ML20248D064
| ML20248D064 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 09/25/1989 |
| From: | William Cahill TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| CP-89-023, CP-89-23, TXX-89719, NUDOCS 8910040115 | |
| Download: ML20248D064 (3) | |
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. Log # TXX-89719' "a;',
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File # 10110 C
905.4 N#ELECTR/C Ref. #'10CFR50.55(e)
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'Septemb'er 25,'1989
. becutive Vicel' resident M
U.,S.: Nuclear Regulatory Commission
-Attn; Document Control Desk
- Washington, DC _20555' g
SUBJECT:
. COMANCHE PEAK STEAM ELECTRIC STATION-(CPSES) s
- DOCKET N05. 50-445 AND~50-446
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' POTENTIAL LOSS OF RHR SDAR: CP-89-023 (FINAL REPORT)
Gentlemen:
On) August 17, 1989, TU Electric verba11y' notified the NRC of'a deficiency involving the failure of a non-safety related inverter which has the' potential-u to. adversely ' affect ' Residual Heat Remwal '(RHR) System operability under
'certain conditions.
Subsequently, it was determined-that'similar adverse effects'could result from the loss of non-safety related air supply. After further evaluation' this condition has been determined to be reportable under
- the provisions 'of 10CFR50.55(e). The required information follows.
Description During plant operating modes 5 and 6 the RHR system is required to remove decay heat from the core.
Under certain conditions, this may be accomplished with' reduced reactor vessel water inventory. At this low RCS inventory j
condition, the potential ~ exists for vortexing in the RHR pump suction if 1
+
- maximum flow rate restrictions-are not observed. To preclude vortexing, air l
operated flow control valves are utilized in both RHR Train A and B to
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restrict RHR flow to below the point at which pump vortexing occurs.
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'Each RHR heat exchanger outlet flow control valve in Train A and B is operated
~ by non-safety related instrument air and has its own controller.
Both Train A
-and B-controllers are powered from a single power panel. This power' panel is.
normally energized from a non-safety related inverter, but may be locally transferred to an alternate power source.
This is a conservative design for the: RHR. System Engineered Safety Features (ESF) function of low pressure safety injection,'since the required position for these valves is fully open and the.. valves are designed to fail open upon loss of instrument air or control power.
However, when RCS inventory is reduced and the RHR pumps are being utilized for decay heat removal, if the valve failed open it could
, result in a flow transient which could lead to vortexing in the pump suction 1
line.
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8910040115 890925
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g 400 North Oluve Street LB 81 Dallas. Texas 75201 r
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A TXX-89719 Page 2 of 3 On July.17,1989, one loop of the Unit.1 RHR System was operating in a reduced inventory condition. 'The power panel that provides control power for both RHR I
heat exchanger. ' outlet flow control valves was being fed from the non-safety.
related. inverter described above.
An electrical short resulted in a failure of the inverter.
Without control power, both Train A and B valves failed open, resulting in a RHR pump flow rate which was above the flow limit established for the' existing RCS level.
The cause of this deficiency was the failure to recognize that control valve
. failure could result in vortexing in RHR pumps during reduced inventory operation.
This deficiency is considered limited to the RHR System due to the
. unique design requirements of the RHR System which require it to perform decay heat removal functions in addition to the ESF function of low pressure safety injection, and due to'the unique conditions under which vortexing in this system may occur.
Safety Significance The safety significance of this issue is considerably Nitigated because both RHR loops should not be operated simultaneously during reduced inventory-operation.
In'the event that an RHR pump becomes inoperatle due to air entrainment, a second pump would be available and, in acco" dance with CPSES procedures, no attempt would-be made to start the standby tump until adequate RCSL level is verified. This minimizes the time period duri g which the inverter or instrument air failure could cause vortexing ano adversely affect both pumps.
Nevertheless, this deficiency could have resultad in circumstances contributing to the loss of both trains of RHR during reduced inventory operations. The failure to recognize the potential for RHR pump vortexing due to control valve failure and failure to provide compensatory design features or specific operating precautions represents s significant deficiency in design which, in the event it had remained uncorrected, could have adversely affected the safety of plant operations. Therefore, this issue is reportable under the provisions of 10CFR50.55(e).
Corrective Actions The appropriate Unit 1 procedure (s) for operation of the RHR System will be revised to require the use of other RHR valves (which are not affected by loss of control power or instrument air) to restrict RHR flow during operating modes 5 and 6 when the RCS is in a reduced inventory condition.
This will ensure that, if the flow control valves fail open, flow will not exceed established limits.
Administrative controls will ensure these valves are fully opened after restoration from reduced inventory RHR decay heat removal.
An engineering review will be performed to analyze other possible failures that could adversely affect RHR system operation during reduced inventory
-operation. These actions will be completed prior to entry into the first reduced RCS inventory condition following the achievement of 5% power level in Unit 1.
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TXX-89719 Page 3 of 3 The Unit 2 corrective action for this condition will be assessed after resumption of the balance of engineering activities for Unit 2.
Sincerely,
/
William J. Cahi 1. Jr.
JLR/vid c - Mr. R. D. Martin, Region IV Resident Inspectors, CPSES (3) 1
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