ML20116K758

Discusses Planned Course of Action to Address Inoperability of Reactor Water Level Probe

ML20116K758
Person / Time
Site:	San Onofre
Issue date:	08/13/1996
From:	Rainsberry J SOUTHERN CALIFORNIA EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-88-17, NUDOCS 9608150214
Download: ML20116K758 (6)

Text

f55 Southem Califomia Edison Company P. O. BOX 128 BAN CLEMENTE, CALIFORNIA 92674-0128 i August 13, 1996 J. L. RAINSBERRY TELEPHONE i SAN ONO R N EA N T G STATON l

U. S. Nuclear Regulatory Commission Attention: Document Control Desk {

Washington, D.C. 20555 Gentlemen:

Subject:

Docket No. 50-361 Reactor Water Level Probe Inoperability San Onofre Nuclear Generating Station Unit 2

References:

(1) Letter from W. C. Marsh (Edison) to USNRC Document Control Desk, dated May 19, 1995;

Subject:

Reactor Water Level Probe Inoperability (2) Generic Letter 88-17, dated October 17, 1988,

Subject:

Loss of Decay Heat Removal-The purpose of this letter is to inform the NRC of Southern California Edison's (Edison's) planned course of action to address inoperability of the Reactor Water Level Probe (RWLP). Compensatory actions being taken in place of the RWLP are specified to allow reduced inventory operations and testing of the replacement system during the Unit 2 Cycle 9 Refueling Outage. These actions are necessary because of repeated RWLP failures. TN actions being taken at Unit 2 are identical to those used at Unit 3 to rJdress a similar RWLP failure (see Reference 1).

BACKGROUND: ,

Currently, San Onofre Units 2 and 3 are each provided with five methods to determine reactor coolant system water level when the RCS is in a reduced inventory condition: (1) Refuelin Junction Thermocouple Probes, (3) thegReactor Water Level VesselIndication, Water Level(2)Sightglass, the Heated (4) water inventory control, and (5) the Reactor Water Level Probe (RWLP).

Because the fifth method, the RWLP, is no longer functional the Draindown Level Monitoring System (DLMS) will be installed in place of the RWLP during the Units 2 and 3 Cycle 9 Refueling Outages. Each of these methods is described below.

Refueling Water Level Indication The Refueling Water Level Indication (RWLI) system provides the primary control room indication for monitoring the Reactor Coolant Syst 9608150214 960813 PDR ADOCK 05000361 P PDR g O

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i Document Control Desk reduced inventory operations in Modes 5 and 6. There are two differential pressure transmitters, one wide range and one narrow range. The wide range and narrow range transmitters share common sensing lines, which are connected to a drain line on the hotleg and to the vent line from a condensation chamber ,

on the pressurizer. An adjustable Hi/Lo level alarm is provided for each differential pressure loop which actuates a common alarm in the control room.

The RWLI is only connected during Modes 5 and 6. During Modes 1 through 4 the RWLI instrumentation piping is isolated and capped off (See Figure)

Heated Junction Thermocouple Probes The Heated Junction Thermocouple (HJTC) Reactor Vessel Level Monitoring System provides an alternate indication of reactor vessel water level. Two safety-related HJTC probes are provided (Channels A and B), The HJTC probes remain in the reactor vessel during Modes 1 through 5 to provide direct reactor vessel water level indication and are primarily used during design basis accident conditions. Each HJTC probe has eight sensors. Three of the eight sensors are located within the hotleg region at 42, 21, and 0 inches above the bottom of the hotleg. During reduced inventory operations the 21-inch sensor provides a control room low level indication and alarm as a supplement to the RWLI and DLMS indication. The 21-inch sensor and alarm is located above the level where vortexing of the shutdown cooling (SDC) suction line would occur under normal flow conditions.

Reactor Vessel Water Level Sightglass A sightglass is installed during Modes 5 and 6 to provide direct reactor vessel water level indication. The sightglass, which must be read locally, is located inside containment. The sightglass uses the same common sensing lines as the RWLI narrow range and wide range transmitters.

Water Inventory Control San Onofre has performed extensive benchmarking of the water inventory required to change water level from one level to another. The volume of water removed from the reactor vessel is monitored closely when the reactor coolant system is drained for reduced inventory operations. San Onofre has had excellent success and agreement between actual conditions and expected values when using the benchmarking described above. This method represents an additional diverse measure to validate the reactor coolant system level control during drain down.

Reactor Water Level Probe The Reactor Water Level Probe (RWLP) provided a secondary means of control room indication for monitoring the reactor coolant system level during reduced inventory operations in Modes 5 and 6. The RWLP was a heated junction thermocouple based reactor vessel water level detection probe which resided in an in-core instrument guide path and provided inputs to the Critical Functions Monitoring System (CFMS) for monitoring the reactor vessel water level during reactor coolant system reduced inventory operations. Heated junction thermocouples were spaced three inches apart from five inches below the top to

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four inches above the bottom of the hotleg in the hotleg region. The presence of water at a specific elevation was detected by monitoring the temperature difference between the heated junction thermocouple and the bulk water temperature. In addition to control room indication, the RWLP also provided an adjustable low level alarm via the CFMS. The RWLP remained in the reactor vessel during Modes 1 through 5, however it was only used during Mode 5. The RWLP was installed in the reactor vessel upper guide structure and removed with it during refueling.

l On November 26, 1994, during Unit 3 Cycle 7, the Unit 3 RWLP failed a routine quarterly test. This test failure indicated that the RWLP, which is only used during Mode 5, was not functional. Subsequent investigation found stress corrosion cracking in the RWLP sheath material (Inconel 600) pure. Thiswater ;

cracking resulted in water intrusion into the RWLP and subsequent short circuits within the RWLP heaters and thermocouples. This was the second RWLP to fail at Unit 3, the first having failed during Cycle 6 from what is believed to be the same cause. On December 12, 1995, the Unit 2 RWLP failed its routine quarterly test from what may be the same cause as the Unit 3 RWLP failures. The RWLPs were designed to last for the life of the plant; however due to repeated failures Edison will be substituting a different design (see i following) for the RWLP.

Draindown Level Monitoring System I

The Draindown Level Monitoring System (DLMS) is the system selected by Edison to meet Edison's commitment to the recommendations of Generic Letter 88-17 and thus replaces the RWLP. The DLMS uses quartz crystal pressure transducers to measure referance pressure and Reactor Coolant System (RCS) head which are used to calculate water level. A reference transducer is attached near the top of the pressurizer. Two RCS head transducers are used to measure coolant head. One transducer is connected to a two-inch header connected to the '

bottom of one hotleg. (TheRWLIisalsoconnectedtothesametwo-inch header.) The other transducer is attached to the sixteen-inch shutdown (See cooling)

Figure. pump suction lineprovide The transducers attached to signals input the bottom of the opposite to a dedicated hotleg.

personal computer located in the computer room which will calculate and display reactor l vessel water level based on the transducer input. The DLMS also transmits the '

calculated level to the CFMS for display and provides an adjustable low-level  !

alarm.

Regulatory Commitments ,

1 Generic Letter (GL) 88-17, Loss of Decay Heat Removal, (Reference 2) recommends the following:

" Provide at least two independent, continuous RCS water level indications whenever the RCS is in a reduced inventory condition."

Edison met this recommendation by installing the RWLP as the second independent reactor coolant system water level indicator. The RWLI is the primary level indicator. The NRC accepted the RWLP in Inspection Report Nos. 50-361/91-34 and 50-362/91-34 as meeting the recommendation in GL 88-17.

o Document Control Desk The Units 2 and 3 Updated Final Safety Analysis Report (UFSAR)

(Section 7.6.2.4 " Reactor Water Level Probe") states Edison's use of the RWLP as a backup to the RWLI to meet the recommendation in GL 88-17. The Units 2 and 3 UFSAR will be updated to delete the RWLP and add the DLMS to meet Edison's commitment to the recommendations of GL 88-17.

Previous to installation of the RWLP, Edison used the HJTC probes for alternate reactor coolant system level indication. The NRC staff, however, noted that the HJTC probe did not have the level resolution necessary for all reduced inventory operations and thus did not satisfy the recommendation in GL 88-17.

DISCUSSION:

As noted above, Edison has experienced repeated failures of the RWLP probes.

Because of the repeated RWLP failures and high replacement costs, Edison will be replacing the RWLP with the DLMS during the Unit 2 Cycle 9 Refueling Outage, which is currently scheduled to begin in late November 1996. The DLMS will also be installed at Unit 3 as a replacement for its failed RWLP during l the Unit 3 Cycle 9 Refueling Outage. To allow time to install and test the DLMS, Edison plans to use the same action plan used to address RWLP inoperability used at Unit 3 during the Cycle 8 Refueling Outage (see Reference 1). This plan is outlined below.

l Planned Compensatory Actions to Address RWLP Inoperability It is planned to perform the Unit 2 Cycle 9 Refueling Outage and any unscheduled reactor coolant system reduced inventory operations during the current Unit 2 Cycle 8 without the RWLP probe with the provision that the ,

following compensatory measures be implemented: I

1. Provide an Alternate Method of Correlating RWLI When decreasing reactor coolant system level the RWLI narrow range indication will be correlated using an alternate method. This method will . utilize the reactor vessel water level sightglass.

2. Change Procedures for Operator Response to Abnormal Conditions The water level during reactor coolant system reduced inventory operations is nominally well above 21 inches above the bottom of the hotleg (i.e., about 26 inches). At the 21-inch level the time available for operator responses to abnormal conditions will be greater than it would be at the current minimum water level when the RWLP is operable.

In addition, the following will be implemented during reduced inventory operations while the RWLP is unavailable:

A. RWLI narrow range control room indication and alarm will be maintained functional; and

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B. At least one channel with 21-inch (No. 6) HJTC Low Level indication and alarm will be maintained operable; and C. If a HJTC probe 21-inch low level alarm is received, Operations will:

(1) stop draining; and (2) validate the alarm using the other HJTC #6 (if available--if a second HJTC #6 is not available then the alarm will be assumed valid); and (3) if the alarm is valid, add water until the alarm clears. l If a second 21-inch HJTC probe low level alarm is received during the I same midloop operations, Operations will immediately refill and maintain level above 21 inches using a method which is independent of RWLI (e.g.,

HJTC #5 at 42 inches above the bottom of the hotleg, a camera positioned ,

within the steam generator, or through some other means). l These actions will be taken until RWLI instruments are verified to be operable.

Acceptability of Edison's Proposed Course of Action A 10 CFR 50.59 Safety Evaluation found that, with implementation of the planned compensatory measures, operation in a reduced inventory condition with the RWLP inoperable does not present an unreviewed safety question.

Therefore, Edison concludes that using the HJTC probe as a backup to the RWLI during the period that the RWLP is inoperable and being replaced is acceptable.

Previous Edison reactor coolant system reduced inventory practices included reducing the reactor coolant system water level to as low as 17.5 inches above the bottom of the hotleg. Operation with the reactor coolant system water level this low required operation within an area in which the HJTC probes do not provide indication. Raising the midloop level was made possible in 1993 when the level required for installing nozzle dams was increased from 17.5 to 26 inches by procurement of a new type of nozzle dam which is positioned higher in the steam generators. This change has significantly reduced the risk associated with midloop operations and has made it possible to utilize the HJTC probe as backup to the RWLI while at midloop when the RWLP is out of service. This increase in midloop operating level gives further assurance of the acceptability of use of the HJTC probe as a backup to the RWL1 during the period that the RWLP is inoperable and being replaced.

In addition to the above, existing station procedures require at least two means of providing inventory makeup during reduced inventory operations (this requirement is ncrmally met by maintaining one High Pressure Safety Injection (HPSI) pu p operable and one additional HPSI or Containment Spray pump available .

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SUMMARY

1 Edison believes it is acceptable to perform reduced inventory operations with <

the RWLP probe inoperable for the following reasons: l e The primary means of reactor vessel water level indication (RWLI wide and narrow range indication) will be available and is reliable; and e The reactor vessel water level sightglass is normally monitored while reducing reactor vessel level and will be monitored when reducing reactor vessel level until the level is stable and correlates satisfactorily; and e The HJTC probe provides an independent indication of an unanticipated reactor coolant system draindown and will alert operators to such an event; and e The nominal reactor coolant system level during reduced inventory operations has been increased to 21 inches above the bottom of the hotleg; and e Operating procedures will be modified to improve operator response to unanticipated reactor coolant system draindown events; and e A 10 CFR 50.59 Safety Evaluation shows that performing reduced inventory operations while the RWLP is inoperable with the compensatory measures in place does not present an unreviewed safety question.

It is expected that the DLMS will be av'ailable for use at each unit by the end of their respective Cycle 9 Refueling Outages.

If you have any questions or would like additional information on this subject, please let me know.

Sincerely, s

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C Enclosure cc: L. J. Callan, Regional Administrator, NRC Region IV J. E. Dyer, Director, Division of Reactor Projects, Region IV K. E. Perkins, Jr., Director, Walnut Creek Field Office, NRC Region IV J. A. Sloan, NRC Senior Resident Inspector, San Onofre Units 2 & 3 M. B. Fields, NRC Project Manager, San Onofre Units 2 an,d 3 ik

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