05000529/LER-2014-002
| Palo Verde Nuclear Generating Station (Pvngs) Unit 2 | |
| Event date: | 11-6-2014 |
|---|---|
| Report date: | 04-21-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 50600 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 5292014002R01 - NRC Website | |
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All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This Licensee Event Report is being submitted pursuant to 10 CFR 50.73 (a)(2)(i)(A) to report the completion of a plant shutdown required by plant Technical Specification (TS) Limiting Condition for Operation (LCO) 3.1.5, Control Element Assembly (CEA) Alignment, resulted when CEA 15 dropped fully into the core and could not be realigned with the other CEAs of its group within the time allowed by LCO 3.1.5, Condition A. The plant shutdown was completed within the time requirement of LCO 3.1.5, Condition C.
The initiation of the plant shutdown required by plant TSs was reported to the Nuclear Regulatory Commission (NRC) pursuant to 10 CFR 50.72 (b)(2)(i) on November 6, 2014, via the event notification system (EN 50600).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
Each PVNGS reactor has 89 CEAs arranged in nine radially symmetric groups which include two shutdown groups, five regulating groups and two part strength groups. The shutdown and regulating CEAs provide the required reactivity worth for immediate reactor shutdown upon a reactor trip. The regulating CEAs also provide reactivity control during normal operation and transients. The part strength CEAs are used for control of axial power distribution and are not credited for shutting down the reactor. Full strength CEAs are installed in either 12-finger or 4-finger configurations. The dropped CEA 15 described in this report is a full strength 4-finger CEA in regulating group 5.
Each CEA is moved and held by its associated control element drive mechanism (CEDM) (EIIS: AA). The CEDM is an electromechanical device which uses induced magnetic fields, through lift and gripper electrical coils, to operate magnetic jacks to move, hold, and release the associated CEA. The CEDM coils are arranged in a "coil stack" in the CEDM housing installed on the reactor vessel head. Each CEDM is surrounded by a sheet metal cooling shroud producing an annulus through which air can flow. The air flow through the cooling shroud serves to remove heat from the CEDM.
Two motor generator sets are connected in parallel to a common bus. This common bus supplies 240 volt AC power through reactor trip switchgear (RTSG) (ENS: AA) to the CEDMCS. The output from the RTSG is directed through 23 power switch assemblies. The power switch assemblies contain silicon controlled rectifiers which convert the 3 phase AC input voltage to a stepped DC output voltage to the CEDM coils of all 89 CEAs. One power switch assembly provides power to operate the CEDM coils for four CEAS, with the exception of the power switch assembly for CEA 1 which powers only the coils for CEA 1.
The operation of each CEDM coil is controlled by electronic logic circuits which determine the sequence to supply power to the coils. Under normal operating conditions, while a CEA is not in motion, the CEA is held by the upper gripper coil which is continuously energized at a low voltage level. The reactor protection system (RPS) (EIIS: JC) provides for rapid and reliable shutdown of the reactor to protect the core and the reactor coolant system pressure boundary from potentially hazardous operating conditions. Shutdown is accomplished by either manual or automatic generation of reactor trip signals which open the RTSG breakers and de-energize the CEDM coils, releasing the spring loaded magnetic jacks, allowing all CEAs to be inserted into the core by gravity.
3. INITIAL PLANT CONDITIONS:
On November 6, 2014, PVNGS Unit 2 was in Mode 1 (Power Operation), at 100 percent power, normal operating temperature, and normal operating pressure. There were no other structures, systems, or components inoperable at the time of the event that contributed to the event.
4. EVENT DESCRIPTION:
At 1116 on November 6, 2014, the Unit 2 control room received alarms and indications that CEA 15 of regulating group 5 had dropped from the fully withdrawn position to the fully inserted position. The 88 other CEAs were not affected and remained fully withdrawn.
Operations personnel entered the CEDMCS malfunction abnormal operating procedure and entered the applicable Conditions for the following LCOs:
- 3.1.5 - Control Element Assembly (CEA) Alignment
- 3.1.7 - Regulating Control Element Assembly (CEA) Insertion Limits
- 3.2.1 - Linear Heat Rate (LHR)
- 3.2.3 - Azimuthal Power Tilt (Tq)
- 3.2.4 - Departure From Nucleate Boiling Ratio (DNBR) TS LCO 3.1.5, Condition A, for one CEA misaligned from its group requires a plant power reduction and restoration of CEA alignment. The TS required power reduction was initiated at 1121 and completed at 1158. In parallel with the power reduction, plant personnel began efforts to diagnose and correct the CEDMCS malfunction.
Following initial troubleshooting and repair attempts it was concluded that CEA 15 could not be repaired and realigned with the other regulating group 5 CEAs within the 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> time limit specified in TS LCO 3.1.5, Condition A. At 1316, TS LCO 3.1.5 Condition C was entered which requires entry into Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Operations personnel continued the power reduction and performed a reactor shutdown in accordance with normal operating procedures by manually tripping the reactor at 1636 from 22 percent power. Following the plant shutdown, Operations personnel verified normal plant response and Unit 2 was stabilized in Mode 3 at normal operating temperature and pressure with all CEAs fully inserted and no actuations of plant engineered safety features systems. Additionally, all TS LCOs associated with the full insertion of CEA 15 were exited.
An investigation was initiated to determine and correct the cause of the dropped CEA.
Troubleshooting activities performed in Mode 3 isolated the malfunction to the CEDM coil stack for CEA 15 on the reactor vessel head. A cooldown of the reactor coolant system was necessary to support the maintenance activities and Unit 2 entered Mode 5 at 1859 on November 7, 2014.
The faulty upper gripper CEDM coil on CEA 15 was replaced. Following completion of maintenance and retest activities, Unit 2 was restarted and entered Mode 1 at 0332 on November 13, 2014.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
This event did not result in a potential transient more severe than those analyzed in the Updated Final Safety Analysis Report or result in the release of radioactive materials to the environment. There were no actual safety consequences as a result of this event and the event did not adversely affect the health and safety of the public.
The power reductions and subsequent plant shutdown performed for this event were conducted within the time limitations and power restrictions specified in the TSs, Core Operating Limits Report and abnormal operating procedures so that specified acceptable fuel design limits were not exceeded. The risk significance of this event was consistent with the probabilistic risk analysis for an uncomplicated reactor trip which has a conditional probability of core damage of 1.2E-7. This risk value is small in comparison to other postulated initiating events.
The condition would not have prevented the fulfillment of a safety function and the condition did not result in a safety system functional failure as defined by 10 CFR 50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
The direct cause of the event was a failed upper gripper coil on the CEDM for CEA 15 which resulted in unlatching and full insertion of the CEA.
Root cause analysis determined that operation at elevated temperatures resulted in accelerated thermal degradation of the upper gripper coil insulation and internal shorting of the coil.
7. CORRECTIVE ACTIONS:
As an immediate corrective action, the failed upper gripper coil (Combustion Engineering, Model R5000) was replaced on November 8, 2014.
To prevent recurrence, a preventive maintenance strategy with the following attributes was implemented:
- Establish an initial coil voltage setpoint band,
- Monitor online CEDM coil voltage and current,
- Adjust online voltage as directed by engineering review.
8. PREVIOUS SIMILAR EVENTS:
In the previous three years, similar events related to malfunctions of CEDMCS equipment that resulted in a plant shutdown were reported in LERs 50-528/2011-004, 50-528/2011-005 and 50-530/2012-001. The corrective actions from these events would not have prevented this event. This event was due to operation at elevated temperatures resulting in accelerated thermal degradation of the upper gripper coil insulation and internal shorting of the coil.
due to a loose terminal lug on the CEA power switch assembly.
subgroup that slipped during post-refueling low power physics testing due to an intermittent failure.
CEA 57 when it stopped moving during low power physics testing.