05000389/LER-2007-002

LER-2007-002, 2B2 Reactor Coolant Pump (RCP) Seal Housing Leakage

Docket Number
Event date:	12-21-2007
Report date:	02-18-2008
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
3892007002R00 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

Description of the Event

On December 21, 2007, while performing unrelated maintenance activities on the 2B2 reactor coolant pump (RCP) [EIIS:AB:P], the 2B2 RCP was observed to be leaking at the seal injection line. Seal injection was not in service. This was determined to be pressure boundary leakage and a short notice outage (SNO) was entered to repair the leak Corrective actions taken to date include replacement of the seal package containing the degraded weld, preliminary destructive metallurgical examination of the weld, and removal of the piping cold-spring. The preliminary metallurgical analysis confirmed the failure mechanism to be outside diameter-initiated fatigue.

Cause of the Event

Visual and liquid penetrant examinations indicated the presence of a linear indication at the pipe side toe of the weld, which is indicative of outside diameter­ initiated fatigue. Preliminary results of the destructive metallurgical examination confirmed this mechanism. Results of completed analysis, for piping vibration sensitivity to pump excitation, show these lines meet ASME III requirements. The presence of the piping cold-spring is being investigated as a contributor to the failure.

The review of St. Lucie and industry operating experience indicates that small bore socket weld failures in primary systems have been a persistent and continuing challenge. The predominant failure mechanism throughout the industry is high cycle fatigue with fractures originating from both the inside diameter at the root of the weld and the outside diameter at the toe of the weld.

The fatigue failures have been caused by vibration of the piping system for which the driving force is machinery-induced vibration and/or hydraulically-induced vibration.

Mitigating strategies have involved replacing the susceptible socket welds with 2:1 taper socket welds or with butt welds. These welds are less susceptible to failure from vibration than a standard socket weld but will not prevent eventual failure in all cases. The failed weld was a j-groove weld with a 1:1 fillet cap.

Upon completion of forensic metallurgical testing of the failed pipe,'a root cause evaluation will be completed and results communicated in a revision to this LER.

Analysis of the Event

This event is reportable under 10 CFR 50.73(a)(2)(ii)(A) any event or condition of the nuclear power plant, including its principal safety barriers, being seriously degraded.

Analysis of Safety Significance Seal injection is not used in the St. Lucie RCP design except for refueling outage filling operations. At the time of the discovery, seal injection had been isolated from the charging system. The RCP leak was discovered during maintenance in Mode 3.

If the leak had not been discovered at that time, normal RCS leak rate monitoring would have discovered the increased leakage long before a significant degradation of the piping had occurred. Experience has demonstrated that with this weld failure FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) mechanism, there is ample time from the discovery of increased RCS leakage to allow a controlled shutdown to cold shutdown conditions without challenging the emergency core cooling system (ECCS) or small break loss-of-coolant accident analysis (SBLOCA).

Although stainless steel components are not susceptible to boric acid corrosion, the spray from the cracked weld deposited wet boric acid onto the external surfaces of carbon and low alloy steel components, including pressure-retaining bolting for a short period of time prior to identification and isolation of the leakage. The leakage was contained within the 2B2 pump shroud. The boric acid was subsequently cleaned from the components in the area. The inspection results of the affected area revealed no material degradation or wastage, and as such, boric acid corrosion of components due to this leak is not a concern.

Based on the above, the safety consequences of the event are judged to be low and there was no adverse impact on the health and safety of the public.

Corrective Actions

The corrective actions taken to date include replacement of the seal package containing the degraded weld, preliminary destructive metallurgical examination of the weld, and removal of the piping cold-spring. Extent-of-condition inspections were performed including liquid penetrant tests on all other Unit 2 RCP seal package nozzle connections (upper, middle, lower, controlled bleed-off, and vapor seal leak­ off) and verifying there was no cold-spring present on any of the other lower seal cavity lines on the other three RCPs.

Similar Events 1.LER 335-87014, PSL Unit 1, 10-8-87, cracked weld at lower cavity seal nozzle flange. Cracked weld was attributed to a combination of relatively high RCP vibration and flange misalignment between the nozzle and injection piping.

2.LER 2005-001, PSL Unit 2, 04-8-05, documents the failure of a 3/4-inch socket weld on the 2B1 safety injection piping. This failure was due to a defective weld along with high cycle fatigue.

3.LER 2007-001, PSL Unit 2, 10-16-07, was a cracked socket weld on the 3/4-inch Class 1 2B1 Reactor Coolant Pump (RCP) seal injection piping. The cause for the socket weld failure was low stress high cycle fatigue resulting from RCP vibration and the susceptibility of socket welds to high cycle fatigue failure.

Failed Components

3/4-inch Schedule 160 piping at weld into the RCP seal cartridge