05000382/LER-2014-004
| Waterford 3 Steam Electric Station | |
| Event date: | 10-22-2014 |
|---|---|
| Report date: | 06-03-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3822014004R03 - NRC Website | |
On October 22, 2014, a walkdown of the Emergency Diesel Generator (EDG) [EK] was conducted as part of the NRC Component Design Basis Inspection (CDBI). An NRC inspector identified corrosion on the EDG Feed Tank [DC][TK] vent lines where the vent lines pass through the roof. Visual inspection revealed the corrosion was through wall. It is unknown how long this corrosion has existed.
Waterford 3 is a Combustion Engineering design pressurized water reactor [AC] with two recirculating type steam generators [SG].
The EDG engines are started by means of compressed air, which is admitted to the cylinders in sequence. The air is supplied by the starting system and is admitted to the engine through one or two air operated valves. The valves are opened by the control system in response to either an automatic or manual control signal. The engine will start to turn over and will accelerate under the pressure of the starting air.
When sufficient engine speed is reached, fuel oil will be injected, and the engine will begin to operate as a diesel and will accelerate to a speed at which the starting air will be cut off. Further acceleration will bring the unit to its rated speed.
The engine speed is controlled by the governor, which regulates the fuel oil supply to the engine injectors. Fuel is pumped from the feed tank to the engine fuel headers and injector pumps by an engine driven pump. The storage tank contains the bulk of the fuel supply, and a transfer pump automatically supplies fuel to the feed tank to maintain the level in the feed tank.
Fuel is supplied to each engine by gravity flow from its feed tank. Each feed tank is kept full by transferring oil from the fuel storage tank as required by means of the transfer pump. The pump is started and stopped by level switches on the feed tank.
Fuel from the feed tank is supplied through either of two parallel connected strainers and either of two parallel connected filters to the engine fuel oil supply header. Booster pumps are provided at the engine to ensure that the header pressure is sufficient for satisfactory operation of the engine at all loads.
Follow up analysis has determined that some amount of rainfall less than the postulated Probable Maximum Precipitation (PMP) event could have resulted in sufficient water ponding on the Reactor Auxiliary Building (RAB) [NF] roof to allow water intrusion into the EDG A and B Feed Tanks that could exceed the 0.1 percent water content allowed by the vendor technical manual. This could have affected the operability of both the A and B Train Emergency Diesel Generator Feed Tanks and Emergency Diesel Generators.
System Description:
The booster pumps are of the positive displacement type, delivering a constant volume of fuel. Surplus fuel in excess of engine load requirements is returned to the feed tank via relief valves.
On October 22, 2014, Waterford Steam Electric Station Unit 3 (Waterford 3) was in Mode 1 at approximately 100% power.
INITIAL CONDITIONS
EVENT DESCRIPTION
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CONTINUATION SHEET
Waterford 3 Steam Electric Station
2. DOCKET
A DC motor driven booster pump is provided to ensure that as the engine is started, the fuel header is primed for prompt firing as the engine speed increases. When the speed is sufficient, an engine driven pump will develop sufficient head for continued operation of the engine, and the DC motor driven pump is automatically shut down. Check valves are provided in parallel with each pump so that the fuel can flow past an idle pump.
Fuel is injected into the cylinders at the correct point in the cycle by cam operated injector pumps, which supply a metered quantity of fuel to the corresponding injection nozzles on the engine. The amount of fuel is controlled by the engine governor.
The unavoidable leakage from the injectors, caused by the high oil pressure and small but necessary clearances in the moving parts, is collected by drains and returned by gravity to the storage tank.
Each Feed Tank has a vent and flame arrestor.
TIMELINE
Engineering review of the available meteorological tower data shows peak rain rates of 3.8 inches per hour have been experienced at the site in in the last two years. Based on the follow-up evaluation, these rainfall rates could have potentially resulted ponding on the RAB roof that would allow in water ingress into the EDG Feed tanks. These rain events have been followed by successful EDG monthly tests.
The monthly EDG operability surveillance is performed in accordance with Operations procedure OP-903-068. Part of this procedure requirement is to visually inspect and drain any accumulated water from the EDG Feed Tank.
Based on periodic review of the EDG surveillance date, no water has been observed or drained from the EDG Feed Tanks.
REPORTABLE OCCURANCE
Technical Specification (TS) 3.8.1.1 requires, in part, two separate and independent diesel generators. This requirement is applicable in Modes 1, 2, 3, and 4. An allowed outage time (AOT) of up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is specified, or be in at least Hot Standby (Mode 3) within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Cold Shutdown (Mode 5) within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. The requirement for restoration to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> may be extended to 10 days if a temporary emergency diesel generator is verified available.
Additionally, the two separate and independent diesel generators will each have diesel oil feed tanks containing a minimum volume of 339 gallons of fuel, a separate diesel generator fuel oil storage tank, and a separate fuel transfer pump. If these criteria cannot be met, operability of the remaining A.C. circuits must be tested within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> after. An AOT of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is specified or be in at least hot standby (Mode 3) within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and cold shutdown (Mode 5) within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. The requirement for restoration to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> may be extended to 10 days if a temporary emergency diesel generator is verified available.
T.S. Surveillance Requirement (SR) 3.8.1.1.2.b requires at least once per 31 days and after each operation of the diesel where the period of operation was greater than or equal to 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> that accumulated water from the diesel oil feed tanks be checked for and removed.
Engineering evaluated this condition and determined that rain rates within the design basis rainfall will cause ponding on the RAB roof that would potentially allow in water ingress into EDG Feed Tanks A and B.
Rain rate history was reviewed and determined that peak rain rates of 3.8 inches per hour had been experienced within the last two years at the Waterford 3 site. A rain rate of this intensity for the measured
CONTINUATION SHEET
Waterford 3 Steam Electric Station
2. DOCKET
duration could have resulted in a water intrusion percentage above the 0.1 percent allowed by the vendor technical manual in both the EDG A and B Feed Tanks. If required to operate after a Design Basis rainfall event over the past 3 years both the train A and B EDGs could have been inoperable due to water intrusion into the EDG Feed Tanks through the corroded holes.
This condition is reportable under 10 CFR 50.73(a)(2)(i)(B) because the corrosion, and therefore the potential for water intrusion greater than that allowed by the vendor technical manual, has existed for longer than the AOT of the applicable TSs. This condition is also reportable under 50.73(a)(2)(v)(D) and 10 CFR 50.73(a)(vii)because the corrosion was found on the vent lines for both trains of EDGs.
CAUSAL FACTORS
An Apparent Cause Evaluation was completed for this condition and then subsequently upgraded to a Root Cause Evaluation.
The direct cause of this condition was a failure of the EDG Feed Tank Vent Line pipe protective coating.
The first root cause was a latent design error that did not: a) Provide a life of plant design for steel to concrete interface in wet environment or; (b) Provide a design ensuring concrete to steel interface would be protected from water or; (c) Identify the need for a replacement or mitigation strategy. The second root cause was that the station did not identify the failure mechanism of through wall corrosion combined with postulated precipitation events at the site could challenge the EDG safety functions. During review of EDG and related systems for maintenance rule, the non-safety to safety related interface associated with the EDG feed tank vent lines did not identify this challenge to performance of the EDG safety functions. If this challenge had been identified, then a maintenance strategy would have been considered.
The contributing cause of this event was programs designed to perform walkdowns/inspections on the EDG feed tank roof did not contain adequate scope to ensure monitoring of the EDG feed tank vent lines.
Specifically, the Engineering and Maintenance procedures did not provide the adequate scope to ensure an individual with directions or opportunities to observe and identify degradation, was placed in direct contact with penetrations contained the severely corroded Fuel Oil Feed Tank Vents, thus delaying identification.
A Green non-cited violation (NCV) of 10 CFR 50 Appendix B, Criterion 16 was issued for failing to identify and correct through wall corrosion on both Emergency Diesel Generator Feed Tank Vents. Measures to identify and correct issues including corrosion are identified in the Engineering procedure for Systems Walkdowns, which provides guidance for the conduct of walkdowns of systems, components, and structural commodities, and the procedure governing the Corrective Action Program.
CORRECTIVE ACTIONS
As an interim action, a stainless steel pipe clamp with a rubber liner was installed around EDG Feed Tank vent pipe to prevent water intrusion. In addition, concrete grout pads were installed upstream of each pipe to divert rain water away from the pipes. Work packages to repair the piping are being planned and scheduled for implementation to permanently repair the condition.
The EDG Feed Tank Vent Lines were added to Maintenance Rule to initiate a maintenance strategy. A maintenance rule scoping was performed for all Category 1 systems, focusing on this operating experience (i.e. failure mechanism of non-safety related component that impacted the safety function of a SSC).
A memo was issued to Systems Engineering re-enforcing the requirements to perform system walkdowns of all accessible areas as procedurally required. An action was also assigned to create and document system walkdown plans.
As an extent of condition review, other roof areas were walked down to identify any similar corrosion issues.
SAFETY SIGNIFICANCE
Industrial Safety: There was no industrial safety significance associated with this issue.
Radiological Safety: There was no radiological safety significance associated with this issue.
Nuclear Safety:
The finding was determined to be of very low safety significance (Green). The change to the core damage frequency was approximately 4x10-7/year. The risk-important sequences included a heavy rain event greater than or equal to 6 inches per hour followed by a random loss of offsite power within the next two weeks. The risk significance was mitigated by the tolerance of the diesel generators to water in the fuel oil and the operators' ability to restore offsite power within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the loss of offsite power.
SIMILAR EVENTS
No similar events at Waterford 3.
ADDITIONAL INFORMATION
Energy industry identification system (EIIS) codes and component function identifiers are identified in the text with brackets [ ].