05000498/LER-2009-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982009002R01 - NRC Website | |
I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). South Texas Project (STP) Technical Specification 3.7.1.5 allows one Main Steam Isolation Valve (MSIV) to be inoperable but open in Modes 1 through 3 for four hours before taking action to begin shutdown without extending the allowed outage time using the Configuration Risk Management Program. However, STP Unit 1 MSIV 1D was determined to have been inoperable longer than the allowed outage time without taking action as required.
Consequently, STP Unit 1 was in a condition prohibited by Technical Specifications.
This event is also reportable pursuant to 10 CFR 50.73(a)(2)(v)(C) as a safety system functional failure because MSIV 1D would not have been able to fully isolate a faulted steam generator to mitigate a radioactive release.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
STP Unit 1 was in Mode 1 at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE
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 September 14, 2009, construction of a wooden deck work platform was begun around MSIV 1D to support implementation of a design change. Construction was completed on September 16. The deck was below the actuator flange and on all four sides of MSIV 1D, a position that would have prevented MSIV 1D from performing its design function of closing on demand. This condition was observed by a Reactor Operator performing his rounds on September 17, 2009, and reported to Engineering and Operations management.
Subsequent measurement confirmed that interference between the wooden deck work platform and the valve actuator would prevent valve closure.
Following confirmation of the interference, action was initiated to dismantle the platform. The platform was removed at 14:57 on September 17 and MSIV 1D was restored to operable status. The estimated duration of MSIV 1D inoperability is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
Because MSIV 1D was inoperable longer than the Technical Specification allowed outage time, the condition is reportable to the NRC under 10 CFR 50.73(a)(2)(i)(B).
MSIV 1D would also have not been able to mitigate a radioactive release from a faulted steam generator; therefore, the condition is also reportable to the NRC under 10 CFR 50.73(a)(2)(v)(C) as a safety system functional failure.
All scaffolds in STP Unit 1 and Unit 2 were subsequently inspected, and no other instance of interference was found.
E. METHOD OF DISCOVERY
This condition was identified by a Reactor Operator making his normal rounds of the Isolation Valve Cubicle.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
No safety systems were required to respond during this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Installation of the subject scaffolding began at 1500 on September 14, 2009, and completed September 16 at approximately 1700. The interference with valve movement was discovered September 17, 2009. Operability of MSIV 1D was restored when the interference was removed at 14:57 on September 17. The estimated duration of inoperability is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
Technical Specification Requirements:
Technical Specification 3.7.1.5 requires each MSIV to be operable in Modes 1, 2, and 3.
With one MSIV inoperable but open, power operation may continue provided that the inoperable valve is restored to operable status within four hours or the requirements of Configuration Risk Management Program are met. Otherwise, the plant is to be in Hot Standby within the next six hours and in Hot Shutdown within the following six hours.
Design Description:
Main steam isolation valves only provide a safety function and are not required for power operation. These valves are normally open to allow steam flow through the main steam system. They are designed to fail closed to stop forward and reverse steam flow. Valve closure provides containment isolation or to prevent blowdown of more than one steam generator at a time. Isolation is required in response to the following:
- Main steamline break inside containment
- Break outside containment and upstream of the MSIV
- Steam generator tube rupture MSIV 1D is a 30-inch, ASME Class 2, Wye pattern globe valve actuated by an Air-to Open/Spring-to-Close actuator. The valve strokes from the fully back-seated position to the fully main-seated position in no more than 5 seconds.
Risk Assessment:
This condition contributed a small change in core damage risk and large early release risk based on the guidance contained in NRC Inspection Manual Chapters 0609 and 0612.
Incremental Condition Core Damage Probability is less than 1E-6 per year, and Large Early Release Frequency is less than 1E-7 per year. Therefore, inability to fully close the MSIV during this interval had very low safety significance.
III. CAUSE OF THE EVENT
The root cause of the event was an inadequate procedure for scaffold installation. As exhibited below, the procedure did not ensure an adequate review of the impact of the scaffold installation on the operation of the plant.
4
- The non-standard scaffolding and decking installation process did not require input from Engineering or Operations to identify and document limitations, restrictions, requirements, or considerations near or around equipment important to plant operations. Additionally, no input was required by these departments to periodically verify that previously approved non-standard scaffold and/or decking sketches are still valid.
- The procedure for erection and use of temporary scaffolding did not contain requirements or attributes with the depth and rigor needed for non-standard scaffold and/or decking installation in areas containing plant equipment important to plant operation. Such work warrants written guidance to ensure adequate job quality and work control.
- The pre-job brief was not required to address application of scaffold installation limitations and restrictions consistent with functional requirements of adjacent equipment.
- The scaffold permit process lacked guidance when addressing special applications such as decks and platforms. The guidance should include restrictions on use while at power, limitations for adjacent equipment operation, and cautions for various plant operational modes.
IV. CORRECTIVE ACTION.
Procedures to establish the process and requirements, including pre-job briefs and the permitting process, for installation of scaffolding and other structures in critical areas of the plant have been revised.
V. PREVIOUS SIMILAR EVENTS
There have been no recent events at the South Texas Project applicable to this obstruction by scaffolding.
Similar scaffolding-related events have occurred elsewhere, as documented in recent operating experience reports:
- 0E23556: Main Steam Isolation Valve - Scaffold Interference [Beaver Valley Unit 2 — 10/01/2006]
- 0E25594: Scaffold construction caused inoperable Standby Gas Treatment System Damper [Peach Bottom Unit 3 — 08/17/2007]
- 0E20423: Scaffold poles struck by Main Steam Isolation Valves when valves are stroked [Perry Unit 1 — 03/02/2005]
VI. ADDITIONAL INFORMATION
None.