ML19343A542
| ML19343A542 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 05/17/1976 |
| From: | YANKEE ATOMIC ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML19343A541 | List: |
| References | |
| NUDOCS 8011140576 | |
| Download: ML19343A542 (7) | |
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V' EFFECTS OF THE REDUCED ACCUMULATOR PRESSURE SETPOIf1T Off THE LOCA BREAK SPECTRUM ANALYZED FOR THE YANKEE fiUCLEAR POWER STATION Yankee Atomic Electric Company 20 Turnpike Road Westboro, Massachusetts 01581 i
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1.
IliTRODUCTI0il As discussed in references (1) and (2), it has been determined that the flow capability of the LPSI accumulator as tested at the plant is slightly greater than that calculated in the approved LOCA analysis (reference (3)) and in the more recent LOCA analysis currently being reviewed by the NRC (reference (4)).
The correction proposed by YAEC in reference (1) is to adjust the accumulator pressure at the plant so that its flow capability would agree with the reference (3) and (4) analyses.
The determination of the revised accumulator pressure setpoint made in reference (2) was based on matching the results of the worst break LOCA (0.6 DECLG) as presented in the reference (4) analysis.
This report discusses the effect of the reduced accumulator pressure setpoint on the results of break sizes other than the 0.6 DECLG.
2.
LARGE BREAK SPECTRUM For the large breaks, the onset of accumulator flow into the main coolant system occurs before E0BY (see Taole C3 of reference (4)),
The effect of the reduced pressure setpoint is to delay this onset siightly (approximately two seconds).
The accumulator coolant not delivered during this core bypass phase is thus available for use in extending the duration of the accumulator delivery during the reflood phase.
For example, as presented in reference (2), the effective accumulator empty time increases from 114.79 seconds in the February 20 analysis (reference (4)) to 115.87 seconds.
Since peak clad temperature occurs at 117.1 secon6, the extra 1.08 seconds of high reflood rate will tend to reduce the peak clad temperature.
In conclusion, the effect of the reduced accumulator setpoint on the large breaks is to improve the ECCS performance capability by providing slightly more accumulator coolant when it is more effective (during reflood) It the expense of providing slightly less accumulator coolant when it is less effective (during core bypass).
3.
SMALL BREAK SPECTRUM As break size decreases, the main coolant system depressurization rate decreases. This prolongs the duration of the blowdown phase and thus delays the onset of accumulator coolant injection into the main coolant system.
However, pumped injection becomes more effective, causing blowdown and reflood to occur simultaneously.
The onset of accumulator delivery approaches the calcriated time of PCT.
The onset of accumu-lator delivery becomes the critical parameter rather than the accumulator inventory as discussed in Section 2 above.
The concern is that there will be a sufficiently small break such that with the reduced accumulator pressure, the delay in the onset of accumulator flow will allow the clad
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temperature and/or oxidation to increase beyond the limits of 10CFR50.46.
The Why in accumulator onset becomes larger as the break size and blow-down rats decrease.
Furthermore, there is a sufficiently small break size whicit is entirely accommodated by pumped injection with no assistance from the accumulator.
A small break spectrum analysis has been performed and submitted to the itRC (reference (5)).
In this analysis, the' four-inch pipe rupture was the smallest which relied upon a timely initiation of accumulator injection to_ terminate the clad temperature transient.
In the next smaller break (three' inch pipe), the core was completely recovered by pumped injection alone.
The four-inch pipe rupture was also the most limiting small break.
An approved re-analysis of this. size break has been performed (references (3) and (6)) and the results are reproduced in figures 12B, 12E and 12F for convenience.
0 It is seen that while the peak clad temperature of 1848 F occurred at nine seconds due to premature Df;3, there is a third peak of 1400 F at 230 seconds.
The accumulator began delivery to the main coolant system at 230 seconds (380 psia).
With a reduced accumulator pressure of 352 psia, injection will be delayed to s 252 seconds (312 psia).
Assuming the adiabatic heatup which started at 215' seconds continues to 252 seconds 0
0 the third peak would be expected to increase from 1400 F to about 1680 F.
This is far below the limit of.10CFR50.46 and remains below the 1848 F PCT previously predicted.
3 I
In conclusion, the effect of the reduced accumulator pressure setpoint on the most sensitive small. break does not increase the peak clad temperature above the previously reported value.
4.
CONCLUSION j
The effects of the reduced accumulator pressure setpoint on various postu-lated LOCA break sizes have been reviewed and previously reported results remain valid and within the limits of 10CFR50.46.
5.
REFERENCES (1) Proposed Change No.138 to Facility Operating License No. DPR-3, submitted to the NRC on May 11, 1976.
(2) " Safety Evaluation of the Accumulator Pressure Setpoint for the Yankee Nuclear Power Station", submitted to the NRC on May 11, 1976.
(3) Amendment No. 21 to Facility Operating License No. DPR-3, issued by NRC on December 4, 1975.
(4) Supplement No.'8 to Proposed Change No. 125, submitted.to the NRC on February 20 -1976.
(5). Proposed Change No. 117, " Revised ECCS Analysis", submitted to D0L/AEC on July 31, 1974.
i (6) Supplement No.1 -to-Proposed Change NO.125, submitted to the NRC on Octoberi10, 1975.
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