ML19312C129
| ML19312C129 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 07/15/1975 |
| From: | Parker W DUKE POWER CO. |
| To: | Anthony Giambusso US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 7912060678 | |
| Download: ML19312C129 (7) | |
Text
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Mr. Angelo Giambusso, Director Division of Reactor Licensing U. S. Nuclear Regulatory Comission Washington, D. C.
20555 Re: Oconee Nuclear Station P cket Nos. 50-269, -270, -287
Dear Mr. Giambusso:
With re' gard to Mr. R. A. Purple's letter of May 13, 1975 requesting information concerning the potential for secondary system fluid flow inst bilities, the a tached response is provided.
V y truly yours, /
/1 $ %. _ - l.fi. u t '
William O. Parker, Jr.
P.ST:vr Attachment i
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DUKE POWER C0MPANY OCONEE NUCLEAR STATION UNITS 1, 2,
AND 3 RESPONSE TO MR. R. A. PURPLE'S LETTER OF MAY 13, 1975 July 15, 1975
Question:
1.
Describe all operating occurrences that could cause the level of the water / steam interface in the steam generator to drop below the feed-water sparger or inl(
nozzles, and allow steam te enter the sparger and/or the feedwater giping.
RESPONSE
The design of the B&W once-through steam generator (OTSG) requires that the level of the water / steam interface remain below the feedwater inlet nozzles during operation. However, the arrangement of the feedwater nozzl,s, ex-ternal ring distribution header, and feedwater piping leading up to the header is such that steam cannot enter the feedwater piping.
The piping i
immediately external to the steam generator contains a " gooseneck" or trap arrangement which will always remain filled with water and preclude the possibility of steam entering the feedwater piping.
Question:
2.
Describe and show by isometric diagrams, the routing of the main and auxiliary feedwater piping from the steam generators outwards through containment up to the outer containment isolation valve and restraint.
Note all valves and provide the elevations of the sparger and/or inlet nozzles and all piping runs needed to perform an independent analysis of drainage characteristics.
RESPONSE
Attached are two isometric diagrams showing the routing of the main and auxiliary feedwater piping from the steam generators outwards through containment up to the outer containment isolation valve and restraint.
l Although these figures are prepared for Oconee Unit 1, they are typical of all three units.
Question:
3.
Describe any " water hamraer" experiences that have occurred in the feed-water system and the means by which the problem was permanently corrected.
RESPONSE
During reactor startup, with reactor coolant temperature of 275 F, the startup feedwater valves intermittently open and close to maintain a minimum steam generator level of 25 inches. A water hammer was observed when feedwater flow was established after being secured for a relatively long period of time.
In order to prevent this possibility of water hammer, a small one-inch bypass line around the feedwater control valve maintains a continuous flow rate to the steam generators prior to power operation.
This small flow rate (which begins when the unit is cold) will in itself keep the feedwater lines leading up to the steam generator full of water
and preclude steam from entering the piping. Once power operation begins, the normal feedwater flow rate fills the pipes and feedwater distribution header and nozzles with water and they remain filled throughout power operation.
A minor feedwater hammer has been observed during the loss of normal feed-flow.
This results due to the few seconds required for the emergency feedwater pump to accelerate to rated speed and the necessary automatic valves to properly position. This is considered of minor significance due to the relatively small amplitude of the hammer and the infrequency of loss of normal feedflow.
Question:
4.
Describe all analyses of the feedwater and auxiliary feedwater piping systems for which dynamic forcing functions were assumed. Also, provide the results of any test programs that were carried out to verify that either uncovering of the feedwater lines could not occur at your facility, or if it did occur, that " water hammer" would not occur.
RESPONSE
The Oconee Nuclear Station feedwater system has been reviewed to determine the potential for " water hammer" during anticipated operational occurrences.
It has been concluded that the existing Oconee feedwater system is adequate and does not require changes in design or operation in order to prevent flow instabilities.
Because derign features of the feedwater system preclude the probability of destructive " water hammer" forcing functions resulting from uncovering feedwater lines, no analyses have been performed nor test program conducted regarding this occurrence.
The following considerations support this conclusion:
a.
Neither the main nor auxiliary feedwater ystems have horizontal or downward-sloping pipe runs adjacent to the steam ganerator.
The auxiliary piping remains below the level of its junction with the steam generator.
The main feedwater line rises above its steam generator connection only after downward and horizontal runs which effectively form a loop seal.
Only in the unlikely event cf steam generator shell pressure near the vapor pressure of the water in this pipe could a steam void occur.
b.
The main and auxiliary feedwater distribution headers on the steam generator are designed to remain flooded regardless of steam generator water level, and would in any event be self-venting if steam were intro-duced.
The main ring header is fed from the bottom, external to the steam generator, and empties upward through the vertical inlet lines.
The auxiliary ring headers on Units 1 and 2 are similar in design to the main header. Unit 3 auxiliary header is internal to the steam generator shell but empties through ports near the top of the header.
Hence, none of the feedwater headers can spontaneously drain into the steam generator.
(
c.
Each steam generator has its auxiliary header separate from the main header. Therefore, there is no need to deliver the relatively cool auxiliary feedwater through the normal path for main feedwater.
]
Question:
5.
Discuss the possibility of a sparger or nozzle uncovering and the con-sequent pressure wave effects that could occur in the piping following a design basis loss-of-coolant accident, assuming concurrent turbine trip and loss of off-site power.
RESPONSE
The steam generator water level is below the feedwater inlet nozzles during power operation. However, steam will not enter the feedwater piping and no i
pressure wave effects will occur in the piping following a design basis accident with concurrent turbine trip and loss of off-site power due to the
" gooseneck" arrangement of the feedwater piping directly external to the I
Even when the main feedwater pumps trip and feedwater flow j
rate is zero, the trap remains full of water and precludes any steam in the piping.
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A test was run on Oconee Unit 1 f rom 40 percent power in which the main
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feedwater pumps and the turbine generator were tripped and auxiliary feed-water flow was initiated.
The auxiliary feedwater system on the OTSG (including distribution header and piping) is completely separate from the main feedwater system. The auxiliary feedwater enters the unit through a i
separate header at the top of the tube section.
This test very closely simulates the effects of a loss of off-site power on the secondary plant.
The steam generator and feedwater piping directly adjacent to the generator were monitored for noises using the Loose Parts Monitoring System; no unusual noises were heard, confirming the fact that no water hammer in-the feedwater piping occurred during the test.
a Question:
6.
If plant system design changes have been or are planned to be made to preclude the occurrence of flow instabilities, describe these changes or modifications, and discuss the reasons that made this alternative superior to other alternatives that might have been applied. Discuss I
the quality assurance program that was or will be followed to assure.
that the planned sytem modifications will have been correctly accomplished at the facility.
If changes are indicated to be necessary for your -plant, consider and discuss the effects of reduced auxiliary feedwater flow as a possible means of reducing the magnitude of induced pressure waves,
-including positive means (e.g., interlocks) to assure sufficiently low-i flow rates and still meet the minimum requirements for the system safety j
function.
RESPONSE
i As described above, the probability of significant feedwater flow instabilities in the Oconee once-through steam generators is negligible. For this reason, l
no design changes have been, or are planned, for the Oconee feedwater systems.
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