ML20238A272
| ML20238A272 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/24/1987 |
| From: | Hukill H GENERAL PUBLIC UTILITIES CORP. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| 5211-87-2131, NUDOCS 8708280390 | |
| Download: ML20238A272 (4) | |
Text
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GPU Nuclear Corporation (A Nuclear g,e r,w8o j
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Middletown, Pennsylvania 17057-0191 3
l 717 944 7621 I
TELEX 84 2386
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Writer's Direct Dial Number:
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l August 24, 1987 5211-87-2131 l
U. S. Nuclear Regulatory Commission l
Attention: Document Control Desk Washington, D.C.
20555 Gentlemen:
Three Mile Island Nuclear Station, Unit 1 (TMI-1)
Operating License No. DPR-50 Docket No. 50 289 OTSG Fouling.- Long Term Plan TMI-l has experienced power output limitations due to secondary side fouling l
of the Once Through Steam Generators.
This letter briefly reviews prior GPUN actions on this problem and describes our planning for future remedial action should such generator fouling again inhibit power output.
As background information, Once Through Steam Generator fouling experienced at TMI is not unique but has occurred at a number of plants with B&W nuclear steam supply systems and Once Through Steam Generators.
The fouling is believed to be caused by the deposition of metal oxides in the boiling region of the steam generator. These metal oxides are introduced into the generator principally via the feedwater system. The magnitude of the metal oxides introduction is a function of the oxygen content of the feedwater, water pH, overall water chemistry, and operation of the secondary side plant drain systems.
Substantial effort has been made at TMI to minimize the introduction of these materials into the steam generator.
Once introduced, the metal oxide can deposit in the generator either on the tubes or other surfaces, including j
directly in the broached holes in the tube support plates.
For material 1
deposited directly in the broached hole, it will act to reduce the water / steam flow area and thereby directly increase generator pressure drop.
For material deposited on the tubes, that material, after building to sufficient thickness and subjected to thermal cycles (for example, normal shutdown), can spall from the tubes in the form of loose flakes.
This spalled material can be suspended on the underside of tube support plates during operation, also blocking water and steam flow passages and increasing system pressure drop. This latter form of increased pressure drop predominates at THI-l as determined by direct inspections approximately a year and a half ago.
With sufficient increase in i
l the secondary side water / steam pressure drop, the water level in the Once Through Steam Generator 8708280390 070024 PDR ADOCK 05000289 k
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GPU Nuclear Corporation is a subsidiary of the General Public Utilities Corporation
5211-87-2131 downcomer increases to compensate until such time as the level reaches a minimum level below the main feedwater nozzles.
If that should happen, feedwater heating is severely reduced and instabilities can occur.
This condition is an unacceptable operating point for the generator.
Water level in the downcomer can then only be reduced by either reducing the obstructions caused by debris or reducing steam and feedwater flow rate.
The first time a power limitation was observed at TMI was late 1985.
Subsequently, as part of the Power Escalation Test Program, a turbine reactor trip was performed. When the plant was restarted, steam generator level was low enough to achieve full power operation.
The second time a power limitation was experienced was early in 1987 at which time the plant was limited to about 83% power level.
When the power level limitation was observed, GPUN evaluated all of the available options to determine if near term corrective action could be taken. These options included techniques for trying to redistribute the particulate matter at power by introducing small pressure fluctuations in the generator through turbine throttle / bypass valve operation, deliberate power ramp downs follnwed by power increases, planned reactor shutdown, and a process referred to as " water slap," performed while the plant is shut down.
Over a period of several weeks the various alternatives were evaluated.
Introduction of minor pressure fluctuations and powering the plant down were performed without impact on generator operation.
We also initiated work to be prepared for " water slap" should that become necessary. After many detailed reviews, including a specific safety review, a planned automatic reactor shutdown from a manual turbine trip was performed.
Upon plant restart, generator levels had recovered to permit 100% power operation with adequate margin.
The planned shutdown was performed with a special temporary procedure and existing plant procedures.
Additional Operations personnel were assigned during the shutdown, and site and corporate technical support were also present.
i In the long tenn, GPUN is convinced the only technically viable solution to the generator fouling is to continue maintenance of high quality water chemistry coupled with periodic but infrequent chemical cleaning of the secondary side of the steam generator. As of this date, no Once Through Steam Generator has been chemically cleaned, although one plant is scheduled for cleaning in the Fall.
In fact, very few nuclear steam generators of any type have been cheniically cleaned. GPUN is actively reviewing chemical cleaning options, and we have been instrumental in trying to expedite final EPRI review and qualification of the KWU developed process.
Our own assessment of where we are, what data is and will be available in the industry, and the TMI-1 operating cycles suggests the earliest time we could implement such cleaning would be the scheduled 1990 outage.
Depending upon the final process selected and the needed facilitation, it is conceivable it could not be ready until the following outage.
r 5211-87-2131.!
Giver the unavailability of chemical cleaning in the near term, the only options in the event future steam generator limitations reappear are mechanical " water slap" or techniques which alter water / steam flow paths in the generator such as power transients or the plant trip referred to earlier.
We are continuing to prepare the plant, the procedures, and the er 2 rating staff to perform " water slap" should it be required.
However, wc would also intend to fully reassess technical, operational and safety aspects of all possibilities before any future decisions are made for TMI-1. This would j
include examination of where the plant was in the operating cycle and other i
technical trade-offs including the implication of opening the steam generators
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and subsequently subjecting them to the " water slap" pressure loadings (which have been determined to be greater than those developed by planned manual plant shutdown, although both are well within the Once Through Steam Generator load capabilities).
In summary, GPUN, both independently and in conjunction with others, has and will continue to assess and support evaluations to correct Once Through Steam Generator fouling and do so on a manageable, low risk basis.
Ultimately, we are convinced that chemical cleaning will be required and are actively working to support and evaluate final process qualification. Any future near term course of action will be predicated on a careful, thoughtful review of the circumstances at the time.
We will keep the Staff fully informed of these ongoing evaluations.
Sincerely, e
i 1
. D. F ukill, Jr.
Vice President and Director, TMI-1 HDH/SMK:jh 5156g/0961A l
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