ML20247M001
| ML20247M001 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 07/21/1989 |
| From: | Hunger G PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-004, IEB-88-4, NUDOCS 8908010407 | |
| Download: ML20247M001 (5) | |
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PHILADELPHIA ELECTRIC COMPANY
- NUCLEAR GROUP. HEADQUARTERS 955-65 CHESTERDROOK BLVDc WAYNE PA 19087 5691 (zis) sao-sooo July 21,'1989
. Docket Nos. 50-277' 50-278 License Nos. DPR-44 DPR-56' U.S. Nuclear Regulatory Commiission
' Attn: Document' Control Desk-Washington, DC 20555-
SUBJECT:
Peach Bottom Atomic Power Station, Units 2 an<l.3 Updated Response to 14RC Bulletin No. 88-04, " Potential Safety-Related Pump Lons"
Dear Sir:
NRC Bulletin (NRCS) 88-04, " Potential Safety-Related Pump Loss," issued on May 5, 1988 requested licensees to investigate two potential design concerns involving safety-related centrifugal pumps. The concerns include the potential for a pump to dead-head when it is operating in the minimum flow recirculation mode in parallel with another. pump, and also includes concerns over the design adequacy of pump minimum flow capacities.
The NRC requested that within 60 days of receipt of NRCB 88-04, Philadelphia Electric Company (PECo) provide a written response:
a) to describe the problems and the systems affected, b) to identify any necessary short-term or long-term modifications to plant-
-equipment or to plant operating procedures which are being implemented to ensure safe plant operations,
- c) to provide a schedule for long-tenn resolution of significant problems
-identified as a result of this Bulletin, and d) to provide justification for continued operation of the plants with particular attention being placed on the General Design Criterion 35 of Appendix A to Title 10 of the Code of Federal Regulations (10 CFR 50),
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Document Control Desk July 21. 1989 Page 2 i
" Emergency Core Cooling" and 10 CFR 50.46, " Acceptance Criteria for i
Emergency Core Cooling System for Light Water Nuclear Power Reactors."
By letter dated June 30, 1988, PECo responded to NRCB 88-04 and, as was requested by item (b) above, both short and long-term resolution action items were provided. The Bulletin also requested that w' thin 30 days of completion of any long-term resolution action items, a written response describing those actions be provided.
The attachment to this letter describes the long-term resolution actions we have taken at Peach Bottom Atomic Power Station (PBAPS), Units 2 and 3, to resolve the concerns raised in the Bulletin.
Should you have any questions or require additional information, please do not hesitate to contact us.
Very truly yours, G. A. Hunger, J.
Director Licensing Section Nuclear Support Attachment cc:
W. T. Russell, Administrator, Region I, USNRC T. P. Johnson, USNRC Senior Resident Inspector T. E. Magette State of Maryland J. Urban, Delmarva Power R. A. Burricelli, Public Service Electric & Gas H. C. Schwemm, Atlantic Electric T. H. Gerusky, Commonwealth of Pennsylvania
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Docket Hos. 50-277 50-278 Attachment UPDATED RESPONSE TO NRC BULLETIN 88-04
" POTENTIAL SAFETY-RELATED PUMP LOSS" HRC Bulletin 88-04, " Potential Safety-Related Pump Loss," requested licensees to investigate two potential design csncerns involving safety-related centrifugal pumps. The two broad NRC concerns discussed in the Bulletin included:
1) the potential for a pump to dead-head when it is operating in the minimum flow recirculation mode in parallel with another pump (pump-to-pump interaction),and 2) the adequacy of the minimum flow capacity.
Philadelphia Electric Company's (pef,o's) understanding of these two concerns is as follows.
1)
Pump-to-Pump Interaction, When the minimum flow lines from two or more pumps join at some point to form a comon line, there is a potential for interaction between the pumps.
If the design of the piping configuration has not considered the pump unique performance characteristics, the pump with the higher discharge pressure (stronger pump) could reduce the flow through the pump with tlie lesser discharge pressure (weaker pump) to the point where it is inadequate for long-term integrity.
If each individual pump's minimum flow line is orificed (back-loaded)
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upstream of the junction with the common line, and if the comon line has a t
large enough flow area such that its resistance is a relatively small part of the overall hydraulic resistance, there should be little adverse pump-to-pump interaction. The parallel pumps can be expected to operate individually or in unison with adequate minimum flow.
However, if the minim a flow lines are not individually orificed, but the comon line is oriff ed or contains no orifice, interaction between the two pumps may occur. Tie severity of the degradation of minimum flow through a pump depends on the shape "t the pump characteristic head-flow curves and the mismatch between the pumps.
If the characteristic curve is such that a small change in flow results in a relatively large change in developed head (i.e., moderate to high flow conditions), it is probable that little operational difficulty would 7 4 ult from an undesirable piping configuration. However, if a relatively large change in flow resulted in i
only a smal? change in developed head (i.e., low flow conditions), some problems could be expected in satisfying the minimum flow requirements.
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Docket Nos. 50-277 50-278 2)
Adequacy of Minimum Recirculation Flow The original design basis for sizing the minimum flow recirculation lines for the safety-related pumps at the Peach Bottom Atomic Power Station (PBAPS) was to provide sufficient flow to avoid overheating the pumps due to low flow. However, more recently, pump vendor's guidelines for minimum flow are based on avoiding hydraulic instability in addition to avoiding pump overheating, leading to higher reconsnended minimum flow values than thosc used in original system design. Hydraulic instabilities can occur at low flow rates due to flow separation across the impeller vane, which can lead to asymmetrical shaft and bearing loads in addition to pump and piping vibration. Since the pump vendor guidelines are only applicable for
" continuous" or " intermittent" opcation, there are no new guidelines which specifically address low flow limit: for infrequent operation, as is the case of BWR Emergency Core Cooling System Pumps.
By letter dated June 30, 1988, PECo responded to NRCB 88-04 and, as was requested, both short and long-term resolution action items were provided.
The Bulletin also requested that within 30 days of completion of any long-term resolution action items, a written response describing those actions be provided.
The long-term resolution actions that we have taken at PBAPS to resolve the two minimum flow concerns discussed in the Bulletin are presented below.
I.
Pump-to-Pump Interaction Minimum flow testing of the Residual Heat Removal (RHR) and Core Spray pumps, with both pumps in a loop operating together, was performed during the fall of 1988. Because there are no permanently installed flow measuring instruments in either systems' minimum flow lines, ultrasonic flow meters were used.
Several unsuccessful attempts were made to accurately determine flow in the Core Spray and RHR systems using the ultrasonic flow meters.
Although actual minimum flow data was not obtained while two pumps were operating in parallel in the minimum flow mode, there was little or no difference observed when comparing the discharge pressure of each pump.
Because the pump curves for each pump in a loop are virtually identical, the flow rates at a specific discharge head are quite similar for each pump; thus, little or no pump-to-pump interaction is expected to be occurring.
Surveillance testing regularly confirms proper pump operation (proper head-flow relationship) to ensure pump operability, so that any changes in pump performance which could lead to adverse pump-to-pump interaction would be i
identified.
l Additionally, as discussed in the General Electric BWR Owners Group (OG) j response to this Bulletin dated June 22, 1988, and as stated previously, if each individual pump's minimum flow line is orificed upstream of the junction with the common line, and if the common line has a large enough I
flow area such that itt resistance to flow is a relatively small part of the i
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"l Docket Nos. 50-277 50-278 l
overall hydraulic resistance, little adverse pwnp-to-pump interaction should Because the minimum flow piping of the RilR and Core Spray pumps at occur.
PBAPS employ this type of configuration, we do not expect to see any pump-to-pump interaction.
Based on the above discussion, no further long-term corrective actions are required.
II. Adequacy of Minimum Recirculation Flow As stated in the BWROG response to NRCB 88-04, some pump vendors now suggest that in order to prevent undesirable hydraulic instability, noise, and vibration when operating for extended durations at low flow, the minimum flow for intermittent operation of these types of pumps should be significantly higher than the present minimum flow. These recommended minimum flows (approximately 35% to 40% of the pump rated flow) are intended as operating guidelines for continuous operation. Other guidelines (approximately 25% to 30% of rated flow) are suggested for intermittent operation, where intermittent operation is defined as less than two hours of minimum flow operation in any 24-hour period. For a plant design life of 40 years, this would allow pump operation in low flow for approximately 30,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. The actual time spent in the minimum flow mode is, however, relatively insignificant. Typically, less than five minutes of minimum flow operation occurs during any normal. Emergency Core Cooling System (ECCS) testing, as is the case at PBAPS. There are no new guidelines which specifically address low flow limits for infrequent pump operation.
For P8APS, the originally supplied minimum flow values are adequate to preclude any pump damage resulting from operation of these pumps in the minimum flow mode for short periods of time (i.e., pump start up during surveillance testing). For longer term operation in the minimum flow mode (e.g., low pressure ECCS initiation while the reactor is at high pressure),
the operating procedures instruct the operators not to operate the RHR pumps with a steady state flow less than 4,000 gpm (40% of rated flow). The operating procedures for the Core Spray pumps will be revised and implemented by August 15, 1989, to provide similar operating restrictions.
Based on the above discussion, no further long-term corrective actions are required.
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