ML20054F178
| ML20054F178 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, 05000000 |
| Issue date: | 06/07/1982 |
| From: | Tramm T COMMONWEALTH EDISON CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 4195N, NUDOCS 8206150285 | |
| Download: ML20054F178 (4) | |
Text
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y Commonwoelth Edison one Fatit National Plan Chicago, filinojs O } Address Reply to: Post Offico Box 767 g Chicago, Ithnois 60690 Jun e 7, 1982 Mr. Harold R. Denton, Director Of fice of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Braidwood Station Units 1 and 2 Locked Rotor and Shaft Break Transients NRC Docket Nos.
50-454, 50-455, 50-456 and 50-457
Dear Mr. Denton:
This is to provide additional information regarding reactor coolant pump locked rotor and shaf t break transients at Byron and Braidwood.
Issue 34 o f the Byron SER. Review of this information should close Confirmatory ses o f situations involving either reactor coolant pump rotorThe B seizure or shaf t break.
In both cases the reactor trips in less than one-tenth of a second and the maximum clad temperature is reached in less than four seconds.
over in ten seconds.
The transients are essentially in such a transient and a consequential interruption of power toT plant auxiliaries including reactor coolant pumps could make the transient more severe.
Additional analyses of such events and a re-examination o f the limiting single f ailure were requested.
discussed below, As representative transients of this type have been evaluated and the consequences determined to be acceptable.
Consequential Interruption Scenarios Consequential interruption of power to plant auxiliaries is possiole in only two ways.
Switching of bus feeds at the time of generator trip could involve a breaker failure that would leave one or more reactor coolant pumps without power.
Trip o f the turbine could disrupt grid stability that might result in low voltage or frequency trip of reactor coolant pumps.
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H. R. Denton Ju n e 7, 1982 Switching At Byron and Braidwood the generator trip is delayed thirty seconds past the turbine trip caused by a reactor trip in a loss of flow transient.
During this delay period power flow through the main transformers reverses to maintain generator terminal voltage and supply auxiliary buses that feed reactor coolant pumps.
After thirty seconds these buses are automatically switched to system auxiliary transformers f ed from the 345 kv yard.
This switching is delayed to provide an extra measure of turbine overspeed protection.
It also precludes switching f ailures which could interrupt power to plant auxiliaries during the first few crucial seconds of a locked rotor or broken shaf t transient.
As noted in the FSAR, the locked rotor event concurrent with a loss of offsite power at the time of generator trip has been evaluated.
Without offsite power this transient ultimately results in the coastdown of the reactor coolant flow when the reactor coolant pumps are tripped.
As shown by the FSAR analyses summarized earlier this will have little ef fect because the severity of the transient will have turned around.
Grid Stability The turbine trip associated with a locked rotor or broken shaf t transient would not cause instability on the Commonwealth Edison grid and could not complicate the transient further.
From our operating experience at Zion Station, a plant similar in size, no unit trip has ever caused a noticeable instability in the grid.
System voltage has never dropped to the point of tripping reactor coolant pumps.
If grid instability should ever be induced by a locked rotor or broken shaf t transient, its effect on the plant would be slower than the initiating transient itself.
The grid instability would manifest itself in a reduced frequency.
High fault currents would then occur in the transmission lines.
Au tomatic breaker trips would begin in a cascading manner to isolate the faults.
Since the highest f ault currents would be seen first at points furthest from the station, the station would be the last, or close to the last, point to be isolated from the grid.
This series of events would take longer than four seconds, which is the time o f the peak of the transient, and neither the frequency nor voltage decay to the point i
of pump trip during this time.
The analysis of the locked rotor event is applicable to the broken shaf t event with concurrent loss of of fsite power.
Single Failure The Staf f has also suggested that these events should be analyzed assuming the worst single failure of a safety system active l
component.
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H.R.
Denton 5 Jun e 7, 1982 The single active f ailure assumed 16 t*he Locked Rotor / Pump Shaf t Break analysis presented in the Byron /Graidwood FSAR is thei loss of one protection train used to initiatM'a reactor trip on a,d low flow signal.
This failure, or any otherifa11ure, will not have an e f rect on the transient because o f.the transient's relatively(v.
short duration.
In less than four seconds a fter initiation 'of t ie,1, accident, the safety parameters of concern (ddparturs)from nuclejte boiling ratio, reactor coolant pressure ane' clad temperature) have reached their maximum (or minimum for OliSG) Values and begin to approacn steady state values.
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The following are additiona,1 singl'e active failures that are assumed in other accidents.
As ne.ted, none o f them will increase the severity of the Locked Ro' tor accident.
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Loss o f a safety injection train,- Safety injection is not required to mitigate the effects of this accident.
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1 Loss o f an auxiliary feedwater pump - The Lo'cked Rotor Accident is turned around 'before the
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auxiliary feedwater ( AFW)' pumps could be turaed on.
s PORV s tuck open - The Locked Rotor < Accident ib turned around before the stuck open[PORV w'ould have,
,,. m any e f fect.
A stuck open PORV is analyzed irr Section 15.6.1.
Failure o f a main s team isolation valve (MSIV) -
The MSIVs are not required to mitigate the ' effects of the Locked Rotor Accident.
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j Failure o f a feedline isolation valve *-- The Locked
'l Rotor Accident is turned around before; this. failure.
4 Stuck open secondary, side valve - Radiological effects due to a sttfck open valve on.the, secondary side are of no consequence for therfollowing reasons.
Steam releases assuped,in the radiolo-gical analysis for the locked rotor accident assumed a conservatively high steam release for eight hours.
The conservatisms include (1) no steam dump to the condenser (2) a.qigh rate of decay heat and (3) a high level ~.o f energy stored in i the reactor system structure.
Steam releases computed on the above basis will always exceed releases that could occur while bringing the plant to a shutdown condition after the accident.
The re fo re, a stuck open relief, safety, or dump valve is of no consequence to the radiological releases calculated.
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H..' R. Danton Jun e 7, _198 2
/ Based on the short duration of the transient and a review o f othEP single failures, the loss o f a protection train is -an appropria'te single f ailure for the locked rotor accident both with and without offsite power available.
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in summary, tria gen <erator trip delay and grid stability
- c provide assurance that the events analyzed in the FSAR are appropri-ate # design basis loss of flow' transients.
Failure of one protection
{ }vtral'n is ap appropriate design basis: failure for the transient.
Please direct further questions regarding this matter to
, n th'is o f fice.
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One signed original and fif teen copies o f this letter are provided for your use.
J Very truly yours, j/?<f W M
T. R.
Tramm j
Nuclear Licensing Adminstrator j
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