05000483/LER-2006-008

LER-2006-008, COMS INOPERABILITY, RHR SUCTION RELIEF VALVE FAILURE

Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No.
Event date:	8-30-2006
Report date:	10-30-2006
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor
4832006008R00 - NRC Website

A. REPORTABLE EVENT CLASSIFICATION

10CFR50.73(a)(2)(i)(B) — Operation or Condition Prohibited by Technical Specifications 10CFR50.73(a)(2)(v)(A) — Event or Condition that Could Have Prevented Fulfillment of a Safety Function 10CFR50.73(a)(2)(vii) — Common Cause Inoperability of Independent Trains or Channels

B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT

Plant was in MODE 1 at 100% power at the time the event was discovered.

START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT

See sections II.B and II.0 of this report.

D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES

EJ8708B, Residual Heat Removal (RHR) B suction relief was inoperable for the cold overpressure mitigation system (COMS) function at the same time the pressurizer power-operated relief valves (PORVs) were not operable for the COMS function. Inoperability of the RHR suction relief valve was discovered during surveillance testing on August 30, 2006, after Refuel Outage 14 (Fall 2005). Relief valve EJ8708B lifted at approximately 146 pounds per square inch (psi) high during surveillance testing. Relief valve EJ8708A, RHR A suction relief, lifted within the proper range during surveillance testing. EJ8708A was considered operable but degraded (refer to section I.E., Method of discovery of each component, system failure or personnel error). Inoperability of the pressurizer PORVs for COMS was previously reported in Licensee Event Report (LER) 2006-001-00.

The following information is provided as background material to aid in understanding the event.

Technical Specification (TS) 3.4.12, COMS is applicable in Mode 4 with reactor coolant system (RCS) temperature less than or equal to 275F, Mode 5 and Mode 6 with the head on the reactor vessel. To meet the COMS requirements, TS 3.4.12 allows using one of the following pressure relief capabilities:

a. Two power operated relief valves (PORVs) with lift settings within the limits specified in the PTLR, or b.Two residual heat removal (RHR) suction relief valves with setpoints >1= 436.5 psig and psig, or c. One PORV with a lift setting within the limits specified in the PTLR and one RHR suction relief valve with a setpoint >1= 436.5 psig and d. The RCS depressurized and an RCS vent of >1= 2.0 square inches.

During a plant outage (generally a refueling outage), an RHR suction relief valve is removed from the RHR system and replaced with a pretested valve. Following the end of the outage, the relief valve that was removed from the system is sent to an offsite vendor for testing. The results are then evaluated for impact for the time the valve had been installed.

On Februaryl 1, 2004, an inadvertent Safety Injection was initiated during a plant heatup after a forced outage — LER 2004-004-00. A review of plant data indicates the pressurizer PORVs lifted twelve times before operators terminated the safety injection. The lifting of the pressurizer PORVs caused a pressure transient on the pressurizer relief tank (PRT) relief valve common discharge header. This pressure transient damaged relief valves N66251-00-0003 and N66251-00-0020, which were installed as RHR suction relief valves EJ8708A and EJ8708B. No apparent indications were available to indicate that the relief valves had been damaged.

During Refuel 13 in the Spring 2004, various combinations of the inoperable pressurizer PORVs (LER 2006-001-00) and one inoperable RHR suction relief valve, EJ8708B, and one operable but degraded RHR suction relief valve, EJ8708A, were credited for COMS protection. Refer to the table in section II.B. DURATION OF SAFETY SYSTEM INOPERABILITY for dates and approximate timeframes.

During the cooldown and depressurization for Refuel 14 (Fall 2005), both inoperable Pressurizer PORVs were initially credited for COMS. Later, COMS was credited to PORV A and RHR A suction relief valve.

All four relief paths were documented in the surveillance, but there is no objective evidence the two relief paths not credited were maintained until the next surveillance was performed. Later, COMS was credited to the PORV B and the RHR B suction relief valve. All four relief paths were documented in the surveillances.

On September 20, 2005, an equipment operator unisolated centrifugal charging pump (CCP) A during restoration from engineered safety features actuation system (ESFAS) testing by opening the A CCP manual discharge valve. (Note: This was documented in NRC inspection reports as apparent violation 05000483/2005004-01). At the time, the A CCP was isolated for COMS. It was determined that unisolating the CCP did not constitute an "Event or Condition That Could Have Prevented Fulfillment of a Safety Function" because the pump handswitch was in Pull-to-Lock and an automatic pump start would require "an additional random single failure." Therefore, this issue was determined to be not reportable.

At the time of the event, RCS pressure was being maintained with a nitrogen bubble in the pressurizer, and COMS protection was being provided by the inoperable B pressurizer PORV and the inoperable B RHR suction relief valve.

In October 2005, during Refuel 14, RHR suction relief valves N66251-00-0003 (EJ8708A) and N66251- 00-0020 (EJ8708B) were removed from the system and replaced with N66251-00-0004 and N66251-00- 0019 respectively under plant work documents.

On November 14, 2005, the Pressurizer PORVs were determined to be inoperable for COMS during the investigation of Callaway Action Request (CAR) 200509340. CAR 200509374 was written to resolve this issue.

On August 30, 2006 the RHR suction relief valves serial numbers N66251-00-0020 (EJ8708B) and N66251-00-0003 (EJ8708A) were tested at NWS Technologies in South Carolina. N66251-00-0020 failed its surveillance test, lifting at 596 psig. Lift acceptance criteria is 436.5 to 463.5 psig.� N66251-00- 0003 passed its surveillance test, lifting at 442.1 psig.

On September 12, 2006, a Root Cause Team started investigating the cause of the damaged RHR suction relief valves. On September 22, 2006, the cause of the pin failure of the RHR suction relief valves was determined to be a pressure transient on the PRT common relief valve discharge line. The pressure transient was caused by lifting the Pressurizer PORVs 12 times at normal operating pressure (NOP) prior to terminating a safety injection on February 11, 2004. A night order was issued to alert the Operating Crews.

ERROR

The failure of the RHR suction relief valves was discovered in August 2006 by surveillance testing followed by subsequent disassembly and inspection. Both of the RHR suction relief valves were replaced during the fall 2005 Refuel 14 outage as routine surveillance testing per the in-service testing program.

The individual valves described in this LER were not installed in the plant when the failure was discovered.

On August 30, 2006, the RHR suction relief valves serial numbers N66251-00-0020 (EJ8708B) and N66251-00-0003 (EJ8708A) were tested at NWS Technologies in South Carolina. N66251-00-0020 failed its surveillance test, lifting at 596 psig. Lift acceptance criteria is 436.5 to 463.5 psig. During disassembly and inspection, the assembly pin was discovered broken in three pieces. Two of the pieces were located between the disc button and the bellows bushing, which increased the preload of the setpoint spring and raised the lift pressure to 596 psig. N66251-00-0003 passed its surveillance test, lifting at 442.1 psig. Pin impact marks were found on the disc button.

During disassembly and inspection of N66251-00-0003, the assembly pin was discovered broken in eight distinct pieces and a few fragments. Shavings of the pin jammed between the disc button and insert bushing caused an interference fit of the disc button and insert bushing. A 1/4 inch piece of the pin was located in the valve body at the base of the nozzle ring. The remaining parts of the pin were located between the disc insert and bushing. On both valves, pin impact marks were found on the disc button.

II.� EVENT DRIVEN INFORMATION

A. SAFETY SYSTEMS THAT RESPONDED

Not applicable for this event.

B. DURATION OF SAFETY SYSTEM INOPERABILITY

TS 3.4.12, COMS, is applicable in Mode 4 with RCS temperature less than or equal to 275F, Mode 5 and Mode 6 with the head on the reactor vessel. To meet the COMS requirements, TS 3.4.12 allows using the pressurizer PORVs, Residual Heat Removal (RHR) system suction relief valves, a combination of PORVs and RHR suction relief valves, or the reactor coolant system (RCS) depressurized with a RCS vent >1= 2.0 square inches.

The affected equipment is the pressurizer PORVs when used for COMS function and the RHR suction relief valves. The pressurizer PORVs function at normal RCS operating conditions is not affected.

From February 11, 2004 until the fall of 2005 Refuel 14 outage, EJ8708B RHR suction relief valve was inoperable for COMS. RHR suction relief valve EJ8708A was considered operable but degraded.

Per LER 2006-001-00, the pressurizer PORVs were considered inoperable for COMS before and during the same timeframes the RHR suction relief valve was inoperable.

The following tables summarize when the COMS TS was applicable and what methods were considered to have been used to satisfy the TS.

COMS TS Applicable (3 year history) Start� Stop� Comment 10/23/2002 1452� 10/28/2002 0720� Refuel 12 Cooldown 11/14/2002 1502 � 11/21/2002 0554 � Refuel 12 Heatup 03/22/2003 0034� 03/31/2003 0557� March 2003 outage 04/10/2004 1946� 04/19/2004 0344*� Refuel 13 Cooldown 05/18/2004 0312� 06/07/2004 0514� Refuel 13 Heatup 09/17/2005 1635� 09/22/2005 1713� Refuel 14 Cooldown, 11/03/2005 0421� 11/13/2005 1735� Refuel 14 Heatup 05/23/2006 1200� 06/01/2006 1420� Spring 2006 Outage Potential timeframes for noncompliance with TS 3.4.12:

Timeframe� COMS By Refuel 12 10/23/2002 1452 — 10/26/2002 0206� 2 PZR PORVs 10/26/2002 0206 — 10/28/2002 0720� 2 RHR suction relief valves 11/14/2002 1502 — 11/17/2002 0042� 2 RHR suction relief valves 11/17/2002 0042 — 11/19/2002 0116� 1 PZR PORV and 1 RHR Suction relief valve 11/19/2002 0116 — 11/19/2002 0153� 2 RHR suction relief valves 11/19/2002 0153 — 11/21/2002 0244� 1 PZR PORV and 1 RHR suction relief valves 11/21/2002 0244 — 11/21/2002 0544� 2 PZR PORVs Spring Outage 03/22/2003 0034 — 03/24/2003 0022� 2 PZR PORVs 03/27/2003 2210 — 03/31/2003 0557� 1 PZR PORV and 1 RHR suction relief valve Refuel 13 04/10/2004 1946 — 04/19/2004 0344� 2 PZR PORVs 05/18/2004 0312 — 05/23/2004 0052� 2 RHR suction relief valves 05/23/2004 0052 — 05/24/2004 2308� 2 PZR PORVs 05/24/2004 2308 — 05/26/2004 0007� 1 PZR PORV and 1 RHR suction relief valve 05/26/2004 0007 — 05/28/2004 0919� 2 PZR PORVs 05/28/2004 0919 — 05/28/2004 1025� 1 PZR PORV and 1 RHR suction relief valve 05/28/2004 1025 — 05/29/2004 0143� 2 PZR PORVs 05/29/2004 0143 — 06/01/2004 0025� 2 RHR suction relief valves 06/01/2004 0025 — 06/06/2004 0514� 2 PZR PORVs Refuel 14 09/17/2005 1635 — 09/18/2005 1737� 2 PZR PORVs 09/18/2005 1737 — 09/22/2005 1713� 1 PZR PORV (B) and 1 RHR suction relief valve (B) (Note: Both CCPs were unisolated for approximately 20 minutes on 09/20/2005.) 11/03/2005 0421 — 11/08/2005 0433� 2 RHR suction relief valves 11/08/2005 0433 — 11/09/2005 1143� 2 PZR PORVs (Note: 2 RHR suction relief valves available for some of this time.) 11/09/2005 1143 — 11/09/2005 1502� 1 PZR PORV and 1 RHR suction relief valve 11/09/2005 1502 — 11/13/2005 1735� 2 PZR PORVs The above dates and times were taken from control room narrative logs, watch relief checklists, plant computer data, and work control system data.

C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.

From February 11, 2004 until they were replaced in October 2005 during Refuel 14, the RHR suction relief valve EJ8708B was inoperable for COMS and RHR suction relief valve EJ8708A was operable but degraded for COMS. During Refuel outages 13 and 14, various combinations of RHR suction relief valves and pressurizer PORVs were credited for COMS.

Concurrent with the inoperability of EJ8708B and the degraded condition of EJ8708A, the Pressurizer PORVs were discovered inoperable for COMS as described in LER 2006-001-00. The Pressurizer PORVs were inoperable for COMS for the entire period the B RHR suction relief was inoperable.

Coincident with the degraded condition of the COMS relief paths, both CCPs were capable of injection into the RCS below 275°F for approximately 20 minutes as described in inspection reports 05000483/2005004 and 05000483/2005005, and apparent violation 05000483/2005004-01.

Determination that the one RHR suction relief valve (EJ8708B) was inoperable and the other (EJ8708A) was degraded was made after both valves were removed from the system during Refuel Outage 14.

Determination that the pressurizer PORVs were inoperable for COMs was also an after the fact determination, which was made after Refuel 14. RHR suction relief valve EJ8708A did relieve within the proper range when it was tested. Valve EJ8708B relieved approximately 146 psi high when tested. The pressurizer PORVs were capable of providing some cold overpressure protection for the RCS when they were aligned for that purpose. As discussed in LER 2006-001-00, the response time to a pressure transient had not been tested to verify that it was within the time assumed in the accident analysis. As documented in LER 2006-001-00, the expected pressure increase was approximately 11 psi. During the period the RHR suction relief valves were inoperable or degraded for COMS, RCS pressure did not exceed 436.5 psig (the lower range of the lift pressure) with the RHR loop suctions open. Additionally, Solid Plant Operations were not conducted during this period.

As noted in LER 2006-001-00, the analysis (for cold overpressure protection) includes assumptions that also qualitatively allow this potential pressure increase. These include:

• Safety Factor of 2 for primary stresses

• Assumes an initial flaw equal to 1/4 the reactor vessel wall thickness. Actual NDE techniques detect smaller flaws.

• Conservative mass and heat injection rates.

The bases for TS 3.4.12 states, "Two RCS relief valves are required for redundancy. One RCS relief valve has adequate relieving capability to prevent overpressurization for the required coolant input capability." TS 3.4.12 also notes that two CCPs may be made capable of injecting for .�.pump swap capabilities.

The inoperability of EJ8708B and the pressurizer PORVs and the failure to complete the Required Actions within the Completion Time is a condition prohibited by Technical Specifications. The failure mechanism for the RHR suction relief valves is a common mode failure. The damage to the RHR suction relief valves is a condition that could have prevented fulfillment of a safety function needed to maintain the reactor in a safe shutdown condition. A pressure transient similar to the one that damaged the RHR suction relief valves is possible but unlikely. We cannot predict the effect on the RHR suction relief valve setpoint because of the way the broken assembly pin fragments arranged themselves in the valves. For the specific case discussed in this LER, the damage to the RHR suction relief valves resulted in EJ8708B lifting higher than the required setting. EJ8708A lifted within the proper range. In addition, the pressurizer PORVs were functionally able to provide overpressure relief although they were technically inoperable.

The safety function of providing cold overpressure protection for the RCS was capable of being fulfilled for the conditions in the plant.

The RHR suction piping is designed for 600 psig at 400 degrees F. The as-found lift setpoints for the RHR suction relief valves were not so high as to cause an overpressure concern with the RHR pump suction piping.

Operability is not a concern for the RHR suction relief valves currently installed in the plant. The valves were surveillance tested by NWS Technologies during fuel Cycle 13 and installed in October 2005 during Refuel 14. No events causing the Pressurizer PORVs to lift have occurred since the RHR suction relief valves were installed in October 2005.

The current PRT Relief Valve Common Discharge Header piping design still allows the same degradation of design function to occur if an RCS PORV actuation event occurs at normal operating pressure (NOP). This risk has been identified and Operations has been informed of the potential degradation of the RHR suction relief valves. The current course of action, if a PORV actuation event occurs, is to declare the RHR suction relief valves inoperable for COMS. Longer range solutions are being considered under the corrective action program.

III. CAUSE(S) OF THE EVENT AND CORRECTIVE ACTION(S)

N66251-00-0020 failed its surveillance test, lifting at 596 psig (acceptance criteria is 436.5 to 463.5 psig).

N66251-00-0020 was installed as RHR suction relief valve EJ8708B during fuel Cycles 13 and 14. During disassembly and inspection, the assembly pin was discovered broken in three pieces. Two of the pieces were located between the disc button and the bellows bushing, increasing the preload of the set point spring and raising the lift pressure to 596 psig.

N66251-00-0003 passed its surveillance test, lifting at 442.1 psig. N66251-00-0003 was installed as RHR suction relief valve EJ8708A during fuel Cycles 13 and 14. During disassembly and inspection, the assembly pin was discovered broken in eight distinct pieces and a few fragments. Shavings of the pin jammed between the disc button and insert bushing caused an interference fit of the disc button and insert bushing. A 1/4 inch piece was located in the valve body at the base of the nozzle ring. The remaining parts of the pin were located between the disc insert and bushing. On both valves, there were pin impact marks on the disc button.

The RHR suction relief valves, EJ8708A and EJ8708B, share a common discharge header with the Pressurizer PORVs and Safety Relief valves. The cause of the damage has been determined to be a pressure pulse transient induced in the water solid tailpipe lines of the RHR suction relief valves when the Pressurizer PORVs lifted 12 times over approximately eight minutes following the inadvertent Safety Injection on February 11, 2004. The direct cause of pin failure is compression of the preload spring and shear of the disc assembly pin.

The root cause of the assembly pin failure is that the common relief valve discharge piping to the Pressurizer Relief Tank (PRT) is designed such that the 4-inch RHR suction relief valve discharge piping becomes water solid when PRT pressure exceeds six psig. Once water solid, repeated PORV actuation at normal operating pressure produces sufficient backpressure forces in the 4-inch discharge header to fail the disc assembly pin.

A water solid discharge, or mostly water solid with gas pockets, establishes conditions to transmit a significantly higher backpressure force (shock) on the valve internals than a gas-filled discharge during PORV and or Pressurizer Safety Valve actuation at full operating pressure.

Actions that have been taken or will be taken:

The Operations Department has been informed of the potential for degradation of the RHR suction relief valves. The current course of action, if a PORV actuation event occurs, is to declare the RHR suction relief valves inoperable for COMS.

The PRT Relief Valve Common Discharge Header piping design is such that a pressure transient developed from the lifting of a Pressurizer PORV at NOP is transmitted through water solid piping to the discharge side of the RHR suction relief valves. We are pursuing a change to the common relief valve discharge piping configuration to the PRT to prevent development of a water solid condition.

Actions related to the pressurizer PORVs are discussed in LER 2006-001-00.

Inadequate investigation and corrective action from past events did not identify some root causes. The extent of condition of past events was not fully investigated. The cause of the assembly pin failure discovered on valve N66251-00-0003 under W159359 in 1994 was never determined. Determination of the cause of an earlier assembly pin failure (refer to section IV Previous Similar Events) could have enabled the plant to make necessary design changes. A latent organizational weakness, which was identified in 2005, has been addressed in the corrective action program

IV. PREVIOUS SIMILAR EVENTS

On February 13, 1988 (CAR 198800045), a Low Steam Line Pressure Safety Injection was inadvertently automatically initiated after a plant trip when RCS temperature was allowed to cool down to 491 degrees Fahrenheit (F), 615 psig steam pressure. The Safety Injection flow caused the pressurizer steam bubble to compress, resulting in the Pressurizer PORVs to lift 12 times. The transient in the PRT common relief valve discharge header caused by the lifting of the Pressurizer PORVs damaged the A Train RHR suction relief valve. The damage was not discovered until 1994 during the investigation of CAR 199301774 — EJ8708A lifted at 350 psig causing RCS pressure to lower to 100 psig.

V. ADDITIONAL INFORMATION

The system and component codes listed below are from the IEEE Standard 805-1984 and IEEE Standard 803A-1984 respectively.

• System:� BP. Residual Heat Removal/Low-Pressure Safety Injection (PWR) Component:� RV, valve, relief