05000335/LER-2004-001
| Event date: | 04-15-2004 |
|---|---|
| Report date: | 06-04-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3352004001R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
- NUMBER NUMBER 111111111■MINIMMI.=
Description of the Event
On April 15, 2004, St. Lucie Unit 1 was defueled in a refueling outage and St. Lucie Unit 2 was operating in Mode 1 at 100 percent reactor power. The NRC Branch Technical Position PSB-1, "Adequacy of Station Electric Distribution System Voltages" provides specific design requirements to protect safety-related equipment from damage and/or loss of function due a sustained degraded voltage condition. PSB-1 and GDC 17, "Electric Power Systems," require that degraded voltage relays not actuate on expected switchyard voltage and load transients following a unit trip, loss of coolant accident, startup, or shutdown.
St. Lucie electrical systems response calculations were initially performed in the early 1980s and updated in the early 1990s. These calculations evaluated the electrical systems response to degraded voltages and determined the proper protective relay settings to preclude equipment damage and/or the loss of safety function.
Although there were several design changes that affected the calculations, they were not formally revised because the original computer model used in these calculations had become obsolete and the knowledge and expertise to run the program was no longer available.
An effort was initiated in 2002 to update the subject calculations using new software. Over the next two years, anomalies and non-conservative assumptions were identified during the development of the system model and subsequent voltage drop calculation runs. By the time the calculation upgrade project was coming to closure in 2004, the following issues had been identified and corrected:
1. Certain loads were assumed to be off without adequate bases. These loads included lighting panels, power panels, battery chargers, and shield building fan heaters (December 2002).
2. The impedance of the non-segregated phase bus, which connects the startup transformers (EI/S:EA:XFMR) to the switchgear, was assumed to be zero. The impedance is not negligible and results in an additional voltage drop of approximately 1.2 percent (January 2003).
3. All motors were assumed to start and accelerate to full running load within 5 seconds. This assumption is not valid for several large fan motors (HVS-1, 4, 9), particularly with reduced voltage (December 2003).
4. The St. Lucie Unit 1 containment cooler fans (HVS-lA, B, C, D) were assumed to be 75 HP motors when they are actually rated for 150 HP during accident conditions.
In addition, all four HVS-1 fans were assumed to be running when normal operation has three fans running with the fourth starting on SIRS. Therefore, each train should have included the starting current included for one HVS-1 fan (January 2004) 5. Both St. Lucie Unit 1 HVS-4A and 4B auxiliary building supply fans were assumed to be running. Normally one fan is running and the other starts on a SIAS.
Therefore, each train should haVe included the starting current of the HVS-4 fan (January 2004).
These issues did not compromise the protection afforded by the degraded voltage relays (EIIS:EB:27), but there was a potential for relay actuation and separation of the offsite power source on a safety injection actuation signal (SIAS) with the minimum PSB-1 grid design voltage of 230 KV. As a precaution, the PSB-1 minimum switchyard voltage was increased to 232.5 KV based on preliminary runs using the new FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) � St. Lucie Unit 1 05000335 model and static load conditions (transient motor starting analysis had not been completed). At that time, this condition was not considered to be reportable for the reasons discussed below in the Analysis of the Event.
The required St. Lucie Unit 1 plant modifications to restore the PSB-1 minimum switchyard design voltage of 230 KV were completed by the end of the spring 2004 SL1- 19 refueling outage. These corrective actions included modifications to the startup transformer taps, modifications for tripping various loads on SIAS, and SIAS timing modifications for various safeguards ventilation fans.
The St. Lucie Unit 1 PSB-1 and setpoint calculations were completed April 2, 2004.
On April 9, 2004, FPL initiated a condition report to re-evaluate the previous reportability determination. During this effort, on April 15, 2004, the NRC issued Regulatory Issue Summary (RIS) 2004-05, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," that clarified that the previous condition was reportable.
Cause of the Event
Human performance errors during the development of the early 1990s electrical distribution calculations are the most likely apparent cause of the past condition.
The original calculations contained omissions and nonconservative assumptions that resulted in incorrect conclusions and did not address the basis for many of the assumptions that were made. Furthermore, specific acceptance criteria were not provided to ensure that the degraded voltage relays did not actuate during the SIAS motor starting transients. It is appaient that the primary emphasis was ensuring that the safety-related loads remained functional during the event and the degraded voltage setpoints provided the necessary equipment protection for damage or loss of function.
Analysis of the Event
In 2003, during the electrical distribution calculation upgrades, FPL concluded that a SIAS could result in the separation from the offsite power source at a grid voltage above the minimum grid voltage assumed in the design. However, the onsite and offsite power Technical Specification (TS) 3.8.1.1 does not specify a minimum voltage requirement for the grid (i.e., offsite power system), and neither the TS Limiting Condition for Operation (LCO) nor its associated surveillance requirements specify a grid voltage range or minimum voltage. FPL subsequently revised plant procedures to administratively treat the startup transformers as inoperable when minimum grid voltage could cause separation of the offsite power source during SIAS and initiate a plant shutdown if the allowed outage time (AOT) for the condition is exceeded. The AOT mirrored the TS requirements for the loss of one startup transformer (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) and the loss of both startup transformers (24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />).
Separation of offsite power during SIAS events did not meet the design basis for the PSB-1 analysis. However, it did not represent an unanalyzed event that significantly affected plant safety because offsite power would remain available for post-LOCA recovery, the startup transformers would still be capable of supplying the required electrical loads for all other events, and lastly, offsite power is not credited for any accident mitigation function. Additionally, this condition did not represent a safety system functional failure because offsite power is not credited for any accident mitigation functions in the UFSAR. Furthermore, operation in a mode prohibited by TS was judged not applicable because the governing TS defined offsite
DOCKET
St. Lucie Unit 1 05000335 0 I SEQUENTIAL REVISION YEAR I NUMBER � NUMBER �■
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- .-..-'� power operability as a function of breaker alignment and did not specify a minimum grid voltage.
However, on April 15, 2004, RIS 2004-05, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," was issued. Specifically, RIS 2004-05 described the connection between GDC 17 design requirements and TS operability of the offsite poWer system. RIS 2004-05 states in part: "Plant operators should therefore be cognizant of the capability of (1) the offsite power system to meet plant safety needs during operation and (2) situations that can result in a LOOP following the trip of the plant. If offsite power is not capable of supporting the NPP (nuclear power plant] safety requirements in either situation, the system should be declared inoperable and pertinent plant TS provisions followed." Based on this latest NRC clarification, the past condition is reportable under 10 CFR 50.73(a)(2)(i)(B) as operation in a condition that was prohibited by the station's TS.
Analysis of Safety Significance
- The undervoltage relays on the Class 1E 4160 and 480V buses protect safety-related equipment from damage or loss of function due to sustained degraded voltage conditions or a complete loss of offsite power. There are two levels of undervoltage protection. The first level of undervoltage protection detects a loss of offsite power on the 4160V buses. The setpoint is intended to separate the engineered safety systems from the offsite power source when there is a loss of offsite power. The second level of undervoltage protection is the subject of this LER and detects a sustained degraded voltage condition with a setpoint above the minimum required to prevent equipment damage or loss of function (i.e. greater than minimum pickup and starting voltages).
The St. Lucie Unit 1 480V degraded voltage circuits only initiate load shedding with a concurrent SIAS signal. Actuation of the loss of voltage or degraded voltage relays automatically starts the emergency diesel generators (EDG), disconnects offsite power, closes the EDG breakers, and sequentially starts essential loads. The diesel generators also start on SIAS; however, they are not loaded (the output breaker is not closed) unless a degraded or undervoltage condition also exists. This function is considered safety-related since it ensures operability of safety-related equipment and protects the equipment from damage due to degraded voltage conditions.
PSB-1 places another design requirement on the degraded voltage protection scheme where an accident condition will not cause separation from offsite power when a minimum switchyard voltage is maintained (230 KV). This requirement ensures the reliability of the offsite power source is maintained and prevents unnecessary challenges to the diesel generators.
None of the past non-conservative PSB-1 assumptions compromised protection of safety- related loads afforded by the degraded voltage protective relaying. However, a SIAS condition would have likely resulted in separation from offsite power with nominal switchyard voltages and sequencing of safety-related loads on to the emergency diesel generator. There are several key factors that must be considered when determining whether or not the degraded voltage relays would have actuated, including equipment and bus alignment, initial switchyard voltage, and grid conditions. Various historical scenarios were evaluated using the new electrical distribution model to determine the sensitivity of equipment and bus alignment to the required switchyard voltage to prevent degraded voltage relay actuation. Depending on the equipment and bus alignment scenarios, the switchyard voltage required to prevent relay actuation varies from approximately 234 to 257 KV. Based on the review of the past three FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) St. Lucie Unit 1 05000335 months of switchyard voltage data, the average switchyard voltage is 239.6 KV with a minimum of 234.2 KV and a maximum of 243.6 KV. The switchyard voltage dip following a unit trip can vary anywhere from 2 to 7 KV depending on whether one or both PSL units are on line and the existing grid conditions (load and MVAR capacity).
Therefore, FPL concluded that St. Lucie Unit 1 operated for significant periods of time where a SIAS signal would have resulted in degraded voltage relay actuation for at least one train.
Notwithstanding, this event had no significant effect on the health and safety of the public. The ability to supply power from an offsite source is not a safety-related function and no credit is taken for offsite power to mitigate the consequences of a design basis accident. This condition was only an issue during SIAS events, and even so, this condition had no effect on the ability of the safety-related onsite emergency power system to respond to the loss of the offsite power source.
FPL also completed the St. Lucie Unit 2 electrical distribution calculation upgrade, and the past configuration for St. Lucie Unit 2 was also evaluated with respect to sensitivity for equipment and bus alignment. Initial Unit 2 calculation results showed that with a switchyard voltage of 230 KV and concurrent SIAS, the onsite power system was likely to separate from offsite power and switch to the backup emergency diesel generators. Contingency calculations without any Unit 2 modifications incorporated showed that a switchyard voltage of approximately 232.6 KV was required to ensure reset of the degraded voltage relays. This is approximately 1% above the 230 KV minimum assumed in calculations. However, Case 2 of the evaluation in the St.
Lucie Unit 2 UFSAR Section 8.2 shows the results of an analysis performed for the offsite system response to a situation with Unit 2 off line and a sudden trip of Unit 1. The resulting system response showed a 230 KV switchyard voltage drop from normal 240.12 KV (104.4%) to 235.5 KV (102.4%). Therefore, it can be concluded that there is a very low probability that Unit 2 would operate with a switchyard voltage low enough to result in separation from offsite power for a concurrent degraded grid voltage with SIAS with associated plant trip. In order to ensure the 230 KV switchyard voltage criteria as analyzed in existing calculations is met, the startup transformer tap changes were required to restore Unit 2 to full PSB-1 design capability to meet the 230 KV criterion. These tap changes were completed in March 2004. It was unlikely that the degraded voltage relays for Unit 2 would have actuated to separate from offsite power for a degraded voltage with concurrent SIAS.
Corrective Actions
1. The St. Lucie Units 1 and 2 electrical distribution calculation upgrades are complete.
2. All required corrective actions to restore the original PSB-1 minimum grid voltages of 230 KV are complete.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) iMIM=11
- 111111M.M.
Additional Information
FPL reviewed past spurious St. Lucie SIAS trips that should have actuated the degraded voltage relays. Three spurious SIAS events have occurred since the degraded voltage circuits were installed; LERs96-008, 94-009,94-010. All three events occurred when the units were off line, Mode 5 or 6. For LER 96-008, the spurious SIAS occurred on Train B with the train removed from service, so no engineered safeguards equipment was actuated. The spurious SIAS events documented in LERs 94 009 & 010 occurred on Train B with load center (LC) 1B2 cross-tied to Train A and the degraded voltage relays disabled. The 480V degraded voltage protection is disabled when load centers are cross-tied to the opposite train by procedure, hence no engineered safeguards equipment was actuated. Therefore, the review concluded that the past spurious SIAS events did not result in engineered safeguards equipment actuation that would have actuated the degraded voltage relays and identified the calculation deficiencies.
Failed Components Identified None Similar Events None