05000305/LER-2011-006
| Kewaunee Power Station | |
| Event date: | 08-30-2011 |
|---|---|
| Report date: | 10-31-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3052011006R00 - NRC Website | |
Event Description
On August 30, 2011, routine surveillance testing of the control rod system [AA] was being performed as required by Technical Specification (TS) Surveillance Requirement (SR) 3.1.4.2. During performance of this surveillance, a rod control urgent failure alarm [ALM] was received, indicating an electrical fault in the control rod drive system. The surveillance activity (i.e., physical performance of the surveillance procedure) was temporarily suspended upon receipt of the urgent failure alarm while troubleshooting activities were carried out to address the cause of the urgent failure alarm and allow completion of the surveillance. The urgent' failure alarm was quickly verified to be electrical in nature, such that control rod trippability was not impacted.
As such, compliance with control rod Operability requirements of TS 3.1.4, "Rod Group Alignment Limits", was maintained.
Control Bank A Group 2 was one step outside (below) the insertion limit specified in the Core Operating Limits Report (COLR) at the time of the event; therefore, the limits of TS Limiting Condition for Operation (LCO) 3.1.6 were not being met. However, since operators had been performing SR 3.1.4.2, the provision of the Note in the Applicability section of TS 3.1.6 was being used during performance of the surveillance to maintain compliance with TS 3.1.6.
TS 3.1.6, "Control Bank Insertion Limits", states (in part):
LCO 3.1.6 Control banks shall be within the insertion, sequence, and overlap limits specified in the COLR.
APPLICABILITY: MODE 1, MODE 2 with keff ?_ 1.0.
NOTE
This LCO is not applicable while performing SR 3.1.4.2.
TS LCO 3.1.6 requires that all control banks be within the insertion, sequence, and overlap limits specified in the COLR. The Note in the Applicability section states that this LCO is not applicable while performing SR 3.1.4.2 (because this surveillance requires control rods to be moved such that they temporarily exceed the limits of LCO 3.1.6, which would violate the LCO.
Although troubleshooting activities continued, the cause of the urgent failure alarm was not readily resolved and these activities continued into the following shift. Performance of the surveillance procedure continued to remain suspended during this period and the provisions of the Note continued to be relied upon to meet the requirements of TS 3.1.6. Due to the extensive length of time being taken to complete assessments of potential corrective actions for resolving the rod control electrical issues, Control Bank A was restored to required limits (by withdrawing the Group 2 rods one step) and the surveillance was terminated. This action restored full compliance with TS 3.1.6, thereby ending reliance on the Note for TS compliance.
During subsequent evaluation of this event (including discussions with NRC staff), use of this TS Note to maintain compliance with TS 3.1.6, while SR 3.1.4.2 was suspended for an extended period, was determined to be inappropriate. The Note was determined to no longer apply once physical performance of SR 3.1.4.2 ceased, despite the intent to complete the associated surveillance procedure upon correcting the U.S. NUCLEAR REGULATORY COMMISSION NRC FORM 366A (10-2010) LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
05000305 Kewaunee Power Station problem (surveillance activities effectively ceased when the initial work shift ended about 1800 CDT on August 30). Consequently, TS 3.1.6 was no longer met and the required actions should have been taken.
However, Control Bank A was not restored to within limits until the following day (when the inappropriate use of this Note was recognized).
Condition A of TS 3.1.6 applies when, control bank insertion limits are not met. The associated required action directs that, within one hour, shutdown margin (SDM) be verified to be within the limits specified in the COLR or boration initiated to restore shutdown margin to within limits. Additionally, control banks must be restored to within limits within two hours of entering this condition.
If the required action and completion time of Condition A is not met, Condition C requires that the reactor be brought to MODE 2 with keff The Note in the Applicability section of TS 3.1.6 was not properly applied when performance of surveillance testing of the control rod system was stopped on August 30, 2011. Misapplication of the NOTE resulted in operation prohibited by the plant's TS. Control Bank A insertion limit was not met and the required action for Conditions A and C of TS 3.1.6 were not carried out within the completion time of the TS. Therefore, this event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B) for any operation or condition prohibited by the plant's Technical Specifications.
Condition Reports CR 440417, CR 441025, and CR 441154 all relate to this concern.
Event and Safety Consequence Analysis The insertion limits of the shutdown and control rods are initial assumptions in all safety analyses that assume rod insertion upon reactor trip. The insertion limits directly affect core power and fuel burnup distributions and assumptions of available SDM, and initial reactivity insertion rate.
Limits on control rod insertion have been established, and all rod positions are monitored and controlled during power operation to ensure that the power distribution and reactivity limits defined by the design power peaking and SDM limits are preserved.
The rod cluster control assemblies (RCCAs) are divided among control banks and shutdown banks. KPS has four control banks and two shutdown banks (Control Bank A is divided into two groups, with each group being comprised of four control rods). TS LCO 3.1.4, "Rod Group Alignment Limits," provides control and shutdown rod Operability requirements, while TS LCO 3.1.6 requires that control banks be within insertion limits. Control bank insertion limits are specified in the COLR. The control banks are required to be at or above the insertion limit lines.
The shutdown and control bank insertion limits LCO is required to prevent power distributions that could result in fuel cladding failures in the event of design bases accidents. The limits on control banks physical insertion, as defined in the COLR, must be maintained because they serve the function of preserving power distribution, ensuring that the SDM is maintained, ensuring that ejected rod worth is maintained, and ensuring adequate negative reactivity insertion is available on trip.
The applicability requirements of TS LCO 3.1.6 are modified by a Note indicating the LCO requirements are suspended during the performance of SR 3.1.4.2. This SR verifies the freedom of the rods to move, and requires the control bank to move below the LCO limits, which would violate the LCO.
U.S. NUCLEAR REGULATORY COMMISSION NRC FORM 366A (10-2010) LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
Operation beyond the LCO limits is allowed for a short time period in order to take conservative action because the simultaneous occurrence of either a LOCA, loss of flow accident, ejected rod accident, or other accident during this short time period, together with an inadequate power distribution or reactivity capability, has an acceptably low probability.
SR 3.1.4.2 verifies rod freedom of movement (trippability) by moving the rod 10 steps. A complete trippability test would require that each rod be tripped, which would result in radial or axial power tilts, or oscillations. Moving each control rod by 10 steps will not cause radial or axial power tilts, or oscillations, to occur.
At the time of the event, the reactor was operating in Fuel Cycle 31. The COLR for Cycle 31 specifies an All Rods Out (ARO) (i.e., fully withdrawn) position of 224 steps withdrawn. For Cycle 31 design calculations, a fully withdrawn rod position of 224 steps is one step inside the active fuel region. During the performance of SR 3.1.4.2, Control Bank A Group 2 was only one step outside (below) the insertion limit specified in the COLR. This correlates to 223 steps withdrawn (Control Bank A Group 1 remained at 224 steps).
An assessment was made of an ARO position equal to 223 steps withdrawn versus 224 steps. Parameters assessed were available shutdown margin, control rod trip reactivity, core peaking factors, axial power distributions, and core reactivity. Based on confirmatory calculations performed as part of this assessment, an ARO position equal to 223 steps withdrawn for Cycle 31 instead of 224 steps withdrawn has no adverse impact on reactivity performance or safety analysis parameters and does not adversely impact the safety analyses that support operation during Cycle 31. The assessment concluded that operation with an ARO position equal to 223 steps withdrawn for Cycle 31 would have been acceptable.
Based on the above, the safety significance of this condition was minimal.
Cause
An assessment of this event determined that the Note in the Applicability section of TS 3.1.6 no longer applied once performance of SR 3.1.4.2 ceased (surveillance activities effectively ceased when the initial work shift ended about 1800 on August 30). Although active performance of the SR had been stopped, operators continued troubleshooting activities in an attempt to correct the rod control system and restart performance of the surveillance procedure. Troubleshooting activities continued throughout the following shift and into the next day.
Appropriate recognition that the allowance provided by the Note no longer applied, once performance of SR 3.1.4.2 ceased, did not occur until the following day. The condition was not recognized earlier due to the assumption that the Note provided a broad waiver of TS LCO 3.1.6. Operators were acting under the premise that since the surveillance procedure was suspended and would eventually be completed once the issue was resolved, then the allowanced provided by the Note continued to apply until the surveillance procedure was finally completed.
Control Bank A was not restored to within its insertion limit sooner because of concerns that attempting rod motion with the existing rod control failure could cause rods to be inadvertently tripped and inserted into the core (which would lead to undesirable core flux distribution and possibly necessitate a reactor shutdown).
Corrective Actions
As immediate corrective action, operators restored Control Bank A to within required limits (by withdrawing the Control Bank A Group 2 rods one step) at about 11:00 a.m. on August 31, 2011. This action restored full compliance with TS 3.1.6, thereby ending reliance on the Note for TS compliance.
Troubleshooting activities determined the cause of the rod control urgent failure alarm to have been failure in a rod control power cabinet circuit card. The faulty circuit card was replaced and the rod control system was tested satisfactorily on September 1, 2011.
As a longer term action, activities were initiated to revise the Bases for TS 3.1.6 to clarify the limitations of the Note. Because the same Note exists in TS 3.1.5, "Shutdown Bank Insertion Limits", the Bases for that TS section is also planned to be revised with the same clarification This activity was entered into the KPS corrective action process as CR 441025.
An evaluation was initiated to determine whether additional long term corrective actions were needed to address the cause of this event. This activity was entered into the KPS corrective action process as CR 441154.
Similar Events A review of Licensee Event Reports covering the last three years did not identify any similar events.