ML20005G212

LER 89-007-01:on 890508,RWCU Sys Isolated on Leak Detection Differential Flow Signal While Shifting RWCU Operation from pre-pump to post-pump Mode.Probably Caused by Inadequate Reactor Pressure.Procedure revised.W/900112 Ltr

ML20005G212
Person / Time
Site:	Grand Gulf
Issue date:	01/12/1990
From:	Cottle W, Ruffin R SYSTEM ENERGY RESOURCES, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
AECM-90-0015, AECM-90-15, LER-89-007, LER-89-7, NUDOCS 9001180288
Download: ML20005G212 (5)

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SUBJECT:

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AECM-90/0015 4

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Mr.: Stewart D. Ebneter (w/a)

Regional' Administrator I

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101 Marietta St.,'N.W., Suite 2900 Atlanta,' Georgia 30323

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.Mr. L. L. Kintner, Project Manager (w/a)

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] Yto w r M avecreo svemwou caras y NO l l g l AssTRAc7m,e ,ue ,.A. mi .,...r-, ,oei On May 8,-1989 the Reactor Water Cleanup (RWCU) system isolated on a leak c

detection differential flow signal while shifting RWCU operation from the i pre-pump to the post-pump mode at a reactor pressure of 27 psig. Operators l checked instrumentation to ensure no actual leak had occurred and restored l RWCU to service in the post-pump mode.

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l' An evaluation of the RWCU system operation and design was performed. It was determined that a reactor pressure of 27 psig may not supply sufficient net positive suction head (NpSH) to facilitate switching RWCU operation to the o post-pump mode. Additionally, erroneous flow indications may result when i establishing or ceasing blowdown due to the valve lineup sequence in relation to L the location of the flow element.

Operating Instructions have been changed to require shifting from pre-pump to i post-pump modes, and vice-versa, at approximately 100 psig reactor pressure.

i Valve lineups have also been modified to prevent erroneous flow indications during future blowdown modes of operation.

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A. : -REPORTABLE OCCURRENCE

'On'May 8, 1989 the Reactor Water Cleanup (RWCU)xsystem isolated on a leak detection differential flow signal while shifting RWCU operation from the pre-pump mode _to the post pump mode. The automatic isolation of the RWCU system containment = isolation valves;is reported as an ESF actuation pursuant to 10CFR50.73(a)(2)(iv).

B. INITIAL CONDITIONS The plant was in mode 2, Startup, with a reactor pressure of 27 psig.

C. ' DESCRIPTION OF OCCURRENCE On May 8,,1989, the plant'was-in a startup evolution. At 27 psig reactor pressure, operators began shifting RWCU (EIIS code:CE) operation from the pre-pump mode (pump discharge flow through the heat exchangers) to the post-pump. mode (pump suction flow through the heat exchangers) when erratic differential flow indications and alarms were received. After the transfer was made and the alarms cleared, operators re-established RWCU blowdown to the main condenser. Differential flow indications on both "A" and "B" channels became erratic. At 0919, approximately five minutes after establishing blowdown flow, the delta flow delay timer alarm annunciated. The delta flow timer is set to time out at approximately 45 seconds. Operators attempted to clear the delta flow signal by securing the RWCU pump and closing the filter domineralizer bypass valve, 1G33-F044. The delta flow. timer timed out and the RWCU system containment isolation valves automatically closed.

Operators checked instrumentation to ensure that no actual leak had occurred and restored RWCU to service'in the post-pump mode.

D. APPARENT CAUSE' An evaluation of system operation and design revealed the following:

o The Integrated Operating Instruction requires shifting RWCU operation to the post-pump mode above 25 psig reactor pressure for thermal protection of the seals and additional Net Positive Suction Head (NPSH)

(See LER 88-014). However, a review of this incident has revealed that a reactor pressure of 27 psig may not supply sufficient NPSH to eliminate flow perturbations while switching RWCU operation to the post-pump mode, o Erroneous flow indications may result when establishing or ceasing blowdown to the condenser because of a vacuum in the blowdown line.

This vacuum condition may cause an imbalance in the impulse lines to the flow transmitter.

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o The RWCU leak detection flow transmitters are not temperature compensated and induce minor flow indication errors due to the n = temperature change between different reactor operating conditions.

E. ' SUPPLEMENTAL CORRECTIVE ACTIONS

' Integrated Operating Instructions (10I's) have been changed to require ahifting from pre-pump to post-pump modes of operation (and vice-versa)e at approximately 100 psig reactor pressure, j i

The 101's are in the process of being changed to ensure that the suction  !

flow instrumentation has been filled and high point vented after any l evolution which'may have caused air to enter the RWCU pump suction lines.

The changes.are scheduled to be implemented by 1/30/90. System Operating  !

Instruction 04-1-01-B33-1 was changed to have these instrument lines R vented after the vessel has been in a depressurized condition. The task- i used ,to vent these instrument lines has been changed to not only vent at l the transmitter but,also at the high point on the instrument line. This action should eliminate the possibility of noisy signals caused by air entrapped in the sensing lines.

Additionally, System Operating Instruction 04-1-01-G33-1 was changed to >

L modify the blowdown lineup such that the final condenser isolation valve J

to be opened is the last valve next to the condenser when establishing blowdown. When securing blowdown, this valve will be the first valve I closed to ensure vacuum is not drawn on blowdown flow instrumentation.

The lineup will prevent erroneous flow indications during future blowdown modes-of operation.

5 The magnitude of induced flow indication errors caused by reactor water temperature changes was evaluated and considered insignificant when I. compared to the effects of low NPSH and vacuum conditions in the blowdown line. Adjustments to compensate for temperature changes at this time are L not warranted.

H lL After an evaluation of the system, it was determined that no design ,

l changes were necessary with the operational controls implemented.  !

However', enhancements such as relocating the flow element in the blowdown line will be considered to increase stability of flow indications.

Since commencement of commercial operation, GGNS has experienced problems with RWCU System isolations on differential flow signals. SERI believes the actions described above are sufficient to prevent recurrence of the described RWCU isolations. SERI will continue to monitor the effectiveness of the corrective actions taken.

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There were no adverse safety consequences as a result of this event.

Containment isolation. valves responded as designed. No actual unidentified RWCU leakage was present.

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