05000528/LER-2003-004
| Docket Number09 16 2003 2003 - 004 - 01 05 25 Ri5■ Pvngs Unit 3 05000530 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5282003004R01 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This LER 528/2003-004-01 is submitted to update a reported condition involving cracking in the contact block for handswitches (EIIS: HS) used primarily in the main control room at each unit.
Specifically, a common cause failure resulted in the inoperability of independent trains of safety related components (10 CFR 50.73 (a) (2) (vii)).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The affected handswitches use Honeywell Micro Switch model PTCC contact blocks, which is also part of the PTC line of contact blocks. These switches are used extensively (2310 total switches) on the main control room boards and other various locations to control components in various systems including safety related systems. The current manufacturer of these contact blocks is Senasys, although the affected blocks were manufactured prior to Senasys taking over the product line.
3. INITIAL PLANT CONDITIONS and EVENT DESCRIPTION:
On March 5, 2003 Units 1, 2 and 3 were operating in Mode 1, Power Operation, at approximately 97, 98 and 99 percent rated thermal power respectively. During a maintenance activity in Unit 3 to replace a broken handswitch knob on a main control room handswitch, electrical maintenance personnel noted that the contact block for the switch had a minor crack.
The contact block was replaced and a condition report disposition request (CRDR) was initiated to further evaluate the cracked block condition.
As part of the CRDR evaluation, work orders (WO) were initiated to inspect additional handswitches. By August 27, 2003 the result of this additional inspection had identified several switches with a crack located on one of the two rivets that attach the back plate to the contact block. An operability determination (OD) was initiated on August 27, which concluded that a switch remained capable of performing its design function, including during a seismic event, with only one crack on one of the rivets. Inspection of other similar switches continued in an effort to further identify the extent of the cracking condition. On August 29, a switch was found with two cracks, one on each rivet. Engineering determined that this condition also did not prevent the handswitch from performing its design function and the OD was revised to include this condition.
Final inspection results have identified that out of a total of 2310 inspections 57 hand switches had cracks that were unacceptable.
There were no inoperable systems at the start of the event that contributed to the event.
4. ASSESSMENT OF SAFETY CONSEQUENCES:
The length of time that the control switches had cracks is not known however, no switch failure caused by the cracking condition has been identified. In the unlikely event that a seismic event were to occur it is possible that an unacceptably cracked switch may not have performed its safety function. The following is a list of degraded switches, by unit, which could have had an adverse impact on plant shutdown following a seismic event.
1JSIAHS0678: control room hand switch for containment spray pump A discharge valve to the shutdown cooling heat exchanger. There is another valve in series with this valve that can be operated from the control room. In addition, SIAHV 0678 could be manually operated.
Redundant 100 per cent train B available.
1JSIAHS0685: control room hand switch for low pressure safety injection pump A cross tie valve to the shutdown heat exchanger. This valve could be manually operated. Redundant 100 per cent train B available.
2JSIBHS0613A: control room hand switch for safety injection tank vent valve. Parallel vent valve is available to vent the tank.
3JSIAHS0607A: control room hand switch for safety injection tank vent valve. Parallel vent valve is available to vent the tank.
3JAFBHS0034A and 35A: control room hand switches for valves providing essential auxiliary feedwater to the steam generators from the motor driven essential auxiliary feedwater pump.
These valves could be manually operated. Redundant 100 per cent capable train A system available.
3JSIDHS0654: control room hand switch for shutdown cooling loop 2 suction line isolation valve. Redundant 100 per cent capable train A system available.
The identified cracking condition would not have prevented the fulfillment of any safety function and did not result in a safety system functional failure as defined by 10CFR50.73(a)(2)(v).
The event has not resulted in any challenges to the fission product barriers or resulted in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this condition.
5. CAUSE OF THE EVENT:
The direct cause of this event was over-torquing the termination screws during initial switch installation in the control board panel. The root cause was attributed to the manufacturing process associated with the contact block which made the block susceptible to cracking.
6. CORRECTIVE ACTIONS:
All unacceptably defective Q-Class hand switch contact blocks have been replaced in all three units. The remaining contact bocks have been scheduled for replacement with 13 in Unit 1. All Unit 2 and Unit 3 switches with cracks have been replaced. In addition, the applicable maintenance instructions have been revised to caution technicians about over-tightening the termination screws and to verify crack free contact blocks when working on the associated hand switches.
7. PREVIOUS SIMILAR EVENTS:
There has been no similar event reported to the NRC by the Palo Verde Nuclear Station in the past three years.
8. ADDITIONAL INFORMATION:
None