05000440/LER-2009-001
| Perry Nuclear Power Plant, Unit 1 | |
| Event date: | 06-21-2009 |
|---|---|
| Report date: | 08-20-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 45147 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 4402009001R00 - NRC Website | |
Energy Industry Identification System Codes are identified-in the text as pOq.
INTRODUCTION
On June 21, 2009, at 1750 hours0.0203 days <br />0.486 hours <br />0.00289 weeks <br />6.65875e-4 months <br />, with the plant in Mode 1 (i.e., Power Operation) and the reactor operating at 100 percent of rated thermal power (RTP), an automatic Reactor Protection System (RPS) [JC] actuation occurred due to a turbine control valve fast closure activation. A moisture separator reheater (MSR) [S13] high water level signal resulted in a main turbine trip and fast closure of the turbine control valves. At 2058 hours0.0238 days <br />0.572 hours <br />0.0034 weeks <br />7.83069e-4 months <br />, notification was made to the NRC Operations Center (ENS 45147) in accordance with 10 CFR 50.72(b)(2)(iv)(B), as an event or condition that results in actuation of the reactor protection system when the reactor is critical. This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in a manual or automatic actuation of the reactor protection system including reactor scram or reactor trip.
EVENT DESCRIPTION
On June 21, 2009, the plant was in Mode 1, operating at 100 percent RTP. The plant was in a normal electrical line-up with all three emergency diesel generators operable and available at the time of the event. There were no testing activities, reactivity manipulations, or plant evolutions in progress. Emergency core cooling systems were operable.
At 1750 hours0.0203 days <br />0.486 hours <br />0.00289 weeks <br />6.65875e-4 months <br /> on June 21, 2009, an automatic reactor protection system actuation occurred due to a turbine control valve fast closure activation of the RPS. An invalid high water level signal from MSR 1B was received which caused a main turbine trip and fast closure of the turbine control valves. All control rods fully inserted. No emergency core cooling systems initiated or were utilized to respond to the event. There were no other reportable actuations. At 1755 hours0.0203 days <br />0.488 hours <br />0.0029 weeks <br />6.677775e-4 months <br />, the motor feed pump was started for level control and maintained reactor water level within the normal operating band. The reactor scram was reset at 1803 hours0.0209 days <br />0.501 hours <br />0.00298 weeks <br />6.860415e-4 months <br />. Off-normal operating instructions for turbine trip and reactor scram were exited at 0140 hours0.00162 days <br />0.0389 hours <br />2.314815e-4 weeks <br />5.327e-5 months <br /> on June 22, 2009.
The plant remained in Mode 3 (i.e., Hot Shutdown) while repairs were made. At 2302 hours0.0266 days <br />0.639 hours <br />0.00381 weeks <br />8.75911e-4 months <br /> on June 23, 2009, operators placed the mode switch in Startup/Standby to enter Mode 2 (i.e., Startup).
CAUSE OF EVENT
The turbine trip/reactor scram was the result of three upper micro-switches of the Magnetrol level switch assemblies on MSR 1B closing to energize the MSR high level trip relays in the turbine trip logic.
The micro-switches on the MSR 1B Magnetrol level switch assemblies were replaced in March 2009 during refueling outage 12 (1R12). Following replacement, adjustment of the micro-switch mounting assemblies/linkages was not performed. This placed the MSR level switches in a vulnerable condition such that vibration and temperature changes experienced during plant operation were sufficient to actuate the micro-switches on MSR 1B and complete the turbine trip logic. The resultant turbine trip initiated the RPS logic which caused the reactor scram. An actual high water level condition in MSR 1B did not exist.
The root cause for not adjusting the MSR 1B level instrument micro-switches was that industry operating experience (OE) (0E19872) for improper linkage adjustment of MSR level switches was not evaluated for applicability to Perry. As a result, potential key learnings were not incorporated into plant procedures. In this case, key manufacturer's instruction manual recommendations were not included in the instrument calibration instruction or the 1R12 preventive maintenance work order for performing MSR level switch calibration checks.
The following causes contributed to the event:
- The step sequence specified in the work order performing the switch calibration checks was not followed. No As-Found calibration of the Magnetrol level switches was performed; the Instrumentation and Control (I&C) technicians proceeded to replace the micro-switches. If the intended sequence had been followed, the l&C technicians would have referred to the vendor manual that describes the set-up and adjustment of the micro-switches when replaced. The technicians did not obtain documented permission to skip the As-Found calibration. This skipped step in the process prevented early identification of improper switch adjustments. The manufacturer's instruction manual was not referenced when the micro-switches were replaced.
- Issues associated with maintenance of MSR level switches were not entered in the corrective action process (CAP). These issues included unacceptable micro-switch as-found conditions and micro-switch replacements. As a result, past maintenance experience was not available for trending or use in 1R12 work planning.
- Non-safety plant equipment was not considered as significant by the workers.
EVENT ANALYSIS
The moisture separator reheaters (MSRs) are large cylindrical pressure vessels designed to improve the quality and increase the internal energy of the high pressure turbine exhaust steam.
The MSRs increase steam efficiency through the low pressure turbine. Water level in the MSR tanks is controlled to prevent moisture carryover which could damage the turbine. Water in the MSRs is routed to individual drain tanks.
Perry has four MSR tanks identified as 1A, 1B, 2A, and 2B. Each MSR tank has three Magnetrol Model 402 level switches installed on exterior instrument level trees to provide a turbine trip signal, on MSR high water level. Each level switch has two (upper and lower) M-1 type mirco-switches.
Initial troubleshooting subsequent to the scram determined that the upper micro-switch contacts on the three MSR 1B Magnetol level switch mechanisms were closed, making up the main turbine trip logic from MSR 1B. The lower contacts were found open as expected for normal MSR water level.
Prior to the scram, MSR drain tank level control was in normal band and steady. No MSR 1B level alarms were received to indicate a high MSR water level. The first indication of a problem was the main turbine trip. A review of other level indications associated with MSR 1B did not indicate a high water level condition or any changes in MSR 1B Drain Tank level control parameters. The MSR 1B drain tank parameters were consistent with the 1A, 2A and 2B drain tank parameters.
The reactor scram was uncomplicated in that all control rods fully inserted, reactor coolant pressure and level were maintained within expected parameters, and no emergency systems actuated in response. The RPS functioned as designed.
The scram event, including plant response, is bounded by the Turbine Trip transient evaluated in the Updated Safety Analysis Report (USAR) Chapter 15, Accident Analysis, section 15.2.3. No plant parameters due to the scram challenged the transients described in the USAR. This transient is categorized as an incident of moderate frequency.
A probabilistic risk assessment (PRA) was performed for the scram event. The PRA calculated a change in core damage frequency (delta CDF) in this case to be 1.7E-07. Since Perry's Large.
Early Release Frequency (LERF) is an order magnitude less than the core damage frequency, the corresponding change in LERF will be on the order of 1.0E-08. Plant configurations with changes in CDF of less than 1.0E-06 and LERF of less than 1.0E-07 are not considered to be significant risk events.
Based on the above information, this event is considered to be of low safety significance.
CORRECTIVE ACTIONS
The upper micro-switch mounting assemblies and linkages for the three Magnetrol level switch assemblies on MSR 1B were adjusted to ensure the micro-switch contacts were opening and closing properly and that the switch travel was optimized. The switches were then calibrated satisfactorily. The Magnetrol level switch assemblies on MSRs 1A, 2A, 2B were checked for proper alignment and calibration. Micro-switch trip and reset verifications were performed to confirm proper operation prior to startup; adjustments to the switches were made as necessary. A monitoring plan to check the MSR high level turbine trip contacts during the current operating cycle was established.
The preventative maintenance (PM) tasks and component templates for MSR 1A/B and 2A/B level instruments will be revised to include applicable good practices (such as verifying the "As-Left" contact positions are open) determined from the troubleshooting and maintenance work orders implemented in response to this event. The PM tasks will include the applicable good practices from the monitoring plan orders performed to check the integrity of the MSR logic switch positions.
Instruction ICI-B1-0, Generic Level Instrumentation Calibration (B1-0), will be revised to include manufacturer's instructions for micro-switch replacement and adjustment. The calibration section of the Magnetrol vendor manual will be revised to include the level switch adjustment information from the level switch replacement section.
A lessons learned document will be created, for training purposes, that aligns the weaknesses identified in the evaluation of industry OE for improper level switch adjustments and controller settings.
The following lessons learned frOm this event will be reviewed with l&C personnel:
- Refer to technical manuals and submit procedure change requests as necessary to ensure calibration instructions contain the necessary information.
- Importance of collecting "As-Found" data.
- Importance of generating condition reports to capture failed components requiring replacement to provide a data stream for trending.
- Procedure use and adherence for performing order operations out of sequence and the possibility of skipping a step.
Work orders were created to perform calibration checks or functional checks on similar applications in the plant of Magnetrol level switches having the M-1 type micro-switch assemblies.
PREVIOUS SIMILAR EVENTS
A review of LERs and the corrective action program database for the past three years did not identify any previous similar events or condition reports relevant to MSR failure mechanisms which defined this event. Corrective actions for the following RPS actuations were reviewed. They would not reasonably be expected to have prevented the MSR scram.
COMMITMENTS
There are no regulatory commitments contained in this report. Actions described in this document represent intended or planned actions, are described for the NRC's information, and are not regulatory commitments.