05000277/LER-2010-002

LER-2010-002, Improperly Fastened Rod Hanger Results in Inoperable Subsystem of Emergency Service Water

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2772010002R00 - NRC Website

Unit Conditions at Time of Discovery At the time of discovery of the condition, PBAPS Unit 2 was shut down for its 18th refueling outage (Mode 5 at 0% rated thermal power.) Unit 3 was operating in Mode 1 at 100% rated thermal power.

There were no other structures, systems or components out of service that contributed to the event.

Description of the Event

On 9/22/10, Engineering personnel determined that a rod hanger (33HB-S143) supporting the discharge pipe of the 'A' Emergency Service (ESW) Pump had not been carrying adequate pipe load prior to recent upgrades of the ESW discharge pipe support system performed during the week of 9/13/10. The degraded condition of the rod hanger was initially identified on 9/18/10 during the performance of planned Inservice Inspection (ISI) activities. On 9/18/10, the ISI inspector identified concerns with the rod hanger 33HB-5143 clamp fasteners. When investigated further by Engineering, it was determined that the rod hanger was not carrying the pipe load contrary to the requirements of the associated design drawings.

The as-found condition on 9/18/10 did not cause the 'A' ESW subsystem to be inoperable due to recent unrelated upgrades on the 'A' ESW subsystem supports installed during the week of 9/13/10. However, Engineering determined that prior to the upgrades performed during the week of 9/13/10, the 'A' ESW subsystem rod hanger condition would have been unacceptable due to its degraded seismic capability.

Furthermore, Engineering determined that the degraded condition of the rod hanger was due to a loose nut at the top of the rod hanger where it is fastened to a nearby structural steel member. It was identified that a jam nut (or other staking method) was not installed during initial plant construction. A jam nut would have assured no movement of the load bearing nut on the threaded rod hanger.

This report is being submitted pursuant to:

10CFR 50.73(a)(2)(i)(B) — Condition Prohibited by TS — This event is reportable under this criterion since an 'A' ESW pump discharge piping rod hanger was unacceptable for loading conditions during a seismic condition. Technical Specification (TS) LCO 3.7.2, Emergency Service Water (ESW) System and Normal Heat Sink requires that two ESW subsystems be OPERABLE. If one ESW subsystem is inoperable, then the subsystem must be returned to an OPERABLE status within 7 days. Since the condition was due to inadequate installation during original plant construction, it is assumed that the fastener deficiency has existed for greater than the 7-day completion time for the TS 3.7.2 Required Action associated with one ESW subsystem being inoperable.

The top attachment nut for rod hanger 33HB-5143 was tightened and staked on 9/28/10.

Analysis of the Event

The ESW system (EIIS: CC) is a standby system which is shared between Units 2 and 3. It is designed to provide cooling water for the removal of heat from equipment, such as Emergency Diesel Generators (EDGs) and room coolers for Emergency Core Cooling System equipment, required for a safe reactor shutdown following a Design Basis Accident or transient. Upon receipt of a loss of offsite power signal, or whenever any EDG is in operation, the ESW system will provide cooling water to its required loads.

The ESW system consists of two redundant subsystems.

The affected rod hanger (33HB-5143) supports the discharge pipe (EllS: P) of the 'A' ESW pump downstream of the discharge check valve. The rod hanger consists of a threaded rod fastened at its top to a nearby structural steel member and at the bottom to the circular pipe clamp. Rod hanger 33HB- S143, as well as adjacent restraints, carry piping system deadweight to limit pipe stress to within code allowables including nozzle and anchor loads at the 'A' ESW pump.

There were no actual safety consequences associated with this event. The degraded condition of rod hanger 33HB-5143 (EllS: H) would have only affected the 'A' ESW subsystem during postulated seismic conditions. This condition was determined to not affect the 'B' ESW subsystem. There have been no actual adverse affects as a result of seismic events at PBAPS. If a design basis seismic event had occurred, the rod hanger deficiency could have resulted in overstressing of the 'A' ESW pump (EllS: P) anchor bolts. Therefore, if a design basis seismic event were to occur, continued operability of the 'A' ESW subsystem could not be assured during the design basis seismic event. For other design events (non-seismic related), the adverse condition of the rod hanger would not have significantly affected the ability of the 'A' ESW subsystem to perform its safety function.

Cause of the Event

The original (construction) installation of the rod hanger was deemed to be inadequate since jam nuts were not installed to prevent the loosening of the top fastener which connects the rod hanger to a nearby structural steel member. It is believed that over time the top fastener on the rod hanger loosened since no jam nut was installed. This resulted in the rod hanger no longer carrying the required pipe load.

The underlying cause was determined to be due to an inadequate design drawing. The original construction design drawing did not properly specify that the load bearing members under tension were to be staked, pinned or double-nutted.

Corrective Actions

The top attachment nut for rod hanger 33HB-S143 was tightened and staked on 9/28/10. The drawing associated with this hanger will be upgraded to clearly require staking / jam nut requirements.

A sample of similar hanger drawings were reviewed for staking requirements. Systems reviewed included Residual Heat Removal, High Pressure Coolant Injection, Main Steam, ESW, and High Pressure Service Water. These drawings were found to require jam nuts, tack welds or staking of pipe clamps, lugs and load bearing threaded rod connections such as turnbuckles, weldless eye nuts, clevises, etc. The hanger 33HB-S143 drawing was the only drawing identified that did not include specific requirements for staking the load bearing threaded rod connections.

Other actions are being pursued in accordance with the site Corrective Action

• Program.

Previous Similar Occurrences Previous concerns with the similar hanger on the 'B' ESW subsystem was identified in 2007 and 2009.

For these occurrences, it was determined that the hanger was carrying load and no stress analysis was required or performed. At those times, there were no additional walkdowns performed to identify similar existing conditions.