05000293/LER-2002-003

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LER-2002-003,
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor
2932002003R00 - NRC Website
v • d • e

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

BACKGROUND

The neutron monitoring system consists of the source range monitors, intermediate range monitors (IRMs), local power range monitors (LPRMs), and average power range monitors (APRMs). The monitors function to detect neutrons in the reactor core and provide input to the reactor protection system and rod block portion of the reactor manual control system.

The six APRMs are flow-biased and receive flow-bias input from the respective recirculation system flow converter. The flow-bias for APRMs 'A', 'C', and `E' is provided from the recirculation loop 'A' flow converter.

The flow-biasing for APRMs 'B', 'D', and 'F' is provided from the recirculation loop 'B' flow converter.

In the late 1998 timeframe, the APRMs were modified by the addition of flow control trip reference (FCTR) features which are part of the features described in NEDO-3239-A, "Reactor Stability Long-Term Solution:

Enhanced Option I-A" (Option I-A). In addition to the FCTR feature of the APRMs, the Option I-A features include an instability detection system, referred to as the Period Based Detection System (PBDS), and mandatory licensed operator actions. The PBDS uses input from the LPRMs. The licensed operator actions are described in procedures and the operator training and re-qualification training programs.

APRMs 'A', 'C', and `E' are part of reactor protection system (RPS) channel 'A' and APRMs 'B', 'D', and 'F' input to RPS trip channel 'B'. Technical Specification Table 3.1.1 (RPS) specifies a minimum of 2 operable APRMs for the high neutron flux (flow-bias) trip function for each RPS trip channel when in the RUN mode.

The Table specifies actions to be taken for less than the minimum number of operable instrument channels (APRMs). For the complete loss of the trip function (APRM neutron flux flow-bias), the Table specifies restoration of the trip function within one hour, or initiate the insertion of operable control rods and the complete insertion of all operable rods within four hours or to reduce the reactor power level to the range of the IRMs and place the reactor mode switch in the STARTUP/HOT STANDBY position within eight hours.

On October 2, 2002, at 1227 hours0.0142 days <br />0.341 hours <br />0.00203 weeks <br />4.668735e-4 months <br />, a procedure was initiated to evaluate reactor core flow and recirculation system flow performance. The procedure is performed periodically as a good practice by reactor engineering.

After completing the procedure, the control room was notified, at about 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br /> on October 2, 2002, that a comparison of APRM percent drive flow to the calculated percent drive flow for RPS trip channel 'A' did not meet the (+/-) five (5) percent acceptance criteria of the procedure. When averaged, the flow differed from the RPS trip channel 'A' APRM flow indicated value by 5.1 percent. The RPS trip channel 'B' percent drive flow met the acceptance criteria of (+/-) five (5) percent. The difference (5.1 percent) in RPS trip channel 'A' APRM percent drive flow was in the non-conservative direction relative to the APRM flow-biased settings. It was not clear whether the problem was with the RPS trip channel 'A' APRM circuitry or with the instrument providing the input to the calculated value. A priority maintenance work document was initiated to troubleshoot and determine necessary corrective action. A corrective action program document was written to document the finding.

The condition was identified during operation while at 100 percent reactor power. The reactor mode switch was in the RUN position.

On October 2, 2002, at about 2320 hours0.0269 days <br />0.644 hours <br />0.00384 weeks <br />8.8276e-4 months <br />, a calibration of the recirculation loop 'A' flow converter was initiated to determine whether the flow converter was in calibration.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

EVENT DESCRIPTION

On October 3, 2002, at 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br />, APRMs 'A', 'C', and `E' were declared inoperable for the flow-bias scram and rod block functions. This action was taken because a calibration of the recirculation loop `A' flow converter found the flow converter to be out of calibration. RPS trip channel 'A' was manually tripped and a rod block was manually initiated at 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br /> on October 3, 2002. These actions were taken in accordance with Technical Specifications Tables 3.1.1 and 3.2.C.1 (Rod Blocks) because the flow converter provides flow-bias signals to APRMs for the rod block function and RPS trip channel 'A' scram function. Technical Specifications limiting conditions for operation were entered as a result of APRMs 'A', 'C', and `E' being inoperable.

Although RPS trip channel 'A' was manually tripped within the timeframe specified in Technical Specifications Table 3.1.1 from the time the inoperable condition became known to the on-shift licensed operators, subsequent evaluation conservatively determined the condition existed longer than the timeframes in Table 3.1.1. The subsequent evaluation determined the recirculation loop `A' flow converter was inoperable from the time when the core flow evaluation procedure was initiated, at 1227 hours0.0142 days <br />0.341 hours <br />0.00203 weeks <br />4.668735e-4 months <br /> on October 2, 2002. During the timeframe of October 2, 2002 (beginning at 1227 hours0.0142 days <br />0.341 hours <br />0.00203 weeks <br />4.668735e-4 months <br />) to October 3, 2002 (until 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br />), the licensed operators did not have sufficient information to determine the flow converter was inoperable. This additional time exceeded the timeframes specified in Table 3.1.1.

The NRC Operations Center was notified in accordance with 10 CFR 50.72(b)(3)(v)(A) at 0800 hours0.00926 days <br />0.222 hours <br />0.00132 weeks <br />3.044e-4 months <br /> on October 3, 2002. The notification was made because the results of surveillance testing (calibrations) of recirculation loop 'A' flow converter, initiated on October 2, 2002 at 2320 hours0.0269 days <br />0.644 hours <br />0.00384 weeks <br />8.8276e-4 months <br />, determined the flow converter was out of tolerance and could not be calibrated and was declared inoperable.

A re-calibration of the recirculation loop `A' flow converter was completed by 0833 hour0.00964 days <br />0.231 hours <br />0.00138 weeks <br />3.169565e-4 months <br /> on October 3, 2002.

The calibration results were satisfactory. The flow converter was returned to service, RPS trip channel 'A' was reset and the rod block was reset by 1248 hours0.0144 days <br />0.347 hours <br />0.00206 weeks <br />4.74864e-4 months <br /> on October 3, 2002.

On October 3, 2002 at 1746 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.64353e-4 months <br />, the main control room alarm, "Recirc Flow Converter Failure," annunciated. The alarm is initiated if a difference (mismatch) of greater than six (6) percent occurs between the recirculation loops 'A' and 'B' flow converters. Investigation of the cause of the alarm identified a step change in the output signal of the loop `A' flow converter. The recirculation loop 'A' flow converter was declared inoperable, the RPS trip channel 'A' was manually tripped, and a rod block was manually initiated. A priority maintenance work document was initiated to replace the flow converter and the flow converter was subsequently replaced. A corrective action program document was written to document the failure of the flow converter (at 1746 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.64353e-4 months <br />).

CAUSE

The direct cause for the loss of the RPS trip channel 'A' APRM flow-bias neutron flux trip function was the age-related failure of the recirculation loop 'A' flow converter. The failed flow converter was the original flow converter and was installed in circa 1971, near the end of plant construction. The failed flow converter was a General Electric model number 135B8303G002; serial number 6,342,222. The component in the flow converter that caused the failure was a vacuum tube (V1).

The root cause of the failure of the recirculation loop `A' flow converter was less than adequate preventive maintenance (PM) for the recirculation flow controllers. Specifically, there was no PM activity established to replace the recirculation flow controllers due to age.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

CORRECTIVE ACTION

Corrective actions taken included the following:

  • The RPS trip channel 'A' was manually tripped and a rod block was manually initiated at 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br /> and 1750 hours0.0203 days <br />0.486 hours <br />0.00289 weeks <br />6.65875e-4 months <br /> on October 3, 2002.
  • The failed flow converter was replaced with a new flow converter and the new flow converter was calibrated with satisfactory results. After the replacement and a period of satisfactory operation of the new flow converter, the RPS trip channel 'A' trip circuit was reset and the rod block circuit was reset.

These actions were completed by 0919 hours0.0106 days <br />0.255 hours <br />0.00152 weeks <br />3.496795e-4 months <br /> on October 4, 2002.

The recirculation loop 'B' flow converter was previously replaced with a new converter as a result of its failure in 1997. Since the replacement of the recirculation loop 'B' flow converter, the corrective action program that existed in 1997 has been replaced with a new corrective action process. For an equipment failure, the new process includes an equipment failure analysis that includes an extent review and action(s) to preclude recurrence.

Corrective actions planned include the following:

  • Creating preventive maintenance activities to replace the recirculation flow converters. An in-service time of 12 years has been selected as the initial replacement frequency, based on discussions with the equipment vendor. The frequency may be modified based on experience and/or change in the flow converter design.
  • Performing a review to identify electronic components in selected systems, which if failed, would have potential to challenge Technical Specifications and/or power generation. The focus of the review is to determine the adequacy of equipment trending and performance, subcomponents susceptible to age- related failure and the related PM frequency for replacement, and adequacy of data obtained during the calibration process for trending. Changes to applicable procedures and/or processes may be initiated as a result of the review and may be further modified based on experience or process changes.
  • Additional actions are being evaluated or performed as part of the corrective action program.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PILGRIM NUCLEAR POWER STATION 05000-293 2002 003 00

SAFETY CONSEQUENCES

The condition posed no threat to public health and safety.

The problem with the recirculation loop 'A' flow converter did not affect the local power range monitors (LPRMs) because the LPRMs are not flow-biased. Moreover, the problem with the flow converter would not have prevented the rod block function provided by APRMs 'B', 'D', and 'F' because any one of the six APRMs is capable of initiating the rod block function.

The recirculation loop 'A' flow converter provides input to the Flow Control Trip Reference (FCTR) circuitry of APRMs 'A', 'C', and `E'. The impact of the flow converter failure was that the RPS trip channel 'A' APRM flow-biased trip function was non-conservative (inoperable) for the neutron flux (flow-bias) trip function for about 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> on October 2 (beginning at 1227 hours0.0142 days <br />0.341 hours <br />0.00203 weeks <br />4.668735e-4 months <br />) to October 3, 2002 (until 0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br />), while RPS trip channel 'A' was not tripped. The problem with the recirculation loop 'A' flow converter did not affect the operability of the RPS trip channel 'B' APRMs for the neutron flux (flow-bias) trip function.

The APRM flow-biased scram defines the boundary of the Exclusion Region for reactor core stability purposes. The safety function of the Exclusion Region (scram region boundary) of the reactor core power- flow map is to prevent operation where it is possible for thermal-hydraulic instability events to occur. The basis and methodology for this safety function is documented in NEDO-32339-A Rev. 1, Long-Term Stability Solution, Enhanced Option 1-A. The following assessment addresses transients that could have challenged the safety function.

There are 3 types of transients that can affect reactor core thermal-hydraulic stability: reactor power increases, feedwater temperature reductions, and flow reductions. Because the condition existed when the plant was operating at rated conditions (100% reactor power, 83% core flow) and the APRM scram clamp setting (120%) is not flow-biased, the first 2 transients are not applicable. The flow reduction events that could have occurred during the October 2, 2002 (1227 hours0.0142 days <br />0.341 hours <br />0.00203 weeks <br />4.668735e-4 months <br />) — October 3, 2002 (0150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br />) timeframe while RPS trip channel 'A' was not in a tripped condition are: a trip of one or both recirculation pumps or a runback of both recirculation pumps.

  • For a trip of one recirculation pump, the reactor power and core flow end state would have been in the Restricted Region of the reactor power-flow map. The APRM flow-biased scram safety function would not have been challenged for this transient.
  • For a trip of both recirculation pumps, the reactor core flow would decrease to natural circulation flow conditions. The RPS trip channel 'A' APRM flow-bias trip function would have been delayed but the RPS trip channel 'B' APRM flow-bias trip function was not affected and therefore, the trip of both recirculation pumps would actuate the RPS trip channel 'B' scram function at 40% core flow. Based on the information in the Enhanced Option I-A Methodology, instability develops after termination of the flow reduction and feedwater temperature equalizes at a lower value, in about 5 — 7 minutes. An automatic scram (RPS trip channels 'A' and 'B') would occur before the lower feedwater temperatures were achieved.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

  • For a runback of both recirculation pumps, the Pilgrim Station recirculation MG sets/pumps each have two speed limiters (Speed Limiter #2 and Speed Limiter #1). Speed Limiter #2 has a setting of 44% rated recirculation pump speed. A runback of both recirculation pumps to 44% speed would have terminated in the Restricted Region of the reactor power-flow map, but would not have challenged the scram safety function. Speed Limiter #1 has a setting of 26% of rated recirculation pump speed, which is the minimum recirculation pump speed. For this assessment, it was assumed that a runback of both recirculation pumps to 26% (minimum speed) would have terminated in the Exclusion Region of the reactor power-flow map and consequently, may not have resulted in an automatic scram (RPS trip channels 'A' and 'B' tripped) at the Exclusion Region boundary before achieving lower feedwater temperature.

The safety consequences of the non-conservative flow-biased scram function of APRMs 'A', 'C', and `E', resulting from the failure of the recirculation loop 'A' flow converter, is low due to the defense-in-depth features of the Enhanced Option 1-A Stability Solution. The flow reduction transients that terminate in the Restricted Region of the reactor power-flow map would have actuated the Restricted Region entry alarm because the RPS trip channel 'B' APRMs were not affected. The alarm would alert the licensed operators to follow approved procedures to exit the Restricted Region.

Defense-in-depth features could also have terminated a minimum speed flow reduction transient before an instability event occurred. For a runback of both recirculation pumps to the minimum speed (Speed Limiter #1 setting), the Restricted Region Entry alarm would also alert licensed operators that the reactor was in an operating state where manual actions are required to exit the Restricted Region. RPS trip channel 'A' might not have tripped due to the impact of the recirculation loop 'A' flow converter on APRMs 'A', 'C', and `E'. The FCTR (APRM flow-biased scram) setting, however, has a linear slope section above the highest rod line above the Restricted Region. This section of the linear flow-biased setting is set closer to the highest power- flow control line to initiate a scram in the event of a power transient in the Restricted Region. Thus, if core conditions were changing such that unstable core conditions were to be approached, the resulting reactor power increase would likely cause APRMs 'A', 'C', and `E' to trip and result in an automatic scram (RPS trip channels 'A' and 'B' tripped). If unstable core conditions were approached and APRMs 'A', 'C', and `E' did not trip, RPS trip channel 'B' would have initiated a scram signal (half-scram tripped condition) alarm when core flow decreased below 40%. This alarm would alert operators that an automatic scram (RPS trip channels 'A' and 'B' tripped) should have occurred and the licensed operators would have initiated a manual scram in accordance with training and approved procedures. Finally, the LPRMs are spatially located throughout the reactor core to detect neutron conditions. The LPRMs are not flow-biased and were not affected by the recirculation loop 'A' flow converter. The LPRMs input to the PBDS. The PBDS would be operable and if instability conditions were to develop and actuate the PBDS HI-HI alarm, the licensed operators would have initiated a manual scram in accordance with training and approved operating procedures.

Therefore, the safety consequences of the non-conservative (inoperable) flow-bias settings for APRMs 'A', `C', and `E' are considered to have been low. For all cases of flow reduction events, defense-in-depth features were available and would increase the likelihood a flow reduction event would have been terminated manually or automatically, prior to the occurrence of an instability event.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

REPORTABILITY

This report was submitted in accordance with 10 CFR 50.73(a)(2)(i)(B) because the timeframes for taking the actions specified in Technical Specifications Table 3.1.1 were exceeded.

This report is also submitted in accordance with 10 CFR 50.73(a)(2)(v)(A) because the inoperable recirculation loop 'A' flow converter caused the RPS trip channel 'A' APRMs (flow-bias) to be inoperable.

Consequently, an automatic scram would not have occurred as a result of entering the Exclusion Region as assumed in the Enhanced Option I-A stability solution while RPS trip channel 'A' was not tripped.

SIMILARITY TO PREVIOUS EVENTS

A review was conducted for similarity that involved the recirculation flow converters. The review identified Converters." The cause was not similar to the cause(s) described in this report.

ENERGY INDUSTRY IDENTIFICATION SYSTEM (ENS) CODES The El IS codes for this report are as follows:

COMPONENTS CODES

Alarm ALM Converter CNV Monitor (APRMs, LPRMs) MON

SYSTEMS

Incore/Excore Monitoring System IG Plant Protection System (RPS) JC Reactor Core System AC Reactor Recirculation System AD

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