ML20127G694
| ML20127G694 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 10/18/1973 |
| From: | James Shea, Ziemann D US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML20127G674 | List: |
| References | |
| NUDOCS 9211170408 | |
| Download: ML20127G694 (5) | |
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UNITED STATES ate (IC ENERGY CEDetISSION SAFETT ETALUATION BY THE DIRECIORATE OF LICENSING NORTEIRN STATES POWER CGIFANT-DOCEET NO. 50-263-MONTICELLO NUCLEAR POWER PIANT OPERATION FROM 1640 MWD /T TO THE EMD OF CTCLE 2 Northern States Power Company (NSF) by letter dated October 10.- 1973, requested renoval o 2,1973,{1 s operating restriction approved by our letter which fixed the control rod inventory to that of October which existed at.1200 MWD /STU and that reactor operation at power levels up to 100% be permitted until the 'Wenerte 72" Scram Reactivity
-Curve "B" (Figure 1 of the NSF October 10, 1973 submittal) is ate =4=ad at _2680 MWD /T according to NSP calculations. At that time the control rod inventory would be fixed (i.e., no further control rod withdrawal permitted) until power level decreased to 91% of rated..With reactor power level limited at 91% for the remainder _of Monticello fuel Cycle 2,
- control rod withdrawal could be resumed until all rods are completely withdrawn.- Figure 2 of the October 10, 1973 NSF submittal illustrates 1
graphically this program of reactor operation for the r===4=dar of i
Cycle 2.
The fixed control rod inventory restriation based on full power level conditions at 1*.00 MND/T has resulted in reactor power
-level being' decreased to about 90%.of rated power at approximately 2000 MWD /T average fuel dopletion in Cycle 2.
Modification of the four_' safety / relief valves to reduce-valve opening-response timas from 0 3-to_0.4 second and increasing the four safety valve trip points to 1240 peig.from 1210-1220 pois provided additional margin between peak transient pressure following turbine trip without -
steam bypass and the safety valve set points. According to NEP these changes permit removal of tho' current reactor operating restrictions (1). -
Withdrawal of-control' rods can be reatmed for reactor operation at rated conditions until the calculated scram reactivity curve shape matches the Generic 72 3 Curve calculated by MSP to occur-at the 2680 MWD /T Cycle _.2 ' exposure threshold. The previously accepted (1) omer >
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I limita, i.e., the 05 design margin of 25 poi between the peak transiest pressure following turbine trip without steam bypass and the peak pressure of less than the 1375 psig design overpressure for the primary coolant systan will not be esseeded following simultaneous closure of the four N817s and high flus remeter scram.
By holding or easseding the core control rod inventory at the 2680 1MD/T fuel esposure limit until remeter power with this limiting rod con-figuration decreases to 91% and then continuing to withdraw control rods while maintaining the 91% power level limit, the required peak transiest pressure margins will be retained to the end of Cycle 2.
We have completed our review of the information presented by the NSF l
1etter dated October 10, 1973. and note that 1.
Attainment of "B" curve conditions was conservatively estimated by NsF to oscur at 2250(2) HWD/T in Cycle 2.
A later refinement (3),
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i based on projected plant operation, resulted in an increased exposure threshold to 2400 MWD /T. According to tha October 10, 1973 NSF cubmittal, a more precise calculation based on actual operating history resulted.in an adjustment to 2680 MWD /T to achieve a Scram Reactivity Curve shape equivalent to the Generic 72 B eurve shaim on Figure 1 of the NSF report. The calculations appear to be-very sensitive to reactor operating history. NSF should indicate their intentions for further calculational refine-ments to assure that the Generic 72 3 curve exposure threshold is not exceeded prior to reaching 2600 MWD /T.
2.
Emmala=nistion of the end of Cycle 2 scram reactivity curve resulted in a slower resetivity insertion rate them had been calculated previously as indicated by the differenees between Curve C1 and C2 of the NSF October 10, 1973 submittal. The methods for assuring that the seat recently calculated and most restrictive BOC Curve C2 limits are not aseeeded should be reviewed.
3.
The scram reactivity curves for fuel espesure at 1640. 2680_HWD/T.
and and of Cycle 2 are presented in Figure 1 of the NSF submittal with desism and operational conservatism factors (DCF.and OCF) and the transtant pressure analytical results-have been listed in Table-2 of the sans report using DCF and OCF. The conservatism factors should be quantified and justified.
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The MSIV closure pressure transtease presented in Figures 5, 6,
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10, and 15 are based on opening four relief valves in 0.4 second and all four safety valves with relief valves opening at 1070, 1075, and 1080 psit + II and safety valves opening at 1240 pois +
II. Previous analyses (4) were based on opening three of the four relief valves in 0 2 seeond and only two ef. the four safety 2
valves. Relief and safety valve opening pressures assumed for l
analytiaal purposes in referesse (4) were est specified but l
aseerdias to the FSAR-4-4.4A, the four relief / safety valve set l
potate were originally simulated to be 1080, 1085, 1090, and l
1095 pois with the settel set point to be < 1080 pois and the
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safety valves vera set at their upper limits of 1210 and 1220 pois.
L The assumed retiraulating pump conditions were not specified in earlier analysis (4), although eere inlet flow rematmad above 100% at the 5 seesad time inarement for the turbine trip transient and continued to incrosse above 100% for the 16 second time period shown for the MSN closure transient. The most recent calculations presentsd in the NSF October 10. 1973 submittal show a faster i
core flow drop off but still slower than would be expected for a pump trip, and the indicated flow rate remains above 100% for the MSN closure transient. These apparent conflicts should be clarified.
t We have concluded, however, that there is sufficient conservatism in the NSP calculations to justify removal at this time of the control 2
i rod inventory restriction currently imposed on the Monticello operation I
because there is adequate time for NSF to provide additional supportive information before reaching a limiting amposure threshold..
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We also have considered the necessity for lowertag the overpower trip l
set points when the operating power level limit is reduced to 911 and j
have concluded that for.the transients under consideration such a l
reduction would not enhance safety and there are no changes in safety considerations or accident considerations, including loss-of-coolent -
accidents. that would justify changes to the existing Technical Specifi-cations requirements.
On the basis of our review we' have determined that the control rod inventory restriction that is currently limiting Monticello power
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production (l) may be removed and that continued operation of the Montieello nuclear power plant should be-limited in the manner described by NSF with the provision that additional supportive information is submitted by NSP. to the Directorate of Licensing on a timely basis. We l
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1 have concluded that operation of the plant for the remainder of Monticallo fuel Cycle 2 in the nanner deseribed does not present an unreviewed safety consideration or significant hasards considerations and there is reasonable assurance that the health and safety of the l
public will not be endangered.
James J. Shea Operating Reactors Branch f2
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Directorate of Licensing Dennis L. 2.ismann, Chief Operating Reactors Branch #2 Directorate of Licensing Date I O 873
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AEC Directorate of Licensing letter and Safety Evaluation dated October 2, 1973.
4 2.
NSF letter tu Directorate of Licensing dated June 1,1973.
Prelfatnary calculations show Scram Reactivity Curve B to be liciting at 2250 tic /STU.
3.
NSP 1'stter to Directorate of Licensing dated September 13, 1973.
The NSP letter incirded proposed changes to the Technical Specifi-cations and a General Electric report that described pressure transients caused by turbine trip and main steam isolation valve i
closure.
4.
NSF letter to Directorate of Licensias dated February 13, 1973.
Supplemental Report of a change in the transient analysis as described in the FSAR.
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