05000317/LER-2006-005
| Calvert Cliffs Nuclear Power Plant | |
| Event date: | 12-17-2006 |
|---|---|
| Report date: | 05-29-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3172006005R01 - NRC Website | |
I. DESCRIPTION OF EVENT
On December 17, 2006, at 04:25 a.m. while evaluating the setpoints for the Reactor Protective System (RPS) Rate of Change of Power-High (also referred to as Startup Rate or SUR) Trip enabling function, Operations Department personnel determined that the SUR Trip would not be enabled at the Technical Specification required value of 12 percent Rated Thermal Power (RTP), decreasing, on Unit 1 RPS Channel A and Unit 2 RPS Channel C. This condition was identified by reviewing the results of Surveillance Test Procedures (STP) M-310-1 and STP M-310-2 (Linear Power Channel Calibration). The SUR Trip is enabled by the reset value of the Nuclear Instrument (NI) Level 1 Bistable. Surveillance Test Procedure M-310-1, completed on December 4, 2006, for Unit 1 RPS Channel A indicated that the NI Level 1 Bistable reset at 0.608 VDC and was recorded as 11.9 percent RTP. Surveillance Test Procedure M-310-2, completed on October 12, 2006, for Unit 2 RPS Channel C indicated that the NI Level 1 Bistable reset at 0.59 VDC and was recorded as 11 percent RTP. The NI output voltages are also provided because they are a more accurate measure of reactor power as compared to the non-calibrated power meters on the local NI drawers. Rated Thermal Power can be derived from NI detector output voltage by using a correction factor of x 20. Technical Specification 3.3.1 requires that four RPS bistable trip units, associated measurement channels, and applicable automatic bypass removal features for each Function in Table 3.3.1-1 shall be OPERABLE.
Footnote (a) to Technical Specifications Table 3.3.1-1 states that this bypass "shall be automatically removed when NUCLEAR INSTRUMENT POWER is >1= 1E-4 percent RTP and Unit 1 was operating at 100 percent RTP and Unit 2 was operating at 30 percent RTP at the time this condition was discovered. No systems, structures, or components that contributed to or SUR Channel Trip Units were placed in bypass mode upon discovery of the condition, at 04:25 a.m. on December 17, 2006. Technical Specification Limiting Conditions for Operation 3.3.1A and D were entered at this time. Corrective actions to revise the automatic bypass removal setpoints were completed for Unit 1 and 2 at 11:30 p.m. on December 17, 2006.
Limiting Conditions for Operation 3.3.1.A and D were exited at that time due to restored operability of the affected channels. The STP review also revealed that the bistable which performs the SUR Trip enabling function was tested, its value recorded, but no setting tolerance was specified in the procedure.
Further review of STP M-310 test results on both operating units for the three year period prior to the discovery date of December 17, 2006 indicates that Technical Specification Limiting Condition for Operation (LCO) 3.3.1(D), one or more Functions with one automatic bypass removal feature inoperable, was exceeded for 400 days (Unit 1) and 336 days (Unit 2). During the same period, Technical Specification LCO 3.3.1(E), one or more Functions with two automatic bypass removal feature channels inoperable, was exceeded for 90 days (Unit 1) and 153 days (Unit 2). Unit 2 also met the requirements for entry into Technical Specification LCO 3.0.3 (LCO is not met and an associated ACTION is not provided) when three RPS trip bistable automatic bypass removal features were inoperable for 179 days during the three year period prior to discovery of the condition. The total time for the SUR Trip automatic bypass removal feature inoperability during the previous three years was 490 days (Unit 1) and 668 days (Unit 2).
II. CAUSE OF EVENT
The apparent cause of this event is a failure to recognize that, when Improved Technical Specifications (ITS) were adopted at Calvert Cliffs Nuclear Power Plant in 1997, Footnote (a) to Technical Specification Table 3.3.1-1 added new acceptance criteria to the Technical Specifications for this function. This new criteria should have been incorporated into the STP at that time to ensure that the Technical Specification would continue to be met. The STPs were not revised following the ITS changes to this Technical Specification. This failure to recognize new acceptance criteria involves a human performance issue that was caused by time pressure, multiple tasks, and vague or interpretive guidance related to the ITS conversion project. Current human performance error prevention tools, which were not required or in use at the time when ITS changes were being made, are expected to prevent similar issues from occurring in the future.
These tools include maintaining a questioning attitude, use of peer checks, and maintaining 200 percent accountability.
This condition was documented by Condition Report IRE-019-028.
III. ANALYSIS OF EVENT
The failure to maintain the RPS SUR Trip reset enabling setpoint at or above 12 percent RTP on one or more channels of Unit 1 and Unit 2 represented a condition prohibited by Technical Specifications 3.3.1 (D), 3.3.1(E), and 3.0.3 (Unit 2 only). This condition is reportable under 10 CFR 50.73(a)(2)(i)(B). A review of STP M-310 test results for the three years prior to the discovery date of December 17, 2006, indicates that this condition existed for approximately 490 days on Unit 1 and 668 days on Unit 2. The actual amount of time during which either operating unit was in a condition where RTP was >/= 1E-4 and SUR Trip bypassed was less than 90 seconds over the previous three years.
The plant's safety analysis does not directly credit the RPS SUR Trip function in any accident analyses. This function is indirectly credited in the accident analyses as the justification for not analyzing subcritical events. This trip function serves as a backup to the administratively-enforced SUR limit. The allowable value for the trip is not derived from analytical limits.
There are no actual safety consequences as a result of this condition. The SUR Trip function minimizes several transients initiating from subcritical conditions and is indirectly credited in the accident analyses as a justification for not analyzing subcritical events. The potential safety consequences were low because the subject condition was not a factor during subcritical operations, since the lowest recorded SUR trip reset at the time of discovery of the condition was 11.8 percent RTP. A review of the calibration records over the three year period prior to discovery reveals that the lowest SUR trip reset value, as determined by surveillance, was 9.4 percent RTP. The extremely short duration that the condition may have actually existed over Incremental Conditional Core Damage Probability associated with this issue, using bounding estimates, is less than 6.2E-09. This represents 0.007 percent of the CCNPP Unit 2 annual core damage frequency and is much less than the 1E-06/year threshold. All Unit 1 shutdowns were conducted by rapid control element assembly insertion from above 12 percent RTP during the three-years prior to discovery of the condition. Therefore, Unit 1 did not experience a risk increase from this issue. No Nuclear Regulatory Commission Performance Indicators were affected by this condition.
IV. CORRECTIVE ACTIONS
A. The STP M-310 was reperformed on both operating units to adjust the bistable setpoints upward and restore operability to the affected channels.
B. Revise the Technical Specifications Basis description of the Rate of Change of Power- High Trip to include a description of the system's automatic bypass removal feature.
C. Revise STPs M-310-1 and M-310-2 to include a specification for the reset function of the Linear Range Nuclear Instrument Level One Bistable bypass removal feature.
D. A review of all RPS STPs will be performed to ensure that no additional components have undefined acceptance criteria for Technical Specification required values.
E. Compensatory measures were implemented to ensure STPs M-310-1 and M-310-2 were not used prior to revision or, if necessary, appropriate controls were in place to prevent recurrence of the subject Licensee Event Report event.
V. ADDITIONAL INFORMATION
A. Component Identification IEEE 803 IEEE 805 Component EIIS Function System ID Reactor Protection System NI Level 1 Bistable JD RLY
B. Previous Occurrences
A review of Calvert Cliffs' licensee event reports over the past three years was performed. The review identified no similar reportable events.