ML20196G889

Special Rept:On 970616,scram Setpoint for Low Flow Scram for Heat Exchanger 503A Leg Was 25 Gpm Below TS Limit.Caused by Drifting of Setpoint of Vutronik Monitor Switch DPS-928A. Adjusted Trip Setting & Retested Trip Setting

ML20196G889
Person / Time
Site:	University of Missouri-Columbia
Issue date:	07/15/1997
From:	Mckibben J, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO
To:	NRC (Affiliation Not Assigned), NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9707230154
Download: ML20196G889 (3)

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, Research Reactor Center Research Park Columbia. Missouri 65211

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Telephone (573) 882-4211 FAX [573] 882=3443 UNIVERSITY OF MISSOURI-COLUMBIA July 15,1997 Director of Nuclear Reactor Regulation ATTN: Document Control Desk Mail Station P1-37 U.S. Nuclear Regulatory Commission Washington, DC 20555

REFERENCE:

Docket No. 50-186 University of Missouri Research Reactor License R-103

SUBJECT:

Report as required by Technical Specification 6.1.h.(2) concerning reactor operation with the scram setpoint of one of five safety system channels that provide scrams for low Primary Coolant Flow not within Technical Specification limits due to setpoint drift INTRODUCTION

- On June 16,1997, while performing compliance testing of the Primary Coolant Flow safety channel connected to the heat exchanger 928A flow transmitter, the scram setpoint for the low flow scram for heat exchanger 503A lag was 25 gpm below the Limiting Safety System Setpoint (LSSS) of 1625 gpm required by Technical Specification 2.2. This Primary Coolant Flow Scram is one of four required by Technical Specification 3.3. An additional backup to these Primary Coolant low flov terams is provided by the core differential pressure scram. The reactor safety system was capable of performing its safety function if an actuellow flow condition had occurred while operating at full power, because the remaming four safety system channels that provide scrams for Low Primary Coolant Flow were operable.

These additional safety channels are: (1) Primary Coolant Flow from FT-912A;(2) Primary Coolant Flow from FT-912E;(3)Ileat Exchanger Differential Pressure for heat exchanger 503B leg (DPS-928B); and (4) Core Differential Pressure DPS-929.

DESCRIPTION On June 16,1997, the reactor was shutdown for scheduled maintenance. Compliance testing (Compliance Procedure CP-23) of the Primary Coolant Flow safety channel connected to flow trans-mitter DPS 928A revealed that its scram trip setpoint was occurring at approximately 1600 gym.

This was below the 1625 gpm LSSS for each heat exchanger leg of the primary coolant system. The trip point for the scram from DPS-928A is normally set for 1675125 gpm. The previous compliance test performed on DPS-928A was completed on December 2,1996, satisfactorily. These tests were done at six month intervals.

The calibration test portion of CP-23 indicated that flow transmitter DPS-928A was operating properly. The problem was determined to be in the lioneywell Vutronik dual monitor switch or dual trip unit which initiates the scram trip for the safety system channel connected to flow transmitter DPS-928A. This trip unit also has a second, separate trip setpoint to cause the 546A/B convective cooling valves to open. The second setpoint trip operated as expected at the proper flow of 1675 i 25 gpm and did not require adjustment.

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l e Letter to Director of Nuclear Reactor Regulation July 15,1997 Page two The Operations Engineer and Chief Electronics Technician adjusted the trip setpoint of the scram on the Honeywell Vutronik monitor to 1675125 gpm. After this adjustment, the DPS-928A scram test was performed again and the scram occurred at 1675 25 gpm.

Each week since June 16, the scram test portion of Compliance Procedure (CP-23) was performed for DPS-928A and revealed that the Vutronik trip unit tripped at 1675125 gpm.

Additionally, on June 23,1997, the compliance procedure (CP-4A/B) was performed for the Primary Coolant Flow channels connected to FT-912A and FT-912E (the safety system channels for primary coolant flow developed from the flow orifice in each of the two primary heat exchanger legs).

The compliance tests of these channels revealed their trip points to be within specification.

ANALYSIS The setpoint on the Vutronik monitor switch for DPS-928A appurs to have drifled down prior to its test of June 16,1997. There was no trend to predict this from previous tests. The last three tests of DPS-928A indicated a trip setpoint of 1660,1655 an/.1660 gpm, respectively. The retest each of the following weeks has indicated no further drift.

Primary coolant flow rate, reactor power, pressurizer pressure, and primary coolant temperature are the set of measurable opemting (process) variables used to develop the reactor safety limit curves for MURR. The limiting safety system setpoints (LSSS) are the settings specified for each of these variables, so chosen that automatic protective action will corr;ct the most severe abnormal situation anticipated before a safety limit is exceeded.

Hazards Summary Addendum 4, Appendix H, Bases for LSSS for Modes I and II Operation, provides postulated transients with three process variables at their LSSS and allow the fourth to depart from its LSSS to show the safety margin inherent in the L3SS value. An example that shows the conservatism of the primary coolant flow LSSS,is postulated Mode I operation with pressurizer pressure reduced to the LSSS of 75 psia, reactor power and coolant inlet temperature raised to their LSSS of 12.5 MW and 155*F, respectively. The safety limit curves predict that the primary coolant flow rate could be reduced to approximately 2400 gpm (1200 gpm/ heat exchanger leg) before DNb vould occur, implying a safety margin of 800 gpm below the LSSS of 3200 gpm on the coolant flow through the core. Because 50 gpm of primary coolant flow is diverted to the cleanup system, the LTSS of 1625 gpm per loop corresponds to the actual core flow rate of 3200 gpm for 10 MW (Mode I) operation.

This example demonstrates that even under extreme conditions (as opposed to steady state nominal conditions of pressurizer pressure greater than 78 psia, reactor power at or below 10 MW, and coolant inlet temperature 123 F)if flow had been reduced in one heat exchanger leg of the primary coolant systern f o 1600 gym, the reactor would not have violated a safety limit. If a loss or reduction of primary coolant flow had occurred, any of the four additional safety system channels were operable to perform the safety Sction at primary coolant flow rates (or differential pressures equivalent to primary flow rates) gr, e than required by the primary flow LSSS. There was no period of reactor operation with reduced flow since the last satisfactory test of this safety system channel scram on December 2,1996.

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Letter to Director of Nuclear Reactor Regulation

_ July 15,1997 Page three CORRECTIVE ACTION Immediate corrective action was the adjustment of the trip setting to 1675125 gpm and subsequent retesting to ensure it consistently tripped at that setpoint. The other trip function for natural convection valves 546A and 546B functioned properly at the correct trip setpoint with no adjustments.- Follow-up action included testing the trips of the other Primary Coolant Flow channels to verify that they functioned at their setpoint. The DPS-928A Primary Coolant Flow channel scram trip has been testad weekly for the last four weeks and each time the unit tripped at the proper setpoint of 1675 25 gpm.

The Chief Electronic Technician bench tested a aplacement dual trip unit that is evaluated to be an equivalent replacement part. This new unit was installed in place of the existing Honeywell dual trip unit for DPS-928A on July 14,1997.

Sincerely, ENDORSEMENT:

O q Reviewed and Approv d k

i e Walt A. Meyer Jr. J. Charles McKibben Reactor Manager Interim Director 3

xc: Mr. Alexander Adams, Jr., USNRC .

Mr. Tom Burdick, NRC Region III Dr. Elaine Charlson, Associate Provost, UMC Reactor Advisory Committee Reactor Safety Subcommittee Sl/ tb I (,b/ 7/lSfY7 omm EmNm Noury Public-Notary Seal STATEOFMISSOURI BooneCounty My Commission Expires: Aprl! 14.1999

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