ML20198Q228
| ML20198Q228 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 01/15/1998 |
| From: | Sorensen J NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9801220376 | |
| Download: ML20198Q228 (5) | |
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Northern States Power Company Prairlo Island Nuclear Generating Plant 1717 Wakonade or. East Welch. Minnesota 55089 January 15,1998 10 CFR Part 50.60 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50 306 DPR 60 Request for Exemption from the Requirements of 10 CFR 50.60 The Northern States Power Company requests an exemption f'om certain of the requirements of 10 CFR 50.60, Acceptance criteria for fracture prevention measures for light water nuclear power reactors for normal operation", in accordance with the provisions of 10 CFR 50.12(a)(2)(iv), which allows the NRC to grant an exemption from
/
the requirements of 10 CFR 50 if the exemption would result in benefit to the public health and safety that compensates for any decrease in safety that may result from the d
granting of the exemption. Specifically, this exemption is requested to allow the application of American Society of Mechanical Engineers (ASME) Code Case N 514, b,h
" Low Temperature Overpressure Protection", in determining the acceptable low temperature overpressure protection (LTOP) system setpoint for Prairie Island Units 1 and 2.
10 CFR 50.60 requires that all light-water nuclear power reactors meet the fracture toughness and material surveillance program requirements for the reactor coolant g 1220 g g h h,
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USNRC NORTHERN STATES POWER COMPANY
Appendix G to 10 CFR 50 defines the pressure / temperature (P/T) limits during conditions of normal operation, including anticipated operational occurrences and system hydrostatic tests, to which the pressure boundary may be subjected aver its service lifetime.10 CFR 50.60(b) specifies that alternatives to the described requirements in Appendices G and H to 10 CFR 50 may be used when an exemption is granted by the Commission under 10 CFR 50.12,
- Specific Exemptions",
in nccordance with the provisions of 10 CFR 50.60(b) and 10 CFR 50.12, Prairie Island r6 quests the application of the ASME Code Case N 514 for specifying allowable pressure margins in the determination of the Low Temperature Overpressure Protection System (LTOP) setpoint. This permits the LTOP pressure relief setpoint to be established such that the maximum pressure at the reactor vessel material's most limiting location is limited to 110% of the pr ssure determined to satisfy ASME Section XI, Appendix G, Article G-2215.
Prairie Island proposes that the application of code case N 514 is consistent with the provision in 10 CFR 50.12(a)(2)(lv), which allows the NRC to grant an exemption from the requirements of 10 CFR 50 if the exemption would result in benefit to the public health and safety that compensates for any decrease in safety that may result from the granting of the exemption, in that the slight reduction in safety margin for the P/T allowances is out weighed by safety gains in operational considerations, as discussed below.
To prevent low temperature overpressure transients that would produce pressure excursions exceeding the Appendix G P/T limits while the reactor is in conditions below normal operating temperatures, Prairie Island has installed a low temperature overpressure system. The Over Pressure Protection System (OPPS)is the Prairie Island specific nomenclature for the LTOP system. The OPPS is designed to provide the capability, during reactor operations at low RCS temperature, to automatically limit pressure transients. W.'th the RCS temperature less than the OPPS enable temperature
. and OPPS active, the OPPS will signal the Power Operated Relief Valve (PORV) to open, whenever the RCS pressure is greater than the OPPS pressure rellsf setpoint.
- The OPPS pressure relief setpoint is a constant value that applies for all RCS temperatures below the OPPS enable setpoint, nurm.we L
USNRC HoRTHERN STATES POWER COMPANY
' was Page 3 The OPPS pressure relief setpoint must be set low enough that the OPPS system will prevent the pressure in the RCS from exceeding the Appendix G P/T limits during a pressurization transient. To prevent the PORVs from lifting as a result of normal operating pressure surges with the reactor coolant system in a water solid condition, the operating pressure must be maintained soraewhat below the PORV setpoint. In addition, in order to prevent damage to the reactor pump seals, the operator must maintain a certain differential pressure across them. Hence, the operators must operate the plant in a pressure window that is between the minimum pressure required to preserve reactor coolant pump seals and a maximum pressure which does not challenge the PORV setpoint, With these operational considerations in mind the OPPS pressure relief sotpoint we.s chosen as 500 psig and the OPPS onable temperature was chosen as 310 'F.
Analysis has shown that these values bound the ASME Appendix G brittle fracture limits curve from an indicated RCS temperature of approximately 200*F through rated operating temperature. Analysis has also shown that those values would bound the ASME Code Case N 514 brittle fracture limits curve from the minimum bo't up RCS temperature through rated operating temperature.
Although Prairie Island could avoid the use of ASME Code Case N 514 by reducing the OPPS pressure relief setpoint, use of the 500 psig setpoint is beneficialin providing added safety to the operation of the nuclear plant. This is based on the following reasons:
1.
Reducing the OPPS pressure setpoint below 500 psig will increase the probability that the RCP's #1 seal will fail as a result of OPPS operation.
Tripping an RCP would reduce the equipment available to provide for decay heat removal.
The PORV takes several seconds to stroke open or closed, so the pressure relief from OPPS operation is not instantaneous. Because of this, the RCS pressure response to each mass injection transient and heat injection transient has an overshoot and an undershoot. With an OPPS setpoint of 500 psig neither injection transients' resultant pressure overshoot will violate the *ASME nawa.we
UsNRC NORTHERN STATES POWER COMPANY
'1/15/98 P:go4 Appen fix G with ASME Code Case N-514" brittle fracture limits curve, and the challenge from the resultant pressure undershoot to the RCP #1 seal required minimum differential pressure will be minimized.
2.
Reducing the OPF'S pressure setpoint below 500 psig will reduce the available pressure operating band and thereby increase the probability that OPPS will be inadvertently initiated by pressure fluctuations induced by typical system evolutions (e.g. starting a Reactor Coolant Pump, putting the Residual Heat Removal system into service, shifting operating charging pumps, etc.). Nominal RCS pressure is maintained between the OPPS pressure setpoin; wd the minimum RCS pressure necessary to protect the #1 seal on each RCP.
The use of code case N 514 incurs a slight reduction in the margin relative to the limits set by Appendix G. However in the development of the Appendix G P/T curves there are numerous conservatisms:
1.
A factor of safety of 2.0 on the primary membrane (pressure) stresses.
2.
A margin factor shift of two standard deviations applied to Rtr,dt.
3.
All dynamic crack initiation and crack arrest toughness experimental data is bounded by the use of the ASME section ill and XI Appendix G reference stress intensity curves (Kir). The use of the reference stress intensity curve bounds the crack initiation fracture toughness (Kic) properties by a factor of 1.2 to 2.5, depending on vessel temperature and Rtndt.
4.
The analysis for the OPPS setpoint includes a detailed instrumentation uncertainty analysis which meets the requirements of WCA?-14040 For the reasons stated above, the code case has been approved by the ASME code committee and the content of this case hss been incorporated into Appendix G of Section XI of the ASME Code and published in the 1993 Addenda to Section XI.
Further, the NRC has incorporated the code case into Draft Guide 1050 which will be Revision 12 of Regulatory Guide 1.147 when approved.
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uSNRC NORTHERN STATES POWER COMPANY
' it15/98 Pcge 5 The operational safety reductions itemized in the discussion above would be eliminated by maintalning the OPPS pressure relief setpoint at 500 psig. This setpoint can only be established if ASME Code Case N 514 is used to relax the dictates of ASME Section XI Appendix 1. Based on this, Northern States Power Company believes that the use of ASME Code Case N 514 would result in a net benefit to the public health and safety, it j
is requested that Northern States Power Company be granted an exemption to 10 CFR 50.60 and be permitted to use ASME Code Case N 514 in development of the OPPS setpoints for the Prairie Island Nuclear Generating Plant.
A licensing amendment request which updates the Prairie Island Pressure / Temperature Operating Curves as of May 1,1997 and proposes the use of a Pressure and Temperature Lin,its Report (PTLR)is currently in preparation. The changes tc be proposed by this upcoming amendment submittal are dependent upon NRC approval of the exemption request and the authorization to utilize the ASME Code Case N 514 provided by this submittal. Please contact John Stanton (612-388-1121, Ext. 4083) If you have any questions related to this exemption request.
Sf4f h Joel P Sorenson Plant Manager Prairie Island Nuclear Generating Plant c: Regional Administrator - Region Ill, NRC Senior Resident inspector, NRC rJRR Project Mar:ager, NRC J E Silberg State of Minnesota, Attn: Kris Sanda naum.me