ML20058K489
| ML20058K489 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 06/30/1990 |
| From: | Walt T PORTLAND GENERAL ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9007060171 | |
| Download: ML20058K489 (7) | |
Text
i Portland General Bectric Cciripia rf June 30, 1990 Trojan Nuclear Plant Docket 50-344 License NPF-1 U.S. Nuclear Regulatory Commission Attn Document Control nesk Washington DC 20555 Dear Sirs.t Reduced Minimum Measured Reactor Coolant System (RCS) Flow Portland General Electric Company (PGE) has been informed that the Trojan Technical Specification (TTS) reactot coolant flow requirements do not ensure compliance with the accident analysis flow requirements. On May 18, 1990 Westinghouse indicated that TTS 3/4.2.3, " Power Distribution Limits. RCS Flowrate and FR [ nuclear enthalpy rise hot channel factor ratio]", was nonconservative with respect to the allowed operating region depicted in Figure 3.2-3, " Flow vs. FAH [ nuclear enthalpy rise hot channel factor) Limit for 4 Loops in Operation" (see Attachment 1).
Irrespective of the provision for a tradeoff between RCS flow and FAH_in Technical Specification Bases 3/4.2.2 and 3/4.2.3, Westinghouse stated that a total minimtm. measured RCS flow of 371,700 gallons per minute (gpm) must be maintained while the Plant is operated in Mode 1 to achieve consistency with the analyses of Departure f rom Nucleate Boiling (DNB) events. A horizontal dotted line has been added to Attachment 1 to show the more conservative limitation imposed by the Westinghouse accident analysis. The cause of the discrepancy was a failure by Westinghouse to consider all relevant factors in the RCS flow Technical Specification and communicate these to PGE.
RCS flow measured during the last operating cycle (Cycle 12) was 371,539 gpm. The operating point corresponding to the highest measured FAH value in Cycle 12 is shown on Attachment 1.
The consequences of Cycle 12 operation at a flow less than 371,700 were determined to be acceptable, as shown in the attached evaluation summary (Attachment 2) which bounds Cycle 12 operation.
9007060171 900630
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I' Pontand Gonoral Dectoc Document Control Desk June 30, 1990 Enge 2 During the 1990 refueling outage an additional 1.1 percent of the steam generator tubes were removed f rom service by plugging.
The total RCS flow is estimated to be near 370,500 gpm for operating Cycle 13.
In anticipation of a lower RCS flow than the required minimum measured flow (MMF), an evaluation to support reduction of the minimum flow requirement to 368,000 gpm total measured RCS flow was performed by Westinghouse and is-summarized in Attachment 2.
The evaluation permits 100 percent power operation provided that a 1.5 percent DNB ratio penalty is assessed against the existing DNB margin.
Existing Plant trip setpoints do not require change. However, the Over-Temperature-Delta-Temperature (OTAT) trip safety analysis K1 value was impacted. The margin to the trip setpoint K1 value in the Technical Specifications includes sufficient inst'rument l
uncertainty allowance to preclude the need to change the Technical Specifications.
The low RCS flow trip setpoint in TTS Table 2.2-1 remains unchanged based on a reference value of 92.925 gpm/ loop. The low-flow trip setpoint will be raised if necessary to comper.;&te for any flow shortfall below the referente value.
t Continuous RCS f'.ow indication is provided by differential pressure transmitters on elbow taps downstream of the reactor coolant pumps.
These indicators read flow in percent, and are checked at the beginning of each cycle to read near 100 percent. The accuracy of these indicators is not sufficient to be the bas 1s of the RCS total flow measurment.
The most accurate RCS total flow measurement is obtained by dividing the total reactor thermal power by the enthalpy rise across the vessel. This-method requires the Plant to be at power (Mode 1) in order to obtain a flow value. This flow calculation is performed after the results of the first full power calorimetric are available.
An additional precaution will be taken to ensure that RCS flow is not lower than the 368,000 gpm analysis value for the Cycle 13 startup. A calorimetric-based flow calculation will be performed at 90 percent power.
This will enabic a d2 termination of flow at a condition where ample DNB margin exists should an unanticipated low-flow value occur.
A power escalation decision will be made after the 90 percent power RCS flow measurement results.
If the flow is less than 368,000 gpm at 90 percent power, Trojan will stay at 90 percent power and contact Westinghouse for further analysis.
If the flow is less than 366,390 gpm (this is the thermal design flow for LOCA analyses including 3.5 percent flow measurement uncertainty), then the Plant will be placed in Mode 3 and further analysis will be obtained.
If the flow is at or above 368,000 gpm, then escairtion to full power will proceed, followed by measurement and
Pw land Gereat Electic Document Control Desk June 30, 1990 Paga 3 confirmation of the flow at 100 percent power.
If the 100 percent power flow measurement lo less than 368,000 gpm, then power will be reduced to I
90 percent and Westinghouse will be contacted.
These limits assure safety margin limits ars not exceeded.
In conclusion, the results of the attached evaluation support operation with RCS flow 2. 368,000 gpm with no Technical Specification setpoint changes. The additional RCS flow measurements at 90 percent power provide assurance that 100 percent power operation is consistent with the safety analysis. To complete resolution of this issue, PGE will submit a License Change Application on July 13, 1990 and propose revised Technical Specifications consistent with the Westinghouse evaluation.
i, Sincerely, T. D. Walt Acting Vice President Nuclear Attachments et Mr. John B. Martin Regional Administrator, Region V U.S. Nuclear Regulatory Commission Mr. David Stewart-Smith l
State of Oregon Department of Energy Mr. R. C. Barr NRC Resident Inspector Trojan Nuclear Plant
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TROJAN. UNIT 1 3/4 2-9a Amendment No.g, 76,153
'tay 24,1989 i
-l Trojen Nuclear Plant Document Control Desk 4
Docket 50-344 June 30, 1990 License NPF-11 Page 1 of 3 TROJAN NUCLEAR PLANT REDUCED MINIMUM MEASURED FLOW EVALUATIQR_
SUMMARY
INTRODUCILQH The purpose of the. Technical Specification on Reactor Coolant System (RCS) flow is to assure that the meanured RCS flow including measured uncertainties is greater than or equal to the minimum flow acsumed in the Final Safety Analysis Report (FSAR) safety analyses and the Nuclusr Steam Supply System (NSSS) design calculations.
For the FSAR safety analysis Departure from Nucleate Boiling (DNB) related events, the uncertainty is incorporated into the value for the DN.B ratio limit.
The other safety analyses and the NSSS design calculations assume' minimum RCS flows consistent with the thennal design flow (354,000 gpm).
f3ALUAIl0H All of the Trojan FSAR safety anal.yces and the NSSS design calculations have been reviewed to determine the impact of a reduction in the minimum measured flow (MMF) requirement from 371,700 gpm to 368,000 gpm.
This section'aummarizes the eficcts of a reduction in the MMF requirement on the safety analyses and the NSSS design calculations.
HSSE_Dealgn Calculationa As stated above, the NSSS design calculations assume a minimum RCS flow consistent with thermal design flow. To assure that there is no impact on the thermal design; flow, the minimum measured flow requirement must be larger than the thermal design flow plus uncertainties.
The mearurement uncertainty > associated with MMF is 3.5 percent (Technical
' Specification Figure,3.2-3). The proposed change to lower the NMF to 368,000 will, therefore,'not impact thermal design flow'(354,000 gpm).
368,000 - 0.035 X 368,000 = 355,120 > 354,000 Since_there is no impact on the thermal design flow, the NSSS design calculations are not affected by the change to the MMF~ requirement.
Non-DNB Safety _Analyana The non-DNB safety analyses also assume a minimum RCS flow consiacent with thermal design flow.
Since there is no impact on the thermal design flow, the non-DNB safety analyses are also not affected by the change to the MdF requirement.
Trojtn Nuclear Plant Documint Control Desk Docket 50-344 June 30, 1990 License NPF-1 Page 2 of 3 DHfLSa.f.ety Ana1ya.RS The DNB safety analyses currently assume an RUS flow of 371,700 gpm.
Following is a list of these DNB events.
- Excessive Load Increase (FSAR Section 15.1.3)
- Loss of External Electrical Load (FSAR Section 15.2.2)
- Partial Lors of Forced Reactor Coolant Flow (FSAR Section 15.3.1)
- Complete Loss or Forced Reactor Coolant Flow (FSAR Section 15.3.2.)
- Locked Rotor Rods-in-DNB Uncontrolled Rod Control Cluster Assembly (RCCA) Bank Withdrawal at Power (FSAD. Secticu 15.4.2)
- RCCA Misalignment (FSAR Section 15.4.3)
Startup of Inactive loop (FSAR Section 15.4.4)
Inadvertent Operation of Emergency Core Cooling System (ECCS)
During Power Operation (FSAR Section 15.N.t)
- Accidental Depressurir.ation of the RCS (FLAR Section 15.6.1)
Reducing the MMF from 371,700 gpm to 358,000 gpm.affects the above safety analyses in the following areas (f) the value for the initial RCS flow will be reduced, (2) the DNB ratio safety analysis margin may be impacted, and (3) the core thermal limits presented in the Technical Specification Figure 2.1-1 will be impacted which may result in changes to the protection system setpoints.
Euch of these areas is discussed below.
1.
Initial RCS Flow A basis for this evaluation is that red' Action in the RCS MMF would not significantly affect the system transient response for any of the DNB events. This is valid since the system transient response is primarily governed by the initiating event, i.e.
an increase or decrease in the secondary side heat removal, a decrease in the RCS flow rate, a reactivity anomaly, or an increase or decrease in the reactor coolant inventory.
The system transient response is not affected by the initial RCS flow assumption, unless the reduction in
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Trojtn Nucin t Plcnt Document Control Desk Docket 50-344-June 30, 1990
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e the initial RCS flow is sufficiently large enough to impair the steady-state core heat transfer or steam generator heat transfer capability. This is not the case for the small reduction. identified L.
above for Trojan.
In addition, for.the events in which a decrease in RCS flow occurs, the flow coastdown is determined by the pump characteristics and not the initial RCS flow assumption. Based on the above, tt.e' system transient response for'each of the DNB events remains unchanged.
Since the transient response' remains unchanged, the allocation of DNB margin to offset the reduction in the MMF as discussed in the following section is acceptable.
2.
Safety Analysis DNB Ratio Margin Impact i
The DNB margin is the difference between the transient / accident analysis results and the design limit DNB value.
A reduction in the RCS MMF to 368,000 gpm incurs a DNB ratio-reduction of'1.5 percent. The current available safety' analysis DNB_
i ratio margin is 14.4 percent less 1.5 percent rod bow penalty as referenced in Technical Specification Bases 3/4.2.2 and 3/4.2.3.
In addition to this change, PGE has recently-allocated 8.'3 percent DNF.
l' margin to account for stainless steel rods in the core an6. reanalysis H,
of the loss of all RCS flow accident'using the low-flow trip in lieu of the undervoltage or underfrequency trips.
Therefore the impact on the DNB margin is within the existing available margin and is acceptable.
3.
Protection System Setpoint Considerations, As noted above, a reduction in the MMF.affects the core thermal limits.- The thermal' limits are composed-of both DNB limits and of vessel exit boiling limits as shown in Technical Specification Figure 2.1-1..
Based on the new thermal limits resulting from the l
reduction in.the RCS flow, it was determined that a reduction in the current safety analysis value for the Overtemperature Delta-T (OTAT).K1 gain from 1.42 to 1.41 was required. The margin between the safety analysis value and trip setpoint equation K1 value (1.28) was sufficient such that no change to the OTAT setpoint equation constants was required. There are no changes to overpower AT trip values.
CONCLUSIONS j
The Trojan FSAR safety analyses and NSSS design calculations have been evaluated for the reduction in the MMF from 371,700 gpm to 368,000 gpm.
It has been determined that the conclusions of the safety analyses l
presented in the FSAR remain valid for the reduction in the RCS flow and the NSSS design calculations are unaffected by the reduction in the MMt l-requirement.
GRA/bsh 4922W.0590