05000440/LER-2006-005

LER-2006-005, Decreasing Instrument Air Pressure Results in Manual Reactor Protection System Actuation

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 43049	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation
4402006005R00 - NRC Website

INTRODUCTION

On December 13, 2006, at approximately 0435 hours0.00503 days <br />0.121 hours <br />7.19246e-4 weeks <br />1.655175e-4 months <br />, a Reactor Protection System (RPS) [JC] actuation was manually initiated in response to decreasing Reactor Pressure Vessel (RPV) coolant level and no automatic Emergency Core Cooling Systems (ECCS) response occurred. At the time of the event, the plant was in Mode 1 (i.e., Power Operation) with the reactor operating at 100 percent of rated thermal power, and all ECCS's were operable. At 0549 hours0.00635 days <br />0.153 hours <br />9.077381e-4 weeks <br />2.088945e-4 months <br />, notification was made to the NRC operations Center (ENS Number 43049), in accordance with 10 CFR 50.72(b)(2)(iv)(B) as an event or condition that results in actuation of the RPS when the reactor is critical.

Thisevent is being reported in accordance with 10 CFR 50.73(a)(iv)(A) as an event or condition that resulted in manual or automatic actuation of the RPS.

EVENT DESCRIPTION

On December 13, 2006, at approximately 0428 hours0.00495 days <br />0.119 hours <br />7.07672e-4 weeks <br />1.62854e-4 months <br />, control room operators entered an off normal instruction due to decreasing non-safety Instrument Air System [LD] pressure. The decreasing air pressure caused the Condensate System [SD] minimum flow recirculation valve to fail open as designed. This decreased the level in the hot surge tank causing a transient in the Reactor Feedwater System [SJ] which resulted in lowering RPV level. The operating crew, while responding to the feed water transient, investigated the source of the air pressure decrease and subsequently isolated the air leak at 0433 hours0.00501 days <br />0.12 hours <br />7.159392e-4 weeks <br />1.647565e-4 months <br />. However, instrument air pressure did not recover quickly enough to close the minimum flow recirculation valve and RPV level continued to decrease. Prior to the RPV level decreasing to its low alarm set point, the operating crew inserted a manual RPS Actuation at 0435 hours0.00503 days <br />0.121 hours <br />7.19246e-4 weeks <br />1.655175e-4 months <br />. The reactor was shut down and RPV coolant level was maintained such that no emergency core cooling systems were needed to maintain level. The plant entered Mode 3 (i.e., Hot Shutdown) at 0436 hours0.00505 days <br />0.121 hours <br />7.208995e-4 weeks <br />1.65898e-4 months <br />.

One control rod, 42-55, [AA] did not initially display a full in position indication on the initial SCRAM. At 0436 hours0.00505 days <br />0.121 hours <br />7.208995e-4 weeks <br />1.65898e-4 months <br />, operators initiated Alternate Rod Insertion (ARI) [AA] as required by plant procedures and confirmed all rods had full in indication. The Shift Manager subsequently made an Event Notification to the NRC for manual RPS actuation and stated that control rod 42-55 did not insert fully on the initial-- SCRAM. Further review by the plant staff determined that the control rod had fully inserted and only a position indication malfunction had occurred. All control rods inserted within their required scram times. The NRC operations center was updated with this information at 0814 hours0.00942 days <br />0.226 hours <br />0.00135 weeks <br />3.09727e-4 months <br /> Other Plant Response On December 13, 2006, at approximately 0617 hours0.00714 days <br />0.171 hours <br />0.00102 weeks <br />2.347685e-4 months <br />, the Condensate Minimum Recirculation Flow Valve failed closed due to a broken spring in the valve's positioner, resulting in a loss of minimum flow to the Condeneate System. This caused elevated temperatures in the Off Gas System [WF]. Elevated temperattires in the off gas charcoal absorbers were identified when temperatures in the two inlets to the charcoal beds were at approximately 650 Degree's Fahrenheit and increasing. The charcoal beds were subsequently piurged with nitrogen to reduce the amount of oxygen and decrease charcoal bed temperatures to acceptable levels.

On December 17, 2006, at 0027 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> the plant entered Mode 4 (i.e., Cold Shutdown). On December 18, 2006, at 1036 hours0.012 days <br />0.288 hours <br />0.00171 weeks <br />3.94198e-4 months <br />, the plant entered Mode 2 (i.e., Startup). On December 19, 2006, at 1715 hours0.0198 days <br />0.476 hours <br />0.00284 weeks <br />6.525575e-4 months <br />, the plant entered Mode 1 and the generator was subsequently synchronized to the grid at 2131 hours0.0247 days <br />0.592 hours <br />0.00352 weeks <br />8.108455e-4 months <br />.

CAUSE OF EVENT

The cause of the event is attributed to decreasing Instrument Air System pressure from a 45 degree elbow joint separation on a 2 inch copper tubing instrument air header. This resulted in a feedwater system transient and manual. actuation of the RPS due to the decreasing RPV coolant level.

The cause of the air header joint separation was determined to be less than adequate workmanship during plant construction in the assembly of the joint. Analysis of the joint determined that the tube was inadequately inserted into the socket (i.e., the tubing was not inserted far enough into the socket when the joint was soldered). The maintenance history of this section of piping was reviewed to determine if any modifications or reworks had been made to this portion of the system. There had been no work orders on the subject joint. This improperly made-up joint had been in place since plant construction.

Ineffective review and implementation of relevant industry operating experience (OE) was also identified as a root cause. Also, the lack of a program, in the past, to ensure that Modifications and Preventative Maintenance Tasks (PMTs) are implemented as intended was a contributing cause.

EVENT ANALYSIS

The safety significance of this event is considered to be minimal. The operators took conservative action to manually actuate the RPS when it was determined that RPV coolant level control was challenged. The plant safety systems responded as designed. Operator response was appropriate and consistent with operator training. Other plant systems, structures, and components responded as described in the EVENT DESCRIPTION section of this report.

The risk associated with a General Transient has been evaluated and documented in the Probabalistic Risk Assessment (PRA) model and the Updated Safety Analysis Report (USAR). A General Transient is an incident of moderate frequency and does not represent an increase in the core darii4e frequency. Based on the results documented in USAR Chapter 15 (event 7, i.e., manual SCRAM) the risks associated with a manual SCRAM are acceptable. Since the December 13, 2006 SCRAM is bounded by a manual SCRAM, evaluated in the USAR and mitigating functions performed as expected, the event was not risk significant.

CORRECTIVE ACTIONS

The damaged joint was replaced. ; Ultrasonic Testing (UT) was performed on 8% (approximately 345 joints) of the 2" and 3" soldered'copper tubing in the instrument air system throughout the plant to determine the extent of joints assembled with inadequate insertion. The instrument air system's 2" and 3" copper tubes having soldered jOints were targeted because the capacity of the air compressors can adequately make-up the air volume from a leak less than 2" and greater than 3" copper tubing is not used in the plant. This testing resulted in 5 joints being identified that did not meet established insertion criteria. Clamps were placed on these 5 joints to prevent separation. A plan will be developed in order to conduct additional UT inspections of 2" and 3" soldered joints. These inspections will be completed by the end of re-fuel outage 12 in 2009.

Programs and processes are currently in place at Perry which would have effectively reviewed the relevant industry OE. The programs and processes will be verified to ensure that they are being implemented correctly and are effectively reviewing industry OE. Also, the Equipment Reliability process will be strengthened to ensure that issues are implemented as intended and in a timely manner.

The failed Condensate Minimum Recirculation Flow Valve positioner was repaired.

PREVIOUS SIMILAR_EVENTS A historical review of the instrument air system revealed that there was a history of leaking joints in the 2" and 3" tube joints during the early days of plant startup and operations. Portions of the Instrument Air System were modified to prevent joint separation and a repetitive task was initiated to regularly inspect the joints. The task was later canceled. Review of LER's for the past 5 years did not identify any similar previous events.

COMMITMENTS

No regulatory commitments are contained in this report.

Energy Industry Identification System Codes are identified in the text as [XX]