05000498/LER-2007-001

LER-2007-001, Turbine-Driven Auxiliary Feedwater Pump Failed to Start During Surveillance Testing (Supplement 1)

Docket Number
Event date:	12-12-2006
Report date:	08-07-2007
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4982007001R01 - NRC Website

A. REPORTABLE EVENT CLASSIFICATION

This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). South Texas Project (STP) Technical Specification 3.7.1.2 requires that all three motor-driven pumps and one turbine­ driven auxiliary feedwater (TDAFW) pump are to be operable when the unit is in Modes 1, 2, or 3. However, the Unit 1 TDAFW pump was inoperable longer than the allowed outage time and plant shutdown was not accomplished within the required time. This placed STP Unit 1 in a condition prohibited by Technical Specifications.

B. PLANT OPERATING CONDITIONS PRIOR TO EVENT

STP Unit 1 was in Mode 1 at 100% power.

AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT

No other inoperable structures, systems, or components contributed to the event.

D. NARRATIVE SUMMARY OF THE EVENT

On December 12, 2006 at 1046 hours0.0121 days <br />0.291 hours <br />0.00173 weeks <br />3.98003e-4 months <br />, a surveillance test of Unit 1 TDAFW pump 14 was performed. However, when the Control Room Operator attempted to open the steam flow control valve for steam flow to the turbine, the valve did not open, and the TDAFW pump did not start.

Plant personnel were stationed at the TDAFW pump for the surveillance run and test equipment was installed to collect turbine start-up data. As part of the pre-start procedure requirements, the turbine was verified to be properly set-up for the surveillance test including visual inspection of the latch-up lever and trip hook engagement. Visual inspection of the latch-up lever and trip hook found that the two faces were not fully engaged, but the condition was deemed to meet minimum interface requirements documented by previous engineering assessment.

After the attempted start, it was determined that, based on information from the personnel stationed at the pump and the fact that the test equipment did not record any movement of the governor valve or speed indication from the turbine, steam had not been supplied to the turbine. The pump was declared inoperable.

Examination revealed that the trip and throttle valve latch-up lever at the valve actuator had disengaged from the trip hook and the steam flow control valve had remained closed.

Items susceptible to wear and degradation were inspected, including the latch-up lever and trip hook mating surfaces. The mating surfaces showed no wear, but were found to be coated with a layer of grease that was more than the vendor-recommended light coating. The excess grease was removed and a light coating applied. The rod end (ball joint) assembly located at the turbine end of the overspeed mechanical trip linkage was suspect based on the fault tree review and was replaced. Inspection of the replaced part found no degradation that would have affected the operation of the assembly or mechanical trip linkage.

The mechanical overspeed trip linkage was inspected for proper assembly and the impact space for the clevis pin at the trip and throttle valve slip link was found to be incorrect. The procedure for the mechanical trip linkage reassembly lacked adequate detail that may be required to consistently achieve satisfactory impact space set-up.

The overspeed mechanical linkage was disassembled and cleaned, the impact space was adjusted, and testing demonstrated repeatable test results.

The motor-driven auxiliary feedwater pumps were not affected by this condition.

The Unit 1 TDAFW pump was declared operable at 0125 on December 14, 2006.

PROCEDURAL ERROR

This condition was identified during a planned surveillance test of the Unit 1 TDAFW pump.

II. COMPONENT FAILURE

The trip linkage is the interconnecting hardware between the turbine mechanical overspeed trip mechanism and the trip and throttle valve. It translates movement of the tappet nut / head lever into unlatching the trip and throttle valve trip hook and closing the valve upon actuation of the mechanical overspeed trip.

The trip linkage consists of a connecting rod with a clevis and pin on one end and a ball-type swivel rod end on the other. The clevis mates with a slip link lever attached to a common shaft with the trip hook on the trip and throttle valve, and the swivel rod end is attached to the turbine head lever. When the trip tappet is raised, either by the action of the overspeed trip pin or the manual trip lever, the head lever is released. Releasing the head lever allows the trip spring to pull the connecting rod towards the tappet / tappet nut. The slip link lever then rotates the common shaft to the trip hook, disengaging the hook from the latch-up lever and allowing the trip and throttle valve to trip closed.

Impact space is provided between the trip rod pin and the trip rod side of the trip arm slot. The purpose of the impact space is to allow for relative movement between the trip and throttle valve and the mechanical overspeed trip assembly (linkage). Impact space between the clevis pin and the slip link lever inside face allows the connecting rod to build up speed before impacting (i.e., hammer blow) the slip link lever. This hammer blow to the slip link lever is to ensure the trip hook unlatches upon overspeed trip operation (i.e., overspeed trip by turbine shaft-mounted counter weight or manual actuation of hand trip lever). Inadequate impact space can make the linkage more sensitive to factors such as spring force relaxation or additional drag forces due to latch-up lever face corrosion.

Lack of full engagement is an indication that the overspeed trip mechanism linkage does not have sufficient impact space; this can cause the latch-up lever and trip hook to separate when the valve is opened. Once the valve begins to open, the latch-up lever transfers the valve spring pack load to the trip hook. Should the trip hook disengage, the valve spring pack immediately closes the valve.

A. SAFETY SYSTEM RESPONSES THAT OCCURRED

No safety systems were required to respond during this event.

B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY

The Unit 1 TDAFW pump is considered to have been inoperable beginning November 16, 2006.

Repairs to the TDAFW pump were completed and the pump was declared operable at 0125 on December 14, 2006. Consequently, the TDAFW pump was inoperable for approximately 28 days.

C. SAFETY CONSEQUENCES AND IMPLICATIONS

Technical Specification Requirements:

South Texas Project (STP) Technical Specification 3.7.1.2 requires three motor-driven pumps and one TDAFW pump to be operable when the Unit is in Modes 1, 2, or 3. With the TDAFW pump inoperable, or with any two auxiliary feedwater pumps inoperable, the affected auxiliary feedwater pump(s) are to be restored to operable status with 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. If the required action and associated allowed outage time are not met, the unit is to be in at least Hot Standby within the next six hours and in Hot Shutdown within the following six hours.

Because the Unit 1 TDAFW pump was inoperable longer than allowed under the Technical Specifications without entering the appropriate action statements, this event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).

Risk Assessment:

A risk assessment was performed to estimate the core damage risk associated with the event. The assessment considers the as-found condition of the TDAFW pump. In this case, although automatic start of the pump was not available due to the mechanical overspeed latch assembly condition, local operator action could be used to start the TDAFW pump following most reactor trip initiators. For the condition existing from November 16, 2006, to December 13, 2006, including credit in the risk assessment for local operator action for starting the TDAFW pump results in an incremental conditional core damage probability (ICCDP) of 3.3E­ 07.

In addition, the balance-of-plant startup feedwater pump that provides a procedural alternate water supply to the steam generator was available for a significant portion of the exposure period. Although not credited in the quantitative risk assessment, an available startup feedwater supply reduces the core damage risk associated with this event. Results of an assessment of room heatup from the loss of Electrical Auxiliary Building HVAC initiator and steam generator dryout studies support the time needed for operator response to align alternate steam generator makeup sources.

IV. CAUSE OF THE EVENT

Two factors were identified as being root causes because they both contributed to the event:

1. Inadequate maintenance instructions; and 2. The design of the trip and throttle valve linkage leaves no margin for variability.

V. CORRECTIVE ACTIONS

1. Enhanced detail / guidance for adjusting / setting latch-up lever and trip hook interface gap and impact space has been included in maintenance procedures. The enhanced impact space set­ up instructions have been incorporated into training materials for craft personnel assigned to the task to include how to optimize the impact space setting, applicable lessons learned, and operating experience.

2. An evaluation will be performed to determine if a modification to the linkage will improve its reliability. If this modification is not feasible, additional actions will be assessed to address the configuration of the latching mechanism.

3. An operator aid has been developed to assist plant operators in determining that the latch-up lever and trip hook linkage is properly latched.

4. The requirement for 75% latch-up lever and trip hook interface has been reviewed, and is consistent with the vendor manual for generic applications of this type of valve.

VI. PREVIOUS SIMILAR EVENTS

Over the preceding five years, there have been no reportable events involving the TDAFW pump due to similar linkage misalignment.