During a reactor startup performed on September 12, 2017, the operators noted that the inoperable ‘B' channel reactor protection system (RPS) high startup rate (HSUR) trip did not occur as expected when reactor power exceeded the HSUR bypass removal setpoint. The ‘B' RPS HSUR channel was then manually tripped via the bistable removal method and plant startup continued.
Investigation revealed that the setpoint reduction method process used to implement the RPS HSUR channel trip did not account for subsequent nuclear instrumentation (NI) detector failures. Therefore the ‘B' RPS HSUR channel was not in the required tripped condition since the February 2017 failure of its wide range NI detector.
The setpoint reduction method was subsequently revised to ensure inoperable RPS HSUR channels tripped by the setpoint reduction method generate a trip with reactor power less than 15 percent reactor power. A procedure revision is in progress to implement these new rule-based instructions.
This event had no significant impact on the health and safety of the public based on system channel redundancy and procedural controls.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.Description On September 12, 2017, St. Lucie Unit 1 was in a reactor startup in Mode 2 operation. The ‘B' channel reactor protection system (RPS) high startup rate (HSUR) [EIIS:JC] channel was thought to be in the reduced setpoint tripped condition in response to earlier unpredictable operation of the ‘B' channel nuclear instrumentation (NI) detector [EIIS:IG:DET]. At 1522 hours
0.0176 daysduring the startup, the operators noted that the ‘B' channel RPS HSUR bistable [EIIS:JC] did not automatically trip as expected for the existing plant conditions. The operators placed the ‘B' channel of RPS HSUR in a tripped condition in accordance with procedures by removing the bistable from the trip unit assembly and entered Technical Specification (TS) 126.96.36.199, Table 3.3-1, Functional Unit 11, Action 2 with the ‘B' channel HSUR bistable in trip. The reactor startup continued with the channel in trip as allowed by the Technical Specifications (TSs).
Cause of the Event
This event was caused by inadequate processes used to implement the HSUR reduced setpoint trip method. The instruction used did not evaluate all potential failure conditions when setting the HSUR bistable. Investigation showed that the bistable did not trip because the setpoint reduction method (initially) internally tripped the bistable in the presence of an active NI signal. When the ‘B' wide range NI detector subsequently failed low in February 2017, the input signal to the Hi Rate bistable from the rate circuit changed and the bistable trip conditions were no longer satisfied. During power operation this latent condition was partially masked by the greater than 15 percent power automatic bypass signal applied downstream of the comparator output circuitry. Additionally, the automatic bypass of the bistable trip signal below 10-4 percent power was never automatically removed during the startup due to the NI detector being failed low.
Following this investigation, maintenance and engineering personnel determined that the correct method to internally trip the bistable was to set the setpoint to the maximum negative value. This would ensure a trip would occur regardless of NI detector health whenever reactor power was less than 15 percent. The HSUR bypass for affected channels would also continue to be bypassed above 15 percent reactor power. A procedure revision is in progress to implement these new rule-based instructions.
Analysis of the Event
This event is reportable under 10 CR 50.73(a)(2)(i)(B) as any operation or condition that was prohibited by TSs.
The RPS HSUR trip is developed from the nuclear instrumentation (NI) wide range channels, and the trip signal may be automatically bypassed below 10 E-4 percent and above 15 percent power. When the trip is not bypassed, a reactor trip is initiated prior to the reactor power rate-of-change exceeding 2.49 decades per minute as measured by any two of the four wide-range NI channels.
Plant procedures provide two methods for placing an RPS HSUR channel in the trip condition. The first method pulls the Hi Rate bistable from the trip unit assembly. This method can be implemented quickly by control room operators, but has the disadvantage of sealing in a channel trip signal above 15 percent power. The second method has maintenance personnel reduce the bistable setpoint such that the channel would be expected to generate a trip signal with the automatic removal of the bypass between 10 E-4 percent and 15 percent power. This method has the advantage of preserving the automatic RPS HSUR bypass below 10 E-4 and greater than 15 percent power.
Prior to this event, the ‘B' channel wide range NI detector signal had been experiencing unpredictable operation, and the ‘B' RPS HSUR channel was placed in trip using the setpoint reduction method in October of 2016. During the Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
September 2017 plant startup, the ‘B' RPS HSUR channel did not trip as expected when reactor power exceeded the bypass removal setpoint of 10 E-4 percent power. The ‘B' wide range NI detector failed low on February 10, 2017.
The setpoint reduction method (used before the detector failed) was predicated on a baseline NI detector signal not a failed low detector signal; the setpoint reduction method did not account for the static failed detector voltage and its effect on the trip. Additionally, the failed low detector signal did not remove the bypass.
The high rate-of-change of power trip is not credited in any of the Chapter 15 accident analyses. However, the trip is considered in the safety analysis, in that the presence of this trip function precluded the need for specific analyses of other events initiated from subcritical conditions (e.g., events not discussed in Chapter 15).
Subsequent to the ‘B' wide range detector failure on February 10, 2017, Unit 1 was within the HSUR bypass conditions with power greater than 15 percent. On September 11, 2017, Unit 1 was shutdown due to degrading switchyard environmental conditions caused by Hurricane Irma. The inadequately implemented reduced trip setpoint method had no effect during the evolution because the operating procedure used during this shutdown required that the reactor be tripped above 15 percent reactor power while the HSUR bypass was still in effect. In addition, the inoperative channel trip was detected in the subsequent September 12, 2017 startup and actions were taken as directed by the TSs; therefore the inoperative trip had no effect on the subsequent startup.
As previously stated, the HSUR bistable is required for operation during the reactor power ranges of 10 E-4 percent to 15 percent power. Per the design basis, the RPS has four independent measurement channels that monitor parameters and trip at TS prescribed setpoints. In addition, each RPS channel is required to be demonstrated operable by the performance of a successful monthly functional test. The RPS is designed to initiate a reactor trip when the two out of four coincidence logic is satisfied (i.e. high startup rate). Therefore, even with the ‘B' RPS HSUR channel in a nonconforming condition, there is reasonable assurance that the three remaining healthy HSUR channels would have performed the function of the RPS system to trip if TS prescribed setpoints were exceeded.
Based on the discussion above, this event had no significant impact on the health and safety of the public.
1. The ‘B' RPS HSUR channel was recalibrated, placed in trip using the new setpoint reduction method, and the bistable was re-inserted into the cabinet.
2. A procedure revision is in progress to implement the new rule-based setpoint reduction method.
Component: wide range nuclear instrumentation detector JI-002 Manufacturer: Sigma Model: 9222-00ED
|St. Lucie Unit 1|
|LER closed by|
|IR 05000335/2017004 (29 January 2018)|
|3352017003R00 - NRC Website|