05000302/LER-2006-001
| Crystal River Unit 3 | |
| Event date: | 10-20-2006 |
|---|---|
| Report date: | 12-11-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3022006001R00 - NRC Website | |
EVENT DESCRIPTION
At 11:15, on October 20, 2006, Progress Energy Florida, Inc. (PEF), Crystal River Unit 3 (CR-3) was operating in MODE 1 (POWER OPERATION) at 100 percent RATED THERMAL POWER when an investigation concluded that flow through the line serving the Train B Nuclear Services and Decay Heat Seawater (RW) System cyclone separator (RWSP-1B) [BI, SEP] was approximately 0.4 gallons per minute (gpm). A flow of approximately 9 gpm is required through RWSP-1 B to ensure flush water with particles less than 250 microns is delivered to the bearings of Emergency Nuclear Services Seawater Pump RWP-2B [BI, P] and Emergency Decay Heat Seawater Pump RWP-3B [BI, P]. Train B of the RW System was declared inoperable at this time.
The drain flow from RWSP-1A was measured and found to be 11.5 gpm. Therefore, the Train A RW System remained operable.
The normal supply of bearing flush water to the RW pumps is from the Demineralized Water (DW) System [KC]. The Domestic Water (DO) System [KK] supplies bearing flush water to the RW pumps upon loss of the DW System. The Nuclear Services Closed Cycle Cooling (SW) System [KG] can be aligned to supply bearing flush water to the RW pump bearings upon loss of the DW and DO Systems. The emergency source of bearing flush water to the RW pumps during a loss of offsite power event is seawater from the RW pump discharge.
To ensure the RW pumps will operate for their 30 day mission time during a loss of offsite power event, the pump bearing flush water from the RW pump discharge is filtered and separated to a cleanliness of 250 microns. Filtering is accomplished by RW System duplex strainers RWS-3A/3B [BI, STR] that are required to remove particles greater than 1/8 inch to prevent plugging the cyclone separator underflow (drain) port, stopping the filtration process of the cyclone separator.
Separation is accomplished by RW System cyclone separators RWSP-1A/1B that are required to remove particles greater than 250 microns. With the identified low flow condition, the filtration process performed by RWSP-1 B cannot ensure a flush water cleanliness of 250 microns to the RW pump bearings.
Unfiltered water supplied to components in the flush water system such as the bearing flow meters, control valves, and pump bearings, could lead to reducing and potentially eliminating critical water flow to the RW pump bearings. Total loss of water flow to the pump bearings will result in bearing degradation and could eventually lead to bearing failure.
At 11:15 on October 20, 2006, Improved Technical Specifications (ITS) 3.7.9, Nuclear Services Seawater System (RWP-2A/2B), Condition A, and 3.7.10, Decay Heat Seawater System (RWP-3A/3B), Condition A, were entered. Both Conditions require that, with one train inoperable, restore the train to an operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
RWSP-1 B was cleaned and flushed. The drain flow from RWSP-1 B was measured and found to be 10.4 gpm. At 21:58 on October 20, 2006, the actions of ITS 3.7.9, Condition A, and 3.7.10, Condition A, were no longer applicable.
A review was performed of available information to determine when the RWSP-1B might have become clogged. The primary method used to verify flow through the cyclone separator drain line is the monitoring of differential pressure (DP) across the strainers upstream of the cyclone separator. The differential pressure indicator for RWS-3B, RW-103-DPI [BI, PDI], is monitored weekly per Surveillance Procedure SP-306, "Routine Surveillance Log." SP-306 verified the DP indication is less than 50 inches of water and greater than 2 inches of water. Verifying the DP is greater than 2 inches of water was the method utilized to ensure adequate flow through the strainers and cyclone separator. Previous SP-306 data was reviewed and it was revealed that RW-103-DPI indicated 0.0 inches of water starting on July 6, 2006. This low DP indicates that flow blockage was likely at this time.
SP-306 had a contingency note that stated if the DP is less than 2 inches of water, then ensure flow exists by verifying strainer outlet temperature is approximately the same as Ultimate Heat Sink (UHS) [BS] temperature. While verifying temperatures may provide some insight to the flow through the separator, this method may give a false indication if the UHS and ambient temperatures are approximately the same. Using this method of flow determination resulted in the incorrect determination that adequate flow was available from July 6, 2006 until October 20, 2006.
A review was performed to determine if RWP-2A or RWP-3A were removed from service between July 6, 2006 and October 20, 2006. RWP-2A and RWP-3A were removed from service under ITS 3.7.9 and ITS 3.7.10, respectively, for a scheduled Emergency Core Cooling System Outage from 02:00 on July 18, 2006 to 15:00 on July 19, 2006. During this time, both trains of the RW System were technically inoperable.
Train B of the RW System being inoperable from July 6, 2006 through 11:15 on October 20, 2006, is reportable under 10CFR50.73(a)(2)(i)(B). Both trains of the RW System being operable from 02:00 on July 18, 2006 to 15:00 on July 19, 2006, is reportable under 10CFR50.73(a)(2)(v)(B).
This condition is not reportable under 10CFR50.72(b)(3)(v) since the condition did not exist at the time of discovery.
SAFETY CONSEQUENCES
The event was not safety significant. RWSP-1B contained foreign material which inhibited its ability to separate debris from the safety related bearing flush water supply. The normal (DW System) bearing flush water availability was not affected. This condition existed from July 6, 2006 to October 20, 2006. During this period, Train A of the RW System was removed from service for scheduled maintenance activities for approximately 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.
The duplex strainers were installed in 1997 during the CR-3 design outage to reduce the potential for plugging the cyclone separator underflow (drain) port. No occurrences of pump bearing failures due to debris in the bearing flush water have been identified prior to 1997. Based on previous operating experience, CR-3 believes that plugged cyclone separator underflow (drain) port would not have had an immediate impact on pump operation during a loss of offsite power event.
Eventually, debris might lead to a pump bearing failure.
Using the CR-3 current Probabilistic Safety Assessment Model of Record (P-02-0001, Revision 3), this plant condition can be evaluated for risk significance. The following bounding assumptions have been made: (1) since the amount of time that a RW System pump could operate without filtering from RWSP-1 B is difficult to determine, RWP-2B and RWP-3B are conservatively assumed to fail immediately upon the loss of DW and DO System supplies to the pump bearings; (2) RWSP- 1 B is assumed to be unavailable for four months; and, (3) Train A of the RW System is assumed to be out of service for two days.
The delta core damage frequency for RWSP-1 B being out of service is approximately 3.00E-7 per year and the Incremental Core Damage Probability (ICDP) is 1.00E-7 for a four month exposure time. The delta core damage frequency of RWSP-1 B being out of service concurrent with Train A of the RW System unavailable is approximately 3.00E-6 per year and the ICDP is 2.00E-8 for a two day exposure time.
Therefore, the total ICDP for this event is approximately 1.00E-7. This is considered to have very low risk significance.
Based on the above discussion, PEF concludes that inoperability of both RW System Trains for a period of two days did not represent a reduction in the public health and safety. Since the identified condition is reportable under 10CFR50.73(a)(2)(v)(B), this event does meet the Nuclear Energy Institute definition of a Safety System Functional Failure (NEI 99-02, Revision 2).
CAUSE
The cause for this event was improper material selection. The duplex strainer baskets upstream of the cyclone separators are fabricated from Monel Alloy 400, which is susceptible to pitting and crevice corrosion in its installed service condition. Seam welds on one of the two duplex strainer baskets failed due to corrosion caused by a low velocity seawater environment and the strainer material of construction. The duplex strainers are designed to remove debris greater than 1/8 inch.
The oyster shell found stuck in the cyclone separator drain port was 9/16 inch in diameter.
Therefore the duplex strainer did not perform its function.
The vendor that supplied the Model 534 Duplex Strainer packages installed as RWS-3A/3B was Fluid Engineering, a Division of TM Industrial Supply, Incorporated.
The failure of Monel Alloy 400 in low velocity raw water service environments is documented in Electric Power Research Institute document EPRI 1012039, "Materials Reliability Program Materials Handbook for Nuclear Plant Pressure Boundary Applications." This document contains the following statement: "[Monet] Alloy 400 has been found to be susceptible to pitting and crevice corrosion under deposits, probably mainly due to microbiological influenced corrosion (MIC), in raw water service applications.
A contributing cause was that the guidance provided in SP-306 was responsible for not recognizing inoperability of RWSP-1B between July 6, 2006 and October 20, 2006. Operability of RWSP-1B was not questioned until there was a difference between the UHS and ambient temperatures and an operator noted that the strainer housing for Train A was sweating and the strainer housing for Train B was not sweating.
CORRECTIVE ACTIONS
1. The debris was removed from RWSP-1B and the piping between RWSP-1B and duplex strainer RWS-3B was flushed to remove any foreign materials. These actions were completed on October 20, 2006 under Work Order 968626-01.
RWS-3B was opened and inspected. Both strainer baskets were intact. However, a failed strainer basket had been replaced in September 2006. The foreign material found in RWSP-1B is postulated to have originated from this previous strainer basket failure and subsequently migrated to the cyclone separator drain port.
2. An extent of condition review was performed for applications of Monel Alloy 400 exposed to a low velocity seawater environment. The only RW System components identified were the components associated with duplex strainers RWS-3A and 3B. There are a few other Monel components, such as piping and valves, downstream of the duplex strainers that have not suffered noticeable corrosion attack due to the fact that these components are located in a higher velocity portion of the system. Monel Alloy 400 is limited in the Circulating Water System [NN] to the main circulating water pump bolting hardware. This hardware has been inspected numerous times with no degradation noted. There are no other known locations at CR-3 where Monel Alloy 400 exists in a seawater application.
3. Other actions associated with this event are being addressed in CR-3 Corrective Action Program Nuclear Condition Report NCR 210023.
PREVIOUS SIMILAR EVENTS
No previous similar events involving equipment inoperability due to a failure of Monel Alloy 400 component in a low velocity seawater environment have been reported to the NRC by CR-3.
ATTACHMENTS
Attachment 1 - Abbreviations, Definitions, and Acronyms Attachment 2 - List of Commitments ATTACHMENT 1
ABBREVIATIONS, DEFINITIONS AND ACRONYMS
CFR Code of Federal Regulations DO Domestic Water System DP Differential Pressure DPI Differential Pressure Indicator DW Demineralized Water System EPRI Electric Power Research Institute gpm gallons per minute ICDP Incremental Core Damage Probability ITS Improved Technical Specifications NCR Nuclear Condition Report NEI Nuclear Energy Institute NRC Nuclear Regulatory Commission PEF Progress Energy Florida, Inc.
RW Nuclear Services and Decay Heat Seawater System RWS RW System Strainer RWSP RW System Cyclone Separator RWP-2B Nuclear Service Seawater System Pump RWP-3B Decay Heat Seawater System Pump SP Surveillance Procedure SW Nuclear Services Closed Cycle Cooling System UHS Ultimate Heat Sink NOTES: Improved Technical Specifications defined terms appear capitalized in LER text {e.g., MODE 1} Defined terms/acronyms/abbreviations appear in parenthesis when first used {e.g., Reactor Building (RB)}.
EllS codes appear in square brackets {e.g., reactor building penetration [NH, PEN]}.
ATTACHMENT 2
LIST OF COMMITMENTS
The following table identifies those actions committed to by PEF in this document. Any other actions discussed in the submittal represent intended or planned actions by PEF. They are described for the NRC's information and are not regulatory commitments. Please notify the Supervisor, Licensing & Regulatory Programs, of any questions regarding this document or any associated regulatory commitments.
RESPONSE
SECTION
COMMITMENT DUE DATE
No regulatory commitments are being made in this submittal.