosphere and maximize the energy release to the containment floor. Thus, the containment pressure is underestimated and the containment floor water vapor pressure is overestimated. Since containment pressure is a positive term in the NPSHA equation and the floor water vapor pressure is a negative term, a conservative calculation of NPSHA results.
These assumptions were implemented by use of Westinghouse mass and energy data in a LOCTIC analysis which employs the pressure flash break effluent modeling.
The pressure flash model assumes that the break effluent expands at constant enthalpy to the containment total pressure. The saturated vapor component goes to the containment atmosphere, and the saturated liquid component goes to the sump, unmixed with the containment atmosphere. This assumption neglects the evaporative cooling effect that the liquid component will realize. This modeling therefore assumes that the steam and liquid components of the break effluent are perfectly mixed, and that the liquid component becomes saturated at the containment pressure before falling to the containment sump.
In this manner, the energy contained in the sump water is maximized, which is conservative for NPSH calculations.
e ATTACHMENT 2 SURRY RESPONSE e Serial No. 97-594A Response to NRC GL 97-04 Page6 References for Attachment 2
- 1.
- 2.
- 3.
NRG Bulletin No. 93-02, Debris Plugging of Emergency Core Cooling Suction Strainers, USNRC, May 11, 1993.
Letter from Virginia Electric and Power Company to USN RC, dated June 10, 1993, Serial No.93-307, Response to NRG Bulletin 93-02.
Letter from Virginia Electric and Power Company to USN RC, dated August 30, 1994, Serial No.94-509, Surry Power Station. Proposed Technical Specification Changes to Accommodate Core Uprating.