IR 05000346/1987025

Corrected Pages to Augmented Insp Team Insp Rept 50-346/87-25

ML20238D081
Person / Time
Site:	Davis Besse
Issue date:	12/29/1987
From:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
To:
Shared Package
ML20238D071	List: IR 05000346/1987025 ML20238D068
References
50-346-87-25, NUDOCS 8801040089
Download: ML20238D081 (6)

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III. INSPECTION EFFORTS Failed Feedwater Flow Control Circuit Problem The initiating event was the failure of a signal amplifier module in Main Feedwater Flow Indicating Loop FTSP2A1. This caused indicated flow to fall to approximately 85 percent of actual flow. This false indication caused the Integrated Control System (ICS) to increase the flow of feedwater to Steam Generator No. 2. This in turn resulted in a cooling of the Reactor Coolant System and a subsequent reactor power increase to 103 percen Licensee Evaluation Main Feedwater Flow is sensed across flow elements FESP2A and FESP2B as shown in Attachment 5. Each flow element has redundant flow trans-mitters for control and indication: FTSP01A1 and FTSP02B1 (preferred)

for flow element FESP2A and FTSP02B2 and FTSP02A2 (alternate) for flow element FESP28. The output of these transmitters is compared to a flow demand by ICS and the error signal is used to position the feed-water regulating valve On November 25, 1986, loop FTSP02A1 could not be properly calibrate The signal amplifier module, which was of non-encapsulated construction, was replaced with a different type of module which was of potted, encapsulated construction. The modules are interchangeable, the encapsulated module being a newer design for harsh environment appli-cations . The cause of the failure of the signal amplifier module was a shorted capacitor that loaded down the output and caused the transformer in the mudule to overhea Licensee Corrective Actions The licensee will send the module to Bailey, the vendor, for further evaluation. The licensee has replaced the failed module with a new module of the previous style (non-encapsulated). The licensee considers the failure of the module as a random failure and not indicative of a trend or design flaw. An encapsulated amplifier module (similar to the one that failed) is presently used in the complementary flow channel as well as several level transmitter applications. The encapsulation of the module is a standard Bailey design to acconnodate a harsher environment and has been in service in the plant for approximately one yea AIT Review

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The AIT reviewed calibration and maintenance history for these modules and the main feedwater flow indicating loops. Based on this and the licensee's review the team concluded that the signal amplifier module failure was random. As noted above, the encapsulated design is standard, has been in service in the plant and has proven to be reliable. The AIT considers the licensee's corrective action in this regard to be adequat ,

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The licensees procedure MP10410.69," Periodic Maintenance of 13.8kV and 4.16kV Breakers" covered floor tripper adjustments. The preven-tive maintenance schedule on 13.8kV and 4.16kV breakers is performed every five years and had not yet been performed on this breaker. For added assurance, the licnesee checked all 13.8kV and 4.16kV breakers to ensure that the floor tripper interlock has adequate clearance between the floor trippper cam and the breaker tripping trigge It appears that the malfunction of the position switch on Main Disconnect 34620 may have contributed to the failure of the Bus A transfer during the September 6,1987 event and would have Jefinitely prevented future bus transfers. Since the 13.8KV Buses A and B tran-ferred successfully on the reactor trip of August 21, 1987, the auxiliary position switch had to have been closed at that time. This switch might have become open during cycling Main Disconnect 34620 following the reactor trip on September 6,1987. The licensee can not be sure if this problem occurred before or after September 6,198 The licensee will also evaluate generator-transformer protection relaying for comon mode failure to ensure operability of the bus transfer schem AIT Review The AIT reviewed the schematic diagrams of the bus transfer scheme, the proposed retesting program, the maintenance procedure for 13.8kV and 4. 16kV breakers, and the Technical Specifications related to these breakers. The team also witnessed portions of the trouble-shooting and testing conducted by the licensee. The team noted that the licensee had implemented a preventive maintenance program prior to the event which would have identified the misadjusted interloc Breaker HX01A was scheduled for this preventive maintenance in mid-September. The team determined that the failure of non-safety Bus A to auto transfer was of minimal safty significance because safety Bus C1 was re-energized by the diesel generator as design. The team concluded that the licensee has taken adequate corrective actions to ensure that the bus transfer circuitry will function as designe The AIT noted that the bus auto-transfer of 13.8kV Buses A and B from their normal source, the Unit Auxiliary Transformer, to their reserve sources, Startup Transformers 01 and 02, is not periodically tested in accordance with Technical Specification 4.8.1.1.1B (the manual testing requirement of 4.8.1.1.1B is performed during unit startups and shutdowns). From the team's discussions with the licensee staff, it appears that periodic surveillance testing is performed each refueling outage by auto-transferring 13.8kV Buses A and B between Startup Transformer 01. and 02 and not from the Auxiliary Transformer to the Startup Transformers. The AIT believes that the licensee has misinterpreted this surveillance requiremen ,

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testing by Babcock & Wilcox showed that.the failed disc collars possessed very low impact strength, a marginally acceptable chemical composition, and grain structure which indicated that the casting did not receive the proper cooling cycle. The licensee will refurbish all eighteen MSSVs during the forthcomin with new wrought and machined (stronger material) disc gcollar refueling outage All stems will be replaced and enhanced quality control will be applied in the area of stem to disc interface, collar to stem interface, and parts identification, i i

The licensee.will continue their engineering evaluation into the failures of the disc collars and the apparent restriction of the failures to the "B" heade AIT Review The AIT reviewed the maintenance procedure for disassembling Main Steam Safety Valves, Babcock & Wilcox metallurgical reports, Dresser Engineering Report SV-231, licensee preliminary reports on the MSSV failures, and a video tape and photographs from the disassembly of valve B2, which failed during a reactor trip on March 13, 1987. The i team also inspected B1 and B3 prior to their disassembly, witnessed 1 the disassembly of B3, and conducted a telephone interview with a i B&W metallurgis The team noted that the licensee was aggressively pursuing resolution of the problem prior to this latest occurrence and that prior to this trip, plans had already been made to refurbish all the MSSVs during the forthcoming refueling outag The number of safety valve lifts at Davis-Besse appears to be excessive. A study to develop methods of reducing the number of lifts occurring during a reactor trip would be beneficial since it '

appears to the team that the frequency of lifts bears on the failure rat '

The AIT considers the licensee's corrective action program for MSSV failures to be adequate. The team concurs with the licensee's evalua- '

tion that six valves could fail in the described mode and would not )

negate the plants capability to control temperatur . Startup Feedwater Control Valves Problem Following the reactor trip and approximately two hours later the operators experienced difficulty in controlling steam generator levels. In the immediate case the Integrated Control System (ICS)

Rapid Feedwater Reduction (RFR) circuit responded to the reactor trip by closing the Startup Feedwater Control to a " target" position. When ICS control of feedwater flow shifted from RFR to low level limits, the steam generators were at a higher water level than desired. This resulted in auto-closing of the Startup Feedwater Control valves with a large error signal which caused

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5. Turbine Bypass Valve Problem

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At approximately 3:08 p.m. Turbine Bypass Valve SP13A3 failed ope Licensee personnel in the vicinity of the valve observed that it would rapidly open, remain open ten to fifteen seconds and then slam shut. System piping (attachment 11) deflected several inches with each cycle. This sequence was repeated three or four times until SP13A3 failed ope The increased steam flow associated with SP13A3 being open and the Startup Feedwater Control valves being isolated resulted in Steam Generator No. 2 level falling to about 23 inches with a resultant SFRCS actuation. Both Turbine Driven Auxiliary Feedwater Pumps started as required and proper steam generator level was quickly restored. The pumps were shutdown and returned to standby after control of steam generator levels was regaine Licensee Evaluation Visual inspection of SP13A3 revealed extensive damage to the positioner: linkages, limit switches, cams, pipe nipples, and pins were bent and/or broken and bolts were missing or sheared of The licensee determined that SP13A3 failed due to over-travel of the positioner cam and linkage. On an opening cycle the cam follower rode past the mechanical stop and onto a cutaway section of the ca This unintended configuration set up a positive feedback in the positioner which on the next opening cycle drove the valve into the oscillations witnessed by the licensee personnel in the area. The successive impacts from the full-stroke oscillations further damaged the positioner and the valve failed fully open. Probable causes of the positioner failure include improper alignment of the mechanical stops and cam positions and entrained water in the piping upstream of the valves flashing to steam as it passes the valve plug and i creating excessive forces in the upward direction on the valve ste The alignment procedure for these valves was non specific in setting the mechanical limits of the valves being aligned. This apparently 4

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resulted in a situation where the electropneumatic signal to the positioner is significantly less than the mechanical full open. When the valve opened fully, the positioner cam apparently rotated be. 'd '

the drop-off onto the cutaway. The procedure does not clearly sp.cify the hook-up of the test rig which could result in only the positioner calibration being checked, independent of mechanical position. If the ,

electrical to pressure (E to P) converter was misadjusted, this would also result in a situation where the electrical full open signal is less than the mechanical full open and the cam could rotate beyond the drop-off. The setting ranges specified in the procedure may provide  !

enough cumulative error to degrade the calibration of the E to P '

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Review of the system history revealed a higher incidence of repairs

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to the "A" train turbine bypass valves. Walkdowns of the piping showed less high pressure drain capacity in the vicinity of the "A" valves and that SP13A3 is closest to the header. This could result in entrained water entering these valves. When the valves open, the low pressure of the condenser will cause this water to flash to steam under the disc, causing excessive upward forces on the ste This upward force could also contribute to the over-travel of the cam and linkag Licensee Corrective Actions The remaining five valves were inspected on September 9, 1987 and two turbine bypass valves showed mechanical open greater than 120 percent of electrical open and one valve with mechanical open at 75 percent of electrical ope The positioner for SP13A3 was repaired and refurbished. The E to P converter and positioner was calibrated and operated manually, pneumatically, and electrically to verify operatio Four of the other five valves were also calibrated (13B3 required no calibration)

and all were tested to verify operation. A full-flow stroke test of the turbine bypass valves will be performed at full main steam pressure while the plant is in Mode 3. The licensee determined that the same positioner configuration was used on other air-operated valves ( the Startup Feedwater Control Valves) in the plant and these valves were also adjuste Corrective actions beyond the immediate repairs, calibration, and testing of the valves includes evalution of the high pressure drain piping for possible addition of another drain trap upstream of the turbine bypass valve header, possible redesign of the positioner cam to eliminate the drop-off, and revision of the calibration procedure to clearly establish the test rig hook-up, provide tighter alignment criteria, and verify mechanical and electrical limit AIT Review The AIT reviewed the maintenance and calibration history of the turbine bypass valves, the calibration procedure, IC 2700.23,

" Control Valves and Accessories Calibration", the vendor instruction manuals for the valve / positioner loop components, and licensee engineering evaluations related to the problems with the turbine bypass valves. The team also witnessed the repairs to SP13A3 and recalibration of some of the valve The team noted that there was a history of problems with the operation of the turbine bypass valves. The licensee was aware of the problem and was attempting to resolve it. Prior to this event the valves and their positioners had been rebuilt under the guidance of a vendor technical representativ _ _ _ _ _ _ _ _ _

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IV. SUMMARY Safety Significance After careful review of the transient and the subsequent equipment I malfunctions which occurred, the AIT concluded that this event did not pose any hazard to the public nor did it raise any significant safety concerns regarding the operation of the facility. The safety significance was evaluated with respect to operator performance, safety system performance, and licensee response. The AIT found that with the exception of the two errors noted in Section III, the operators responded properly to minimize the consequences of the even During and immediately after the reactor trip, all safety systems functioned as required; the problems encountered with safety systems developed during the post-trip recovery. The licensee's response to the event was aggressive, thorough, and indicative of commitment to safet Event Reporting The AIT evaluated event reporting with regard to the notifications required by 10 CFR 50.72 and communications with the resident inspectors and regional management. The team reviewed NRC and licensee transcripts of the 10 CFR 50.72 notification call, listened to a copy of the NRC Headquarters Duty Officer recording of the call, and interviewed licensee personnel, the Senior Resident Inspector, and members of regional managemen The team concluded that the event was properly reported within the applicable time requirements. The amount and content of the information transmitted was adequate with the exception that the licensee did not specifically state that Emergency Diesel Generator No. I had auto-started and loaded in response to the loss of voltage on Essential Bus C1 and that Service Water Pump 1-1 had failed to auto-start when Bus C1 was re-energize Conclusions / Recommendations The AIT recommends the following with regard to both plant specific and generic aspects of this event: The licensee should te-evaluate their interpretation of Technical Specification 4.8.1.1.1B and develop a surveillance procedure which specifically tests the auto-transfer of power from the Unit Auxiliary Transformer to the Startup Transformer . Region III should determine whether or not the licensee has misinterpreted 4.8.1.1.1B and failed to perform adequate surveillance of the electrical power distribution syste _ - _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ .