ML20245K932
| ML20245K932 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 06/30/1989 |
| From: | Richter M COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-004, IEB-88-4, NUDOCS 8907050311 | |
| Download: ML20245K932 (7) | |
Text
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4 / 72 West Adams Street. Chicago,llhnois
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2 ddress Reply to. Post OMce bow 7-L/ Chicago, Illinois 60690 0767 June 30, 1989 U.S. Nuclear Regulatory Commission Attn Document Control Desk Washington, D.C.
20555
Subject:
Zion Station Units 1 and 2 Supplemental Responoe to NRC Bulletin 88-04 i
Docket Nos. 50-295/304 I
1 Referencess (a) NRC Bulletin 88-04, dated May 5, 1988.
(b) W.E. Morgan letter to U.S. NRC, dated July 11, 1988.
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Dear Sirs Reference (a) requested that licensees investigate and correct as applicable two miniflow design concerns. The first concern involves the potential for the dead-heading of one or more pumps in safety-related systems s
that have a miniflow line common to two or more pumps or other piping configurations that do not preclude pump-to-pump interaction during miniflow operation. A second concern is whether or not the installed miniflow capacity is adequate for even a single pump in operation.
In Reference (b), when addressing pump-to-pump interaction in safety-related systems during miniflow operation at Zion Station, Commonwealth Edison (Edison) indicated that the Residual Heat Removal (RHR) pumps may exhibit this condition and that further review would be necessary.
1 Additionally, when addressing the concern on the adequacy of the minimum flow l
bypass lines for safety-related centrifugal pumps at Zion Station, Edison l
indicated in Reference (b) that pump vendor information was being pursued in accordance with the direction given in Reference (a).
This letter presents, for Zion Station, the action taken to address the pump-to-pump interaction concern and provide pump vendor information on minimum flow line capacity.
Edison has confirmed, through further evaluation, that the RHR System could experience pump-to-pump interaction.
Although the RHR pumps do not share a common minimum flow line, the discharge paths for the pumps are l
l hydraulically coupled, creating the potential for the " stronger" pump to l
l dead-head the " weaker" pump (see attached figure). This situation can occur l
when both pumps are operated simultaneously, at minimum or low flow i
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8907050311 890630 DR ADOCK 0500
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~ conditions, following a Safety Injection signal (where Reactor Coolant System pressure remains above pump shut-of f head) or during testing.
As reported in Reference (b), similar pump-to-pump interaction will not occur in the Charging, Safety Injection, Containment Spray and Auxiliary Feedwater Systems.
For each unit, testing (a dual pump minimum flow test) was conducted on the RHR System to quantify the possible interaction between the RHR pumps when operated simultaneously in a minimum flow (recirculation) configuration.
When operated individually, each RHR pump had a minimum flow rate of approximately 375 gallons-per-minute (GPM).
During simultaneous operation of the RHR pumps for each unit, the " weaker" pump's minimum flow rate decreased, from approximately 375 GPM, to approximately 200 GPM.
Following the initial performance of the dual pump minimum flow test, an evaluation was performed by Westinghouse (Zion Station NSSS vendor) to ensure that RHR pump integrity would not be impaired by operation at the reduced minimwn flow rate caused by simultaneous operation of both RHR pumps.
The following assumptions were made to perform the evaluation.
Both RHR pumps would autostart following a Safety Injection signal due to a, mall LOCA, stemnline break, stemn generator tube rupture, or spurious safety injection, where Reactor Coolant System (RCS) pressure could remain above RHR pump shut-off head.
Both RHR pumps would take suction from the Refueling Water Storage Tank (RWST) and operate in minimum flow (the weaker pump at a reduced I
minimum flow), with no component cooling water (CCW) to the RHR heat exchangers for thirty (30) minutes.
After 30 minutes, operators would establish CCW flow to the RHR heat exchangers and be able to maintain whatever temperature was present at the RHR pump suction.
Since initiation of CCW is controlled procedurally at Zion Station during all instances of RHR oteration, Edison believes the 30 minute time period is appropriate and conservative.
Additionally, the ability for CCW to prevent an increase in RHR pump Juction temperature is conservative.
The evaluation determined that a minimum flow rate of 50 GPM for an RHR pump would prevent cavitation and potential pump f ailure within 30 minutes based on the assumed conditions. Therefore, at this time, the " weaker" RHR pump for each unit at Zion Station has adequate minimum flow to preclude short-term (sudden) failure.
To prevent the hydraulic coupling of the RHR pumps, a check valve will be installed for each pump downstream of the minimum flow line, but upstream of the cross-tie line (as shown on the attached figure).
Installation of the check valves is expected to be completed by the conclusion of the Spring 1991 refuel outage for Unit 1, and the conclusion of the Spring 1990 refuel outage for Unit 2.
In the event that outage constraints or parts availability prevents installation of both check valves for a unit as scheduled, the installation of the check valve for the unit's " weakest" pump will be given priority.
As requested by Reference (a), Edison will notify the NRC within 30 days of completion of the check valve installation for each unit.
. To ensure integrity of the RHR pumps until installation of the check valves for each unit, a duni pump minimum flow test will be performed which will obtain flow data during simultaneous operation of both pumps in the minimum flow condition.
This." dual pump" test will.be performed following every " cold shutdown" condition of the unit, and the results will be evaluated to detect any degradation in " weak" pump performance. The acceptance criteria for'this test is 80 GPM, which is consistent with the flow value (50 GPM) determined by the NSSS vendor in the engineering evaluation (taking into consideration instrumentation tolerances).
In the event the flow of one of the RHR pumps should decrease below the acceptance criteria for the dual pump test, the Emergency Operating Procedures for Zion Station will be revised (for the unit of concern) to ensure that only one (1) RHR pump is operating during the initial injection phase of an accident condition when the RHR System is in minimum flow operation.
It should also be noted that RHR pump performance, which includes vibration monitoring, is routinely verified and trended during the monthly operability surveillance. This surveillance, required by the Technical Specifications, operates the RHR pumps individually at minimum flow conditions when the unit is at power, and verifies compliance with the respective pump curve.
1 Edison believes the probability of significant RHR pump degradation due to this hydraulic coupling condition, prior to the installation of the check valves for each unit, is remote. The station has reviewed the surveillance history of the RHR pumps, with particular emphasis placed on the
" weakest" pump for each unit, and no abnormal degradation or adverse trend has occurred.
In addition, the amount of time the RHR pumps operate simultaneously in the minimum flow condition is minimal (i.e., during the recently implemented " dual pump" test and the rare occurrence of a Safety Injection signal).
It should be noted that a flow rate decrease was detected for one of the " weaker" pumps following the initial performance of the " dual pump" test.
This flow rate change was attributed to instrumentation inaccuracies. There was no indication of pump degradation based on a review of other pump parameters and surveillance data.
During a small break LOCA condition, the RHR pumps may operate
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simultaneously in a minimum flow configuration for an extended period of
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time. This situation was addressed by the evaluation performed by the NSSS vendor, which determined a minimum flow rate of 50 GPM for the " weaker" pump was adequate to prevent cavitation and potential pump failure for up to 30 minutes (at which time credit is taken for CCW availability to the RHR heat exchanger). As indicated previously, Edison believes the time period for CCW 4
initiation (CCW initiation is controlled by emergency procedures), and the
_1 ability of CCW to prevent an increase in RHR pump suction temperature, is l
conservative.
Based on the analysis, the pump-to-pump interaction is not l
rendering any RHR pump inoperable since the " weaker" pump _ flow rate is greater l
l than 50 GPM.
Therefore, at this time, Edison believes both RHR pumps are able
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to perform their safety function and the Emergency Core Cooling design j
requirements are satisfied.
In the event one of the RHR pumps were rendered j
inoperable during an accident condition, the other RHR pump would be able to perform the system's design / safety function. The Zion Station accident-analysis allows for a single pump f ailure since the remaining pump has j
sufficient capacity to provide the necessary flow rate for accident j
mitigation. The probability of random f ailure of the remaining pump is i
extremely low.
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It should also be noted that during small break LOCA conditions, l
during which the RHR pumps operate simultaneously in a minimum flow L
configuration, provisions in the emergency procedures will allow the RHR pumps to be shutdown if plant conditions warrant.
1 At this time,.while the configurat!on of the RHR Systems for Zion Station f
are satisfactory for an interim basis, Edison believes the permanent resolution of the hydraulic coupling condition will be the installation of the pump discharge check valves (as described previously).
Edison believes that until the permanent resolution is accomplished, the probability of significant RHR pump degradation is remote and the necessary mechanisms are in place to ensure any pump degradation 1s-detected.
As previously mentioned, Edison indicated in Reference (b) that pump vendor information was beino pursued to address the concern on the adequacy of the minimum flow bypass 11<4; for the safety-related pumps (Action 3 of bulletin).
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Edison contacted Pacific Pumps, the manuf acturer of the pumps for the 3
Charging, Safety Injection, Containment Spray and Auxiliary Feedwater Systems at Zion Station. The minimum flow values for the pumps were submitted to the manufacturer for evaluation. Pacific Pumps, in a ?btter to Edison dated-August 31, 1988, indicated that based on originally provided minimum flow l
values, a catastrophic type failure is not a concern.
Pacific Pumps also indicated that damage caused by low flow operation is not immediate but l
cumulative in nature, and recommended that the duration of operation at the minimum flow values be limited to approximately 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> per_ month. Operation hi Zion Station at minimum flow rates is well under 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> each month.
i Edison also contacted Ingersoll-Rand, the manufacturer of the RHR
-j pumps at Zion Station. The minimum flow values for the pumps were submitted to the manuf acturer for evaluation.
Ingersoll-Rand, in a letter to Edison dated August 23, 1988, initially indicated that a minimum flow r9te of 500 GPM would be adequate to prevent short-term (sudden) damage. Th!a 13 wrate corresponded to the value presented in the vendor manual ~ for the jump.
l Ingersoll-Rand also indicated that a study would be conducted zu an effort to relate flow rates to service life.
Edison provided RHR pump data for this study and recently received the Ingersoll-Rand minimum flow evaluation for i
those pumps.
Ingersoll-Rand indicated that operation of an RHR pump near, or j
below, its recommended minimum flow rate has a time dependent cumulative j
effect on the overall useful life of the pump.
Hydraulic recirculation i
occurring at operation below design flows will cause noisy and rough operation. This will lead to a gradual degradation of the pump due to several possible causes such as: cavitation damage, increased hydraulic loads (leading to shortened bearing life and/or shaf t damage), and excessive vibration.
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Ingersoll-Rand recommended the following minimum flow rates for the' Zion Station RHR pumps:
- Continuous operation - 1,000 to 2,000 GPM, Short period operation - 300.to 500 GPM, and
- Start /Stop operation - 100 GPM.
Where
- Continuous operation is defined as pump operation exceeding a 3 hour3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> duration in a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> time period.
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- Short period operation is defined as pump operation of 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> or less.
- Start /Stop operation is' defined as pump operation of.
30 minutes or less, and is based primarily on thermal rise considerations.
These recommended minimum flow rates were based on pump fluid temperature rise
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and Ingersoll-Rand's experience with the hydraulic effects of low flow operation.
Ingersoll-Rand also indicated that to fully determine the-adequacy of a minimum flow condition, performance monitoring was important to detect any pump degradation and in-service wear.
It should be noted that when
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developing these recommended minimum flow rates, Ingersoll-Rand did not take j
into consideration the RHR System configuration, therefore, the effect of CCW.
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on pump fluid temperature was not addressed.
As indicated previously, the minimum flow rates for the Zion Station RHR I
pumps during individual pump operation, as determined by test, is approximately 375 GPM.
This flow rate satisfies the recommended minimum flow rate specified by Ingersoll-Rand for' " start /stop" and "short period" operations. Edison believes this minimum flow rate is adequate based on the j
following.
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- A review of the surveillance history of the RHR pumps has revealed no abnormal degradation or adverse trends.
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- Individual RHR pump operation, in the' minimum flow condition, J
occurs primarily during the monthly surveillance test and the recently implemented " dual pump" test. l Typically, these tests do not operate an RHR pump in the minimum flow condition for greater j
than 30 minutes.
In the event a RHR pump is required to operate l
for greater than 30 minutes, the test procedures address initiation of CCW flow (through precaution statements) to the l
appropriate RHR heat exchanger.
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- Mechanisms (the monthly surveillance test and the recently l
in.plemented " dual pump" test) are in place to ensure any pump l
degradatica is detected. This performance monitoring is consistent with the intent of the pump vendor recommendation.
During simultaneous operation of tl.e RHR pumps, the minimta flow rate for the
" weaker" pump will only satisfy the Ts> commended minimum flow rate specified by Ingersoll Rand for " start /stop" operation. Minimum flow rate concerns during dual RHR pump operation have been addressed previously in this response.
As requested by Reference (a), Edison will:
- notify the NRC within 30 days of completion of the permanent resolution (check valve installation) for the pump-to-pump interaction concern (for each unit), and
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-- maintain the documentation' associated with the Zion Station j;
response.
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1 Please address any questions that you or your staff may have concerning this response to this office.
l Respectfully, M. H. Richter Attachment /lbs9001k15 cc A', B. Davis Resident Inspector - Zion
- Subsf.r bed'and Sworn to ',} ppg { g'h~{,'
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befoj me.this C O D h my F. M of A llAL,
' 19 tary Public, State of Illinois,1 s
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