05000425/LER-2002-001
| Event date: | 10-22-2002 |
|---|---|
| Report date: | 12-05-2003 |
| 4252002001R01 - NRC Website | |
U.S. NUCLEAR REGULATORY COMMISSION DOCKET FACILITY NAME (11 LER NUMBER (61 TEAR I SEQUENTIAL !REVISION
A. REQUIREMENT FOR REPORT
This event is reportable per 10 CFR 50.73 (a)(2)(i)(B) because the unit was operated in a condition .prohibited by the Technical Specifications (TS) for a period of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and 31 minutes. It is also reportable per 10 CFR 50.73 (a)(2)(v)(B) because a condition existed that could have prevented the fulfillment of the safety function of a system needed to remove residual heat. Furthermore, this report is being made per the requirements of 10 CFR 21.21 because defective components were procured whose use could have led to the creation of a substantial safety hazard.
B. UNIT STATUS AT TIME OF EVENT
At the time of the discovery of this event, the units were defueled at 0 percent of rated thermal power. In Unit 2, personnel were preparing for fill and vent of the residual heat removal (RHR) system. In Unit 1, personnel were specifically looking for this condition. Other than that described herein, there was no inoperable equipment that contributed to the occurrence of this event.
C. DESCRIPTION OF EVENT
At Unit 2 on October 22, 2002, while defueled, residual heat removal (MR) pump A was started for dynamic fill and vent of the RHR system. Moments later, the pump tripped.
Personnel found that the pump shaft could not be hand turned. An investigation determined that a back casing ring capscrew had come loose and lodged between the impeller and the back casing ring.
It was found that the pump was last operated on October 11, 2002, when the unit was in Mode 6 (Refueling). It is believed that the capscrew came loose and lodged between the impeller and the back casing ring as the pump was being placed into standby status on October
D. CAUSE OF EVENT
The root cause of this event was determined to be the failure of the manufacturer to properly stake the back casing ring capscrews. Each capscrew is torqued and staking performed as a redundant measure to hold the capscrews in place. Although staking had been performed for the back casing rings inspected, it was inadequate to prevent at least one of the capscrews from backing out of the casing ring after it had lost its torque and become loose.
RI-IR pump B was.inspected and found to have a similar condition. Spare back casing rings in the -- --warehouse were also inspected and found to have a similar condition of inadequately staked capscrews. In Unit 1, on October 7 & 8, 2003, both RHR pumps were inspected and the Train B pump was found to have inadequately staked capscrews.
E. ANALYSIS OF EVENT
From the period of time when the Unit 2 RHR pump A was stopped on October 11, 2002, at 1017 EST until the reactor cavity water level was raised to 23 feet above the vessel flange at 1648 EST, Unit 2 operated in a condition prohibited by TS 3.9.6 because two RHR pumps were not maintained operable. However, the system safety function continued to be met because RHR pump B remained operable.
The spare back casing rings in the warehouse were not placed into service with the inadequately staked capscrews paid did not create a substantial safety hazard.
In addition, the improperly staked capscrews on the three affected RHR pumps in both Unit 1 and Unit 2 represented a condition that could have prevented fulfillment of a safety function of a system needed to remove residual heat. Discussions with pump vendor personnel determined that there are approximately 60 of these pumps in service in the nuclear industry, most with several years of service, and that this was the first failure of this type. Additionally, 15 of these pumps, including the 4 in Units I & 2, have undergone a coupling modification in recent years that replaced the back casing ring. However, it is not known if the back casing ring defect is limited to only those casing rings that were procured for the modifications or if the original pump back casing rings also possess this defect. Nonetheless, this was the first failure of this type for any of the 60 pumps, providing assurance that this event was an isolated occurrence and that RHR pump B was unlikely to also fail due to this mechanism.
Initially, it was not known if the Unit 1 RHR pumps had a similar condition of improperly staked capscrews. However, the following facts were known:
■ As described above, the unlikeliness of this occurrence was reflected in the hundreds of pump service years by this type of pump in the nuclear industry, with no similar events of a capscrew jamming an impeller.
- Per discussion with the vendor, a loose capscrew was more likely to be discharged from an operating pump rather than be caught in the impeller, as occurred here.
- A methodology was developed to ensure the Unit 1 RHR pumps remained operable following each run to ensure that no capscrews jammed the impellers.
These facts provided assurance that the Train B Unit 1 pump would continue to function until it was inspected and repaired during the Fall 2003 refueling outage.
Based on these considerations, there was no adverse effect on plant safety or on the health and safety of the public as a result of this event.
This event represents a safety system functional failure.
F. CORRECTIVE ACTIONS
1) The capscrews on the back casing rings for both Unit 2 RHR pumps were properly staked prior to the pumps being returned to service and the Unit returning to Mode 6.
-2) The supplierof.the spare back casinvings in the warehouse was advised otthe defect. The casing rings themselves were returned to the vendor for reworking.
3) A methodology was developed by January 20, 2003, to ensure the Unit I RHR pumps remained operable after each pump run to ensure the impellers did not become jammed.
4) Each of the Unit 1 pump's back casing rings were inspected during the Fall 2003 refueling outage.
Inadequately staked capscrews were found on the Train B RHR pump. They were properly staked and the pump returned to service.
G. ADDITIONAL INFORMATION
1) Failed Components:
RHR pump manufactured by Ingersoll-Rand, Model number SX2OWDF.
Back casing ring part number: 6B.
Capscrew: 118C.
2) Previous Similar Events:
There have been no previous similar events in the last two years.
3) Energy Industry Identification System Code:
Residual Heat Removal System - BP Reactor Coolant System - AB