ML20203N845
| ML20203N845 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/29/1986 |
| From: | Withers B PORTLAND GENERAL ELECTRIC CO. |
| To: | Varga S Office of Nuclear Reactor Regulation |
| References | |
| TAC-60783, NUDOCS 8605060110 | |
| Download: ML20203N845 (10) | |
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Ekut D W(hers Vce Presdent April 29, 1986 Trojan Nuclear Plant Docket 50-344 License NPF-1 Director of Nuclear Reactor Regulation ATTN:
Mr. Steven A. Varga Director, PWR-A Project Directorate No. 3 U.S. Nuclear Regulatory Commission Washington DC 20555
Dear Sir:
Auxiliary Feedwater System Anticipated modifications to the Trojan Auxiliary Feedwater (AFW) System for improved reliability have been the subject of several discussions between PGE and the NRC since late 1985. During a telephone conversation between Ken Johnston of your staff and Gary Zimmerman on February 24, 1986, Ken identified an NRC concern over the effect of AFW System changes on the NRC's safety evaluation of PGE's responses to Long-Term NRC Recommendations GL-2 and GL-4.
Those NRC recommendations were provided to PGE in a letter dated October 3, 1979. Ken Johnston indicated that submittal of a description of the effect of system modifications on the CL-2 and CL-4 safety evaluation is required. That description is attached. As requested, changes to our 1979 and 1980 exchanges concerning Long-Term Recommenda-tions GL-2 and GL-4 are described, as well as anticipated changes to the Trojan FSAR and compliance with Section 10.4.9 of the Standard Review Plan. A complete description of the AFW System modifications we intend to make, and our schedule for completing them was provided in our letter of November 24, 1985 (B. D. Withers to E. J. Butcher, Jr).
As described in that letter, many of the system modifications have been scheduled for the 1986 refueling outage which began on April 16, 1986.
Please notify us if further information is required.
Bart D. Withers Vice President Nuclear Attachment c:
Mr. John B. Martin Regional Administrator, Region V U.S. Nuclear Regulatory Commission Mr. Lynn Frank, Director State of Oregon Department of Energy
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Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 1 of 9 EVALUATION OF RELIABILITY IMPROVEMENTS TO THE TROJAN AUXILIARY FEEDWATER SYSTEM I.
Summary of Channes Currently, the safety-related auxiliary feedwater (AFW) pumps are protected against a loss of suction common-mode failure by means of low suction pressure trips. In the event of a loss or blockage of their common suction piping or loss of condensate storage tank (CST) inventory due to, for example, a seismic or tornado event, the resulting decrease in AFW pump suction pressure will cause the pump (s) to trip, thereby preventing pump damage. Operator action would then be relied upon to alleviate the cause of the loss of suction or to manually shift the pump suction to the Seismic Category I Service Water System, if required.
Because of recurring operational difficulties with the low suction pressure trips, such as spurious pump trips and an attendant decrease in AFW System reliability, the low suction pressure trips for the safety-related AFW pumps will be replaced.with pump trips caused by low level in the CST.
Associated design changes, such as removing the valve in the common suction piping, installing a flexible coupling between the suction piping and the CST, and providing a tornado barrier for the above-grade suction piping near the CST, will ensure that the safety-related low CST level trip is no less effective in preventing a common-mode failure to the safety-related AFW pumps than the low suction pressure trips.
II.
Evaluation i
A.
FSAR Changes 4
The intended AFW System modifications will require changes to the system description in FSAR Section 10.4.9.
Current description statements that will require correction are as follows:
1.
In Section 10.4.9.1, the second paragraph on Page 10.4-46 indicates that the condensate supply lines from the CST to the pumps do not meet Seismic Category I requirements.
In conjunction with these system modifications, piping from the CST to the safety-related AFW pumps' suction will be analyzed to show that it meets the design requirements of Seismic Category I.
In addition, MD-050, the common suction piping isolation valve, will be deleted from the system design and removed.
4 2.
In Section 10.4.9.2.3.2, the first paragraph on Page 10.4-54 indicates that the AFW pumps will automatically shut off if
s Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 2 of 9 pump suction pressure falls below a preset level. As pre-viously discussed, the AFW safety-related pump low suction pressure trips are being replaced with safety-related auto-matic pump trips due to low level in the CST.
3.
In Section 10.4.9.4, the first paragraph on Page 10.4-56 indicates that all Seismic Category I components of the AFW System were designed, fabricated, installed and tested under quality assurance requirements in accordance with 10 CFR 50 Aprendix B.
The common suction piping from the safety-related AFW pumps to the CST was fabricated and installed as Seismic Category II.
It will be analyzed to show that it meets the design requirements of Seismic Category I.
4.
In Section 19.4.9.5, the third paragraph on Page 10.4-61 indicates that a pressure transmitter is provided in the suction lins of each safety-related AFW pump to provide automatic rump shutoff due to a low suction pressure con-dition. Pamp suction pressure transmitters are still provided. but only for suction pressure indication and low suction pressure alarms.
B.
Long-Term NRC Recommendations In a letter dated October 3, 1979 (Darrel C. Eisenhut to Charles Goodwin), PGE was advised of NRC requirements for the Trojan AFW System that were identified during the course of NRC reviews of operating reactors following the accident at Three Mile Island.
Included were six long-term recommendations for improving the AFW System. Two of those recommendations, GL-2 and CL-4, involve automatic pump protection in the event of loss of suction, and component protection against tornado and seismic events. PGE responses to the NRC acceptance of (NRC letter and SER dated October 23, 1980), and the effects of these design changes on those recommendations are as follows:
1.
Recommendation CL-2 Licensees with plants in which all (primary and alternate) water supplies to the AFW System pass through valves in a single flow path should install redundant parallel flow paths (piping and valves).
Licensees with plants in which the primary AFW System water supply passes through valves in a single flow path, but the alternate AFW System water supplies connect to the AFW System pump suction piping downstream of the above valve (s), should install redundant valves parallel to the
Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 3 of 9 above valves or provide automatic opening of the valves from the alternate water supply upon low pump suction pressure.
The Licensee should propose Technical Specifications to incorporate appropriate periodic inspections to verify the valve positions.
a.
PCE Response (November 26, 1979)
The AFW System design at Trojan employs a common suction line from the primary water source (CST) to the AFW pumps. We are presently evaluating the need for installing a redundant valve parallel to the existing single valve between the CST and the AFW pump suctions.
The results of our evaluation will be available by January 1, 1980.
PGE Response (December 31, 1979)
The AFW System design at Trojan employs a common suction line from the primary water source (CST) to the safety-grade AFW pumps. The single valve (MD050) in this suction line is locked open, is subject to monthly surveillance and is provided with position indication as described in the response to Recommendation GS-2.
In addition, a separate flow path exists from the CST to the electric motor-driven AFW pump currently being installed. Taken in combination with the.inw AFW pump suction trips and alarms (see response to Recommenda-tion GS-4) and the redundant CST level indications and alarms, no additional modifications are considered necessary to increase the reliability of this aspect of the AFW System.
As discussed in response to Recommendation GS-6, current Periodic Operator Tests and Technical Specifi-cations exist to ensure flow path valve position.
b.
NRC Evaluation (October 23, 1980)
.In lieu of the long term requirements presented.
for assuring AFW pump protection in the event of an inadvertently closed valve on the primary AFW supply from the condensate storage tank, the licensee stated that prior to startup from the next refueling outage, low suction pressure alarms and safety-grade automatic trip of the AFW pumps on low suction pressure will be installed.
Operator action can then be taken locally to
Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 4 of 9 i
manually open the normal supply valve or remotely to 4
open the alternate supply (service water system) valves as necessary.
In addition, control room position indication for the single locked open primary AFW supply l
line suction' valve will also be installed. The licensee further notes that the new electric motor-driven AFW pump will be provided with a separate flow path from the condensate storage tank to the pump suction. We conclude that the licensee's response is acceptable, c.
Desi;a channes During the 1986 refueling outage (April 16-June 10, 1986), Valve MD-050 in the common suction line from the CST to the safety-related AFW pumps will be removed.
It will be replaced by a flexible coupling between the AFW suction piping and the CST.
Also, a debris screen will be added to the CST outlet nozzle to prevent blockage.
In addition, as previously discussed, the common suction piping (below grade) will be analyzed to show that it meets the design requirements of Seismic Category I, and the safety-related AFW pumps' low suction pressure trips will be replaced by AFW pump trips caused by low level in the CST.
d.
Comparison The combination of these design changes ensures that AFW 1
System performance and reliability will be equal to or better than the existing system. By replacing the common suction valve with a flexible coupling and effec-I tively providing a Seismic Category I suction line from the CST, the design change ensures that a safety-related AFW pumps loss of suction can only occur due to:
(1) a loss of inventory in the CST, or (2) blockage of the CST outlet nozzle. Provision of safety-grade CST low water level trips ensures that the pumps will be automatically shut off in the event of low CST level. Operator action can then be taken to restore CST level, or to open the supply valves from the alternate AFW supply (ie, the safety-related Service Water System). The new debris screen at the CST outlet nozzle will preclude blockage.
2.
Recommendation GL-4 J
Licensees having plants with unprotected normal AFW System water supplies should evaluate the design of their AFW j
Systems to determine if automatic protection of the pump is Z
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Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 5 of 9 necessary following a seismic event or a tornado. The time available before pump damage, the alarms and indications available for the control room operator, and the time nee-essary for assessing the problem and taking action should be considered in determining whether operator action can be relied on to prevent pump damage. Consideration should be given to providing pump protection by means such as auto-matic switch over of the pump suction to the alternate safety-grade source of water, automatic pump trips on low suction pressure, or upgrading the normal source of water to meet Seismic Category I and tornado protection requirements.
a.
PGE Response (November 26, 1979)
.We are planning to provide a low pump suction pressure trip on the AFW pumps. This modification will be complete prior to startup of Cycle 3.
b.
NRC Evaluation (October 23, 1980)
The concern raised in this recommendation is similar to that of Recommendation GL-2, namely, providing auto-matic protection for the AFW pumps in the event of loss of normal suction supply for whatever reason. We con-sider the measures provided by the licensee and dis-cussed in this SER under Recommendation GL-2 to be acceptable, and these same features to also satisfy the requirements of this item.
c.
Design Changes In addition to the AFW System design changes described in Item 1.c above, the following modifications will be accomplished to protect the safety-related AFW pumps normal water supply (ie, the CST) and other components from damage during a seismic event or a tornado.
+ A tornado missile barrier will be installed around the above-grade suction piping near the CST.
The safety-related AFW pumps' suction piping at the tank nozzle will be attached by means of a flexible metal bellows-type coupling that will allow seismic displacement of the tank without inducing loads in the suction piping.
(The CST is Seismic Category II.)
+ In addition to the stiffness provided by the bolting ring (located just above the tank nozzle), stiffeners will be added to the CST wall in the vicinity of the
Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 6 of 9 suction nozzle to preclude significant tank deforma-tion (in the nozzle vicinity) during a seismic event.
+ The CST level transmitters will be safety-grade, and will be supported from the tank footing.
+ The CST level taps and transmitters will be housed in breakaway enclosures such that impact by a tornado missile will cause the entire enclosure to carry away. The level sensitg circuitry will be designed so that such an event will cause the level indication (for that channel) to fail low (conservative).
+ The CST level taps and transmitters will be physi-cally spaced around the tank such that the tank curvature precludes them both from being damaged by the same missile.
+ Wiring for the CST level transmitters will be run through buried or physically separated conduit to preclude damage by seismic or tornado events.
+ A new debris screen will be added on the inside of the CST at the turbine and diesel AFW pump common suction nozzle for the purpose of preventing the suction line from becoming clogged, d.
Comparison In the event of a seismic or tornado event during, or associated with, a Plant trip that calls upon the AFW System to operate, the safety-related AFW pumps will still be protected from a loss of suction by the safety-related low CST level trips. Since the suction piping (below grade) will be upgraded to the require-ments of Seismic Category I, and there are no valves to fall, the only potential locations for loss of suction failure modes are the CST (Seismic Category II) and the tank-to-piping coupling at the tank nozzle. The flexible coupling installed at that location will preclude a scenario where the suction piping and tank are displaced from each other (eg, due to a seismic event), producing a situation where the pumps could lose suction but would not immediately trip on low CST level. The low CST level trips protect the safety-related AFW pumps from damage in the event of other seismic / tornado events affecting the CST.
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Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 7 of 9 v
In the event of a tornado-driven missile that ruptures i
the CST, the following indications are available in the control room to alert the operators:
+ AFW pump control indications,
+ AFW pumps' suction pressure indications and alarms, 4
+ Low CST level alarms,
+ Low CST level AFW pump trip alarms,
- Low-low CST level alarms, and
+ Steam generator level indications and alarms.
i These indications will provide ample warning of an impending loss of AFW, enabling the operators to take the necessary steps to alleviate the problem or transfer suction to the Service Water System.
C.
Standard Review Plan Section 10.4.9 of the Standard Review Plan (NUREG-0800) identi-fies the design requirements and the required functional per-formance capabilities of PWR AFW Systems. Included are requirements for physical separation or shielding to protect essential parts of the system from externally generated missiles. Reference is made to SRP Section 3.5.2 which in turn references Regulatory Guide 1.117.
As previously described, the safety-related AFW pump trips on low CST level will be provided by Seismic Category I level transmitters displaced around the tank circumference such that protection from damage by a single tornado missile is provided by the tank itself.
The portion of AFW suction piping that is located above grade will be protected by a tornado missile barrier, and if the tank should be ruptured by a tornado missile, safety-related AFW pump protection will be provided by the CET level trips.
Other AFW System design requirements discussed in SRP Sec-tion 10.4.9, such as the capability of withstanding a single active failure, the capability of automatically initiating AFW flow upon receipt of an actuation signal, sufficient AFW flow capacity and the capability for sustained func~ionability dur-t ing hot shutdown followed by a cooldown, are not affected by these system design changes.
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e Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29. 1986 4
License NPF-1 Attachment 4
Page 8 of 9 III.
10 CFR 50.59 Evaluation A.
These changes will not increase the probability of an accident, nor the probability of a malfunction of equipment important to safety, previously evaluated in the FSAR. These changes will increase the reliability of the AFW System by-improving design features which formerly contributed to system unreliability.
The safety-related AFW pumps' low suction pressure trips are being eliminated (replaced by low CST level trips). The valve in the common suction line (a potential source of common-mode failure for the safety-related pumps) is also being eliminated.
These changes will have no effect on the FSAR Chapter 15 analyses that rely on AFW.
The assumptions made in Sec-tion 15.2.7 (Loss of Normal Feedwater Flow) for example, are still valid. Similarly, the availability of the AFW System to provide a source of feedwater in the event of a main feedwater line rupture (Section 15.2.8) is not adversely affected.
B.
These changes will not increase the consequences of an acci-3 l
dent, nor of a malfunction of equipment important to safety, previously evaluated in the FSAR. Increased AFW System protection against seismic or tornado events is provided by:
1.
Analysis of buried piping, and buried cabling or hazardous area separation.
2.
A tornado missile barrier for the section of pump suction piping that is above grade.
3.
A flexible coupling installed between the Seismic Category I (by analysis) pump suction piping and the Seismic Category II CST.
i 4.
Removal of the valve in the common suction line.
5.
A debris screen inside the CST to prevent the suction nozzle from becoming clogged by debris from the floating roof.
6.
Stiffeners added to the CST in the nozzle area to restrict seismic deformation.
C.
These changes will not create the possibility of an accident nor a malfunction of equipment important to safety, which is different from any previously evaluated in the FSAR. The most significant of these changes involves replacing the low suction pressure AFW pump trips with trips due to low level in the
Trojan Nuclear Plant Mr. Steven A. Varga Docket 50-344 April 29, 1986 License NPF-1 Attachment Page 9 of 9 CST.
The modified design accomplishes the same thing - protec-tion of the AFW pumps from a common mode loss of suction - in a more reliable manner. Reliability of the low CST level trips is ensured by providing fail-safe (ie, in the conservative direction) CST level sensors and Seismic Category I level transmitters.
D.
The margin of safety, as defined in the Bases for the AFW System Technical Specifications (B 3/4.7.1.2) is not reduced by these changes. There will be no adverse effect on operability of the AFW System, nor on the capacity of the AFW pumps.
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