05000528/LER-2007-004
| Palo Verde Nuclear Generating Station (Pvngs) Unit 1 | |
| Event date: | 08-11-2007 |
|---|---|
| Report date: | 10-10-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282007004R00 - NRC Website | |
Note: All times listed in this event report are approximate and Mountain Standard Time (MST) unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER (50-528/2007-004-00) is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) to report operation in a condition prohibited by Technical Specifications (TS).
Specifically, TS Surveillance Requirement (SR) 3.6.6.2 requires Containment Spray (CS) piping be maintained full of water to the 113 ft level in the CS header. Contrary to this requirement, system monitoring determined that a portion of the Unit 1 CS "B" header piping contained a void.
Further investigation revealed that the station surveillance test procedure (STP) failed to adequately assure that this TS SR had been met for the CS headers for any of the Units.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
System Description:
The CS system (EIIS Code: BE) consists of two independent trains that provide a spray of cool borated water into the containment to reduce containment temperature and pressure, provide hydrogen mixing, and to reduce the concentration of fission products in the containment atmosphere during a Design Basis Accident (DBA) which results in actuation of the CS system. Heat is removed from the spray water by the Shutdown Cooling (EIIS Code: BP) Heat Exchangers (SDCHX). The CS system may be actuated either automatically or manually for the initial injection phase, where inventory from the Refueling Water Tank (RWT) (EIIS Code:
BP) provides the borated water source for injection. Upon low RWT water level, a Recirculation Actuation Signal (RAS) automatically realigns pump suction to the containment Emergency Core Cooling System (ECCS) recirculation sump, thereby initiating the recirculation mode of operation.
Palo Verde Nuclear Generating Station TS SR 3.6.6.2 requires CS piping be maintained full of water to the 113 ft level in the CS header. The requirement to fill the header to the 113 ft elevation ensures that the spray flow is admitted to the containment atmosphere within the time frame assumed in the safety analysis. This level also serves as an assumption in analysis for dynamic loading (water hammer) consideration in system design. The CS initial fill process following an outage (from a drained down condition) has been performed utilizing Procedures 400P-9S104, "Safety Injection System Venting," and 400P-9S102, "Recovery from Shutdown Cooling to Normal Operating Lineup." The following figure represents the piping arrangement of the "B" CS piping near the Containment penetration.
CS "B" CONTAINMENT PENETRATION AND PIPING CONFIGURATION
CONTAINMENT PENETRATION
SIBV165 check valve elev. 90' 5 1/2" flow direction
3. INITIAL PLANT CONDITIONS:
On August 11, 2007, Palo Verde Units 1, 2 and 3 were in Operating Mode 1 (Power Operations), at approximately 100 percent power. No equipment was inoperable that contributed to this event.
4. EVENT DESCRIPTION:
On August 11, 2007, the Safety Injection System Engineer was reviewing Unit 1 plant data and noticed that as Containment Building atmospheric pressure increased the level in the CS header decreased, indicative of air entrapment in the header volume. The System Engineer notified Unit 1 Operations personnel of the suspicious indications, and on August 11, 2007, at 23:15 hours Unit 1 Operations personnel declared the "B" CS System inoperable due to suspected voiding in the header, and entered TS Limiting Condition for Operation (LCO) 3.6.6. Engineering personnel performed system troubleshooting including Ultrasonic Testing (UT) analysis and determined that a section of the header did contain a void in the portion identified in the drawing above as "Region of Trapped Air'. Therefore, the header was not "full of water' as required by the TS. Operations personnel filled and vented the header in accordance with procedure 400P-9S102. On August 12, 2007, at 16:09 hours the "B" CS System was restored to operable status and TS LCO 3.6.6 was exited.
During subsequent investigation of the cause of this event, station personnel determined that the STP developed to ensure compliance with the TS SR was inadequate in that it did not completely verify that the header was full of water. As a result, on September 8 at 14:05, all three units entered TS SR 3.0.3 which states that if it is discovered that a surveillance was not performed within its specified frequency, then compliance with the requirement to declare the LCO not met may be delayed, from the time of discovery, up to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or up to the limit of the specified frequency, whichever is greater. This delay period is permitted to allow performance of the Surveillance. The affected STP was revised and reissued on September 21, 2007, to include direction to perform UT analysis of the CS header to verify that the header is full on a monthly basis.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
Unit 1 condition identified on August 11, 2007 The voided section of CS header in Unit 1 did not result in a planned or unplanned shutdown of any unit. However, this condition did result in the Unit 1 "B" Containment Spray system being declared inoperable for nearly 19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> while UT analysis could confirm the location of the unfilled header and the header was appropriately filled and vented. During this nearly 19 hours2.199074e-4 days <br />0.00528 hours <br />3.141534e-5 weeks <br />7.2295e-6 months <br /> of inoperability, Unit 1 remained in Mode 1 within the requirements of TS LCO 3.6.6. Subsequent investigation indicated that this voided condition was created during the initial fill process of this section of header on July 1, 2007, and, therefore, the condition existed for approximately 42 days.
The voided volume in Unit 1 was determined to be approximately 55 gallons and was located downstream of the outboard CS Containment Isolation Valve (CIV). The effect it would have on CS operation following an accident was evaluated and it was concluded that the condition would yield no significant effect from this decrease in Containment Spray header level/volume based on the volume of the void, the location of the void (downstream of the outboard CS header CIV, SIB-671) and the flow capacity of the CS pump (3890 gpm rated flow). The delivery of water to the spray nozzles would have been delayed as a result of the identified void by as much as 1.5 seconds assuming a maximum Containment Building atmospheric pressure of 60 psig. This would have had no effect on the ability of the CS system to perform its designed safety functions.
Additionally, this void did not present a water hammer concern since it was located in the discharge side of the CS pump and the discharge piping is open to the Containment Building atmosphere without obstruction. This would preclude any water hammer event.
The void would have been simply flushed out into the Containment atmosphere.
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. There were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The event did not result in a transient more severe than those analyzed in the updated Final Safety Evaluation Report Chapters 6 and 15. The event did not have any nuclear safety consequences or personnel safety impact.
The condition would not have prevented the fulfillment of any safety function of structures or systems as defined by 10 CFR 50.73(a)(2)(v).
Inadequate STP for all three units Since the STP did not verify the CS piping between the outboard and inboard containment isolation valves was full there may have been past occurrences when the CS piping was not full for an extended period of time. The normal practice for filling the CS headers is to do a fill and vent using normal operating procedures following a refueling outage. Since this portion of the CS piping is not exposed to gas systems (e.g.
nitrogen), if properly filled and vented the piping should remain full during plant operation. A review of internal operating experience did identify previous occurrences of some amount of voiding in the CS headers; however, in each instance the amount of void was determined to not be significant and would not have prevented the CS system from performing its safety function.
6. CAUSE OF THE EVENT:
There are two distinct but related conditions associated with this report, each with separate direct and root causes.
The direct cause of the void in the CS header was that the Unit 1 "B" CS Header was not properly vented during the fill and vent process on July 1, 2007. The root cause of this condition was ineffective use of operating fundamentals during performance of the pre-job briefing, resulting in failure to identify the potential for air entrapment during the fill and vent process on July 1, 2007. Contributing to this root cause was a procedural deficiency in procedure 400P-9S102, which in section 4.7.7.15, directed personnel to "Unlock and throttle open SIB-V891 as needed to vent the Containment Spray Header." In utilizing the words "as needed" the direction was unclear, in that operations personnel concluded that the entire step was optional, rather than the words "as needed" applying only to the extent valve SIB-V891 could be intermittently throttled. Operations personnel involved with the pre-job briefing believed that the CS header had not been drained during the outage, and as such would not require any fill and vent to be performed.
The direct cause for the identified failure to properly meet the TS SR to verify the CS header is full was a procedural deficiency in that STP 40ST-9S113, "LPSI and CS System Alignment Verification" did not adequately demonstrate compliance with TS SR 3.6.6.2.
The root cause of this condition is indeterminate in that greater than 20 years have passed since the STP was developed and no records were located to determine the basis of the original STP.
7. CORRECTIVE ACTIONS:
The Unit 1 "B" CS header was filled and vented and then verified through ultrasonic testing (UT) to be filled. Both "A" and "B" Train CS headers in each Unit were evaluated to assess their current condition for compliance with TS SR 3.6.6.2. In order to confirm that CS headers were full, specific UT was performed on each of the Containment Penetration pipes of each CS Train on September 10, 2007. All CS headers were found to be completely filled except Unit 3 "B", which evidenced a slight void. Operations personnel completed an immediate operability determination and concluded that the "B" train remained operable. The Unit 3 "B" header was filled and then verified to be filled using UT.
On September 13, 2007, a Unit Night Order was entered for all 3 units to inform Operations personnel of the event, and to instruct that all procedural steps with qualifying statements such as "as necessary" or "as needed" shall be reviewed with the Control Room Supervisor (CRS) to ensure that the desired action is understood.
STP 40ST-9S113 was revised and reissued on September 21, 2007, to require performance of UT analysis of each CS header, between the containment isolation valves, to verify that the header is full. The STP was completed on September 23, 2007, for the "A" train of CS for all three Units and for the "B" train in Units 1 and 2 on October 6 and October 8, 2007, respectively, thereby satisfying SR 3.0.3. Unit 3 entered Operating Mode 5 for its 13th Refueling Outage on September 29, 2007, and therefore the requirements of SR 3.0.3 were no longer applicable. As previously stated, UT had been performed on the "B" train of CS for Unit 3 on September 10, 2007, and performance of the revised STP including UT will be completed prior to restart of Unit 3 following the refueling outage.
Operations Department will establish and implement a program to improve the use of operator fundamentals which will include performance monitoring and trending.
A review of each Surveillance Requirement to verify a system, sub-system or component is filled with water (or other fluid) to a specific criterion will be performed to ensure the Surveillance Test adequately demonstrates the Surveillance Requirement criteria are met.
A review all procedures that affect safety related equipment for steps with conditional phrases such as "as needed", "as necessary", "if required", etc. will be performed to ensure the steps cannot be reasonably misinterpreted to permit an unintended action.
8.� PREVIOUS SIMILAR EVENTS:
Emergency Core Cooling System containment sump piping due to a failure to translate the design intent to have the line filled with water into start-up, surveillance, and operating procedures. The corrective actions for that event would not have prevented this event, as the intent to maintain the CS header full of water was clearly captured in the STPs and operating procedures.