ML20236S991
| ML20236S991 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 07/17/1998 |
| From: | Langenbach J GENERAL PUBLIC UTILITIES CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 1920-98-20384, 50-289-98-02, 50-289-98-2, NUDOCS 9807270359 | |
| Download: ML20236S991 (4) | |
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- NUCL' EAR Post Office Box 480 GPU Nuclear,Inc, Route 441 South Middletown, PA 17057-0480
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1920-98-20384
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July 17,.1998 I
U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
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Dear Sir:
Subject:
Three Mile Island Nuclear Station, Unit I (TMI-1)
- Operating License No. DPR-50 Docket No. 50-289 GPU Nuclear Response to NRC Concern Regarding Operation with Makeup
- Pump Suction Cross Tie Valves Open (IFI 98-02-03) t l
The purpose of this letter is to respond to an NRC concern discussed in a conference call on
. May 14,1998 and identified in Inspection Report (IR) 50-289/98-02, dated June 22,1998. As stated in the inspection report, GPU Nuclear changed the position of the Makeup &
Purification /High Pressure Injection System (MU/HPI) pump suction cross tie valves from normally shut to normally open in the fall of 1997. Having taken this action to address several system operation concerns and improve reliability, GPU Nuclear noted and informed the NRC resident inspector that an NRC safety evaluation dealing with a potential generic B&W common mode failure of the MU pumps had incorrectly' documented TMI-1 MU cross tie valves as l
_ normally closed. The IR identified the NRC's concern that the opening of the cross ties would allow the third MU pump to be potentially affected by an unknown Makeup Tank (MUT) low level condition.
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The Inspection Report acknowledges two basic differences between TMI and the Oconee plant, j
where a problem occurred that allowed two MU pumps to become damaged: 1) The TMI-1
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z MUT levelinstrument utilizes a dry reference leg, and 2) At TMI-1 the second MU pump does j
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' not start automatically on low MU pump discharge header pressure, but must be started j
L manually..TMI-1 Operating Procedure (OP) 1104-2, " Makeup and Purification System," limits I
.. and precautions require that the operator verify an adequate suction supply and flow path prior to startirig a MU pump.
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TMI-1 M'UT pressure and level are maintained within an operating envelope to ensure that (1)
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. MU/HPI pump net positive suction head (NPSH) will be greater than the pump NPSH test curve and (2) the gas bubble in the MUT will not expand into the MU/HPI pump suction header
- if the most limiting design basis scenario for each concern is analyzed using conservative
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7 assumptions. In addition, the operating procedure (OP) 1104-2, " Makeup and Purification i
System," further restricts normal MUT operating pressure ud temperature to an " operating 9807270359 9907 d FDR-- ADOCK 05000289 F
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'192Q-98-20384 Page 2 0f4'-
box" within the required operating envelope. Operation within the " operating box" provides additional margin and simplifies the operating limits. Operation outside of the operating envelope is not permitted. OP 1104-2 requires immediate action to restore MUT level and pressure to the unrestricted operating region if these parameters should leave the steady state operating region defined by the curve in Figure 1, " Makeup Tank Level & Pressure Limits," of the procedure. The analysis of design basis events does not credit any operator actions to control the MUT pressure and level once the event begins. If MUT pressure and level are within the operating envelope at the start of a design basis event, NPSH ofthe MU/HPI pumps is assurred.
During the conference call, the NRC asked about the quality and redundancy of the TMl-1
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MUT pressure instrument and how this arrangement might be vulnerable to a single failure, and in particular, the consequence of a failure of the transmitter to respond to changes in pressure I
(fail"as-is") or failure due to a slow drift of the instrument rather than failing high or low.
e A single pressure transmitter (MU17-PT) has been provided for MUT pressure indication in the
~ control room. The instrument loop includes indication on the main console (MUl7-PI) and on the plant computer (A1028), In addition to pressure indication, the MU17-PT transmitter signal is used for MUT gas pressure alarms on the control room overhead panels and on the plant computer., The plant computer alarms on high or low pressure at the limits of the steady state operating region (" operating box") in accordance with the operating procedure which bounds the operating envelope curves.
The MU pressure instrument loop consists of a sensing line off the top of the tank going to a transmitter which feeds a signal to the pressure indicators and alarm bistables. The possible failures and expected responses have been considered as follows:
a) Sensing line failures:
- 1) Leaks: Small leaks in the sensing line would not have a significant effect on indication.
Large tubing leaks would effect MUT pressure as well as indication. The radioactive gas leak would be an early indicator in addition to the MUT low pressure alarm.
- 2) Condensation or water in the sensing line: It is unlikely that any significant water accumulation could occur because the sensing line is connected on the top of the MUT and tank temperature is only slightly above ambient temperature (approximately 100 'F), However, instrumentation and control (l&C) technicians check the drain pots every three months" for liquid and ' drain any liquid that might be present.
I 3 Procedum IC-126, " Makeup Tank Level Transmitter's Sensing Lines' Draining," scheduled by GMS-2 PM Task No.1690.
j 1920-98-20384 j
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b) Tra'nsmitter failures: The transmitter is located in a " mild" environment. There are no adverse temperature, humidity, or radiation conditions. In over twenty years of service, 4
there have not been any significant problems with this transmitter. The transmitter provides a -10 to +10 VDC signal for an input of 0 to 100 psig. The indicator provides a 0 to 100 psig indication for a -10 to +10 VDC input.
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- 1) Loss of power: If power were lost to the transmitter or indicator, the indication would fail to 50 psig, which is well outside the " operating box" (20 to 30 psig) resulting in an alarm.
- 2) Fr"ure Low or High: Failures high or low would be alarmed.
- 3) Excessive Transmitter Drift: This problem would be identified by the weekly comparison with MU pump suction header pressure which allows a tolerance of 3 psig.
NOTE: The MU pump suction header pressure instruments are calibrated each refueling interval.
- 4) Transmitter output Failed "As - Is:" If the output were to fail (" hang up") indicating an otherwise acceptable condition, neither high nor low, the problem would be identified when adding water to the MUT. This ope;ation is performed approximately once each day.
Operations and surveillance activities would help in the early identification of any problems with MUT pressure indication. MUT level and pressure receive significant operator attention. The operator in the control room monitors the MUT pressure regularly and adds water or gas to the
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MUT as needed. Typically, water and gas additions are made daily. Reactor coolant samples are analyzed three times each week for gas concentration. Since RCS gas concentration is most directly controlled by MUT pressure, any unusual variation in RCS gas concentration would 2
lead to an evaluation of MUT pressure indication. Weekly instrument operability checks are
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performed where the control room indications are compared with a local pressure indicator on j
the MU pump suction header; and each refueling interval, the MUT pressure instrument loop is 3
calibrated where the instrument error assumed in the design basis analysis discussed above is
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accounted for. These instrument calibration and operability checks are performed under the controls of a 10CFR50 Appendix B quality assurance program.
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Procurement and
- maintenance requirements for the MUT pressure, Itci md temperature instruments are appropriate for those instruments used to maintain plant operation within operating limits and consistent with the initial condition assumptions in the design basis analysis.
The MUT pressure instrument is not an accident monitoring instmment (i.e., not a Regulatory Guide 1.97 instrument). Typical design practice in the nuclear dustry would not require
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redundancy for such an application.
j in summary, the MUT pressure instrument is used to maintain the tank withi its operating n.
envelope and is not required for any actions to mitigate the event. Cased on our experience with
- Surveillance Procedure 1301-4.1. " Weekly Surveillance Checks."
' Surveillance Procedure 1302-5.17. "Make-up Tank Level Instrumentation."
1920-98,20384 Page 4 of 4 the MU/fiPI system design using the installed equipment, GPU Nuclear has found the MUT pressure indication to be very reliable. A review of postulated failure modes as discussed above demonstrates that any postulated failure of the MUT pressure instrument would not go undetected for an extended period of time. Based on our assessment of the MUP/HPI System, GPU Nuclea; concludes that the potential for MU/HPl pump failure as occurred at Oconee is not applicable to TMI-1.
If any additional information is needed, please contact Bob Knight, Nuclear Safety & Licensing, at TMI (717) 948-8554.
Sincerely, 3'*'+ *6#
James W. Langenbach Vice President and Director, TMI MRK File No. 98106 cc:
Administrator, NRC Region I
-TMI Senior NRC Resident Inspector TMI Senior NRC Project Manager l
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