05000529/LER-2007-002
| Palo Verde Nuclear Generating Station | |
| Event date: | 02-03-2007 |
|---|---|
| Report date: | 04-04-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 43140 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 5292007002R00 - NRC Website | |
Note: All times listed in this event report are approximate and Mountain Standard Time (MST) unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER (50-529/2007-002-00) is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications (TS). While testing Control Element Assemblies (CEA) as required by Surveillance Requirement (SR) 3.1.5.3, Unit 2 entered Limiting Condition for Operation (LCO) 3.0.3 as a result of more than one CEA below the inches withdrawn limit required by LCO 3.1.6. LCO 3.1.6 is applicable in Mode 1 and 2, but is not applicable when performing Surveillance Requirement (SR) 3.1.5.3. See item 9, "OTHER," at the end of this LER for information on the LCO 3.1.6 limit. Notification of this event was made to the NRC on February 3, 2007 at 19:51 via ENS 43140.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The Control Element Drive Mechanism Control System (CEDMCS) (EIIS Code — AA) provides control signals and motive power to the coils of the magnetic-jacks in the Control Element Drive Mechanisms (CEDMs) (EIIS Code — AA) which position and hold the reactor Control Element Assemblies (CEAs) (EIIS Code — AA).
Two motor/generator sets are connected in parallel to supply 240 VAC, 3 phase power through the Reactor Trip Switchgear (RTSG) (EIIS Code — AA). The output from the RTSG is directed through Power Switch Assemblies. The Power Switch Assemblies contain silicon controlled rectifiers (SCRs) which convert the 3 phase, AC input voltage to a chopped DC output voltage. The conversion is controlled by various electronic circuits in the power switch and in the CEDMCS Subgroup Logic Housing. These control circuits determine when to supply power to the CEDM coils and determine the level of the power applied. The level of applied power is controlled by turning the SCRs on at different points in the AC sine wave.
Some of the control circuits involved with the control of the firing angle of the SCRs are on a circuit card called the phase synchronization card. There are two phase synchronization cards in each subgroup. One card controls the firing angle for each of the three phases for high and low voltage for the upper lift and upper gripper coils for all four CEDMs in a subgroup. The other phase synchronization card controls the firing angles for the lower lift and lower gripper coils for all four CEDMs in a subgroup. The circuits on the phase synchronization card are separated such that the failure of a single circuit would normally affect one of the three phases of the high voltage, or the low voltage, to all of the associated coils.
A pulse counter determines CEA position by counting the signals from the CEDMCS system to the CEA to demand CEA movement. The pulse counter indication does not directly measure CEA position and the counter cannot determine if the CEA has actually moved or not. The only way to determine if the CEA actually moved would be to see a change in CEA position as indicated on the Reed Switch Position Transmitters (RSPTs).
The RSPTs consist of reed switches located every 1.5 inches which are activated by a magnet assembly located at the top of the CEA extension shaft. However, since a CEA moves 0.75" every step, the RSPTs may not see every step of CEA movement. The CEA position can be seen on either the CEAPD (CEA Position Display) screen or a Group Display if called up on the CPCs (Core Protection Calculators) (EIIS Code: JC).
3. INITIAL PLANT CONDITIONS:
On February 3, 2007, Palo Verde Unit 2 was in Mode 1 (power operations), operating at approximately 100 percent power. There were no major structures, systems, or components inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On February 3, 2007, while performing Surveillance Test Procedure 40ST-9SF01 (CEA Operability Checks) on Shutdown Group 'B' CEAs in Unit 2, CEA Subgroup 6 (four CEAs) was inserted five inches to 144 inches withdrawn but failed to withdraw upon demand. The failure to withdraw was not immediately noticed by Operations personnel (utility - licensed).
The operators were monitoring the CEA pulse counter indication, CEA position display indication and CEA withdrawal indicator arrows. These indicators responded as expected for the intended CEA motion. Additionally, there were no deviation alarms (and none were expected) for this failure scenario.
The test procedure had the operator inserting the CEAs 5 steps (3.75 inches) in Manual Group. This moves all the CEAs in the selected group (i.e. Shutdown Group B). Then the operator inserted each individual CEA in that group two more steps (1.5 inches) in Manual Individual to complete the 5 inch move, logged the pulse counter reading, then withdrew that CEA 2 steps to realign it with the other CEAs in the group. The CEAs appeared to be in the proper position because each CEA had been pulsed and the pulse counters and the CEA withdrawal indications on the CEDMCS Operators Module were as expected for CEA motion.
After attempting to withdraw the four CEAs in Subgroup 6 to the upper electrical limit position in Manual Individual, it was noticed that the CEA Position Display System indicated that these CEAs did not withdraw. Recognizing that the four CEAs in Subgroup 6 that had not moved and were below the TS LCO 3.1.6 Shutdown Control Element Assembly Insertion Limits, the operators at 12:32 MST entered TS LCO 3.1.6 condition A and TLCO 3.1.204 condition A. (Note: TS 3.1.6 has a note that states LCO 3.1.6 is not applicable during performance of SR 3.1.5.3 which verifies full strength CEA freedom of movement (trippability) by moving each individual full strength CEA, that is not fully inserted in the core, at least 5 inches.) At 13:15 MST, after further review the crew entered LCO 3.0.3 (initiate action to place the unit in Mode 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> and Mode 5 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />). Instrument & Control maintenance technicians performed troubleshooting and determined that the inability to withdraw the CEAs was due to a faulty phase synchronization card.
At 16:16 MST, Unit 2 exited LCO 3.0.3 when the phase synchronization card was replaced and the CEAs were withdrawn to greater than 147.75 inches. (Note: Control Room Staff were making preparations for a plant shutdown by reviewing the Shutdown General Operating Procedure and notifying the Electrical Control Center (ECC) of the possible shutdown but exited LCO 3.0.3 prior to inserting negative reactivity.) 5.� ASSESSMENT OF SAFETY CONSEQUENCES:
This event was initially reported in ENS 43140 as an unanalyzed condition that significantly degraded plant safety.
Further evaluation by Engineering personnel concluded that sufficient reactivity margin remained to ensure the ability to shut down the reactor and maintain it in a safe shutdown condition and the identified condition was determined not to be "serious" or "significant.
During this event, Unit 2 maintained adequate shutdown margin (SDM). The shutdown CEAs must be within their insertion limits in Mode 1, and Mode 2 with any regulating CEA not fully inserted. This ensures that a sufficient amount of negative reactivity is available to shut down the reactor and maintain the required SDM following a reactor trip.
Additionally, the safety analyses assume one CEA with a maximum worth to be stuck following a trip (N minus 1 condition — all rods in except one (N-1 condition)). The CEA group could be tripped throughout the event and the N-1 condition assumption in safety analyses was preserved providing sufficient reactivity to ensure the ability to shutdown the reactor during an event and maintain it in a shutdown condition within the accident analysis assumptions. Therefore, this event did not challenge the shutdown margin of Unit 2 and this LER provides a retraction for the information pertaining to the unanalyzed condition in ENS 43140.
The event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event and the event did not adversely affect the safe operation of the plant or health and safety of the public.
The event did not result in a transient more severe than those analyzed in the updated Final Safety Evaluation Report Chapters 6 and 15. The event did not have any nuclear safety consequences or personnel safety impact.
The condition would not have prevented the fulfillment of any safety function of structures or systems as defined by 10 CFR 50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
The Direct Causes of this event were, 1) Shutdown Subgroup 6 CEAs failed to withdraw after insertion due to a faulty phase synchronization card, and 2) the inability to ascertain CEA movement by Reed Switch Position Transmitter (RSPT) indication when moving CEAs a maximum of two steps by pulse counter indication.
The Probable Cause of the card failure was a low DC resistance of a capacitor on the phase synchronization card. This was determined to be a random electronic failure.
The Root Cause of more than one CEA inserted below the TS Limit was that the method used for performing Surveillance Test 40ST-9SF01 made it difficult to detect a lack of CEA movement when attempting to move a CEA only two steps.
7. CORRECTIVE ACTIONS:
Immediate corrective actions were implemented to:
1. Replace the failed phase synchronization card that was discovered during troubleshooting.
2. Stop the performance of 40ST-9SF01 to evaluate the procedure instructions.
3. Change the Pre-job brief data base to include this event.
Corrective actions to prevent recurrence were implemented by revising 40ST-9SF01 to include the following:
1. Check each CEA has moved by RSPT indication.
2. Ensure that the CEAs are above the T.S. limit on all indications prior to inserting the next CEA.
3. Ensure the CEAs can be moved out prior to testing.
Any additional corrective actions taken as a result of this event will be implemented in accordance with the APS corrective action program. If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
8. PREVIOUS SIMILAR EVENTS:
In the past three years, no similar event was reported for multiple CEAs being inserted beyond TS Limits.
9. OTHER:
On July 20, 2006, APS submitted a request for an amendment to TS 3.1.6, "Shutdown Control Element Assembly (CEA) Insertion Limits." The proposed amendment would revise TS 3.1.6 which currently requires the Shutdown CEAs to be 144.75 inches withdrawn. Through an evaluation it has been determined that this limit is non conservative. The proposed amendment would change this limit to 147.75 inches withdrawn. This limit is currently controlled administratively.