05000346/LER-2003-011

LER-2003-001,

Davis-Besse Unit Number 1
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3462003001R01 - NRC Website

DESCRIPTION OF OCCURRENCE:

On September 17, 2003, with the plant in Mode 3 and the Reactor at 0 percent power, a non-nuclear heat-up teat of the Reactor Coolant System (RCS) [AB] was in progress. This test consisted of heating the RCS to Normal Operating Pressure and Normal Operating Temperature (NOP/NOT), for approximately seven days (reference Serial 2973 for additional information on the NOP/NOT test). At 0336 hours0.00389 days <br />0.0933 hours <br />5.555556e-4 weeks <br />1.27848e-4 months <br /> on September 17, 2003, Containment Spray (CS) Pump [BE-P] 1 was declared inoperable and Technical Specification 3.6.2.1 was entered due to closing the Lower Spray Header Isolation valve during the line-up for recirculation of the Borated Water Storage Tank (BWST) [BP-TK]. A start of CS Pump 1 was scheduled to be performed separate from the NOP/NOT test to obtain various operational data of the CS Pump.

However, at 0350 hours0.00405 days <br />0.0972 hours <br />5.787037e-4 weeks <br />1.33175e-4 months <br />, when a start of CS Pump 1 was attempted, the breaker closed and then immediately opened. Therefore, CS Pump 1 remained inoperable following the test due to the failure to start on demand (via remote control switch).

Technical Specification 3.6.2.1, Containment Spray System, requires two independent Containment Spray Systems be Operable in Modes 1 through 4. With one CS System inoperable, Technical Specification 3.6.2.1 requires the inoperable system be restored to operable status within 7 days or initiate a plant shutdown/cooldown.

The Containment Spray System sprays borated water from the BWST into the Containment Rail atmosphere, after a design basis (full power) loss-of-coolant accident (LOCA), to cool, condense, and reduce the pressure of the post-LOCA Containment atmosphere. After the injection phase of system operation, the system sprays water, which is treated with Trisodium Phosphate to raise the pH of the sump water (to 7), from the Containment Emergency Sump.

In response to the failure of CS Pump 1 to start, a problem solving team which had evaluated a previous breaker failure was re-initiated. The subsequent Problem Solving Plan (for the CS Pump 1 breaker trip) identified four recent occurrences in which either CS Pump 1 or CS Pump 2 failed to start (two occurrences for each, including the September 17, 2003 failure of CS Pump 1), On July 10, 2003, with the plant in Mode 5, CS Pump 2 motor tripped immediately (first of four recent trips, failure one of two for CS Pump 2) upon startup, during post maintenance testing. CS Pump 2 was declared inoperable. Corrective action for the July 10, 2003 failure of CS Pump 2 was to increase the circuit breaker instantaneous trip set point (this action was implemented on both CS Pump 1 and CS Pump 2 breakers). Based on the observed immediate trip reported by Operations and on actuation of the breaker bell alarm device (which opens or closes when the breaker trips because of overcurrent), the motor winding and cable were tested to verify that a fault did not exist. Because no evidence of a fault existed, it was believed that the CS Pump 2 breaker trip was caused by false operation of the Solid State Trip (SST) instantaneous function.

The SST system protects the CS Pump Motor with the following trip functions: Long- Time, Instantaneous and Ground Fault. The Instantaneous trip protects against short circuits of the motor winding or feeder cable.

DESCRIPTION OF OCCURRENCE (continued):

Following the trip on duly 10, 2003, CS Pump 2 was returned to service and operated successfully (at least eight starts) before tripping again on September 11, 2003 (see below).

On September 5, 2003, with the plant in Mode 5, while performing testing, CS Pump 1 failed to start and was declared inoperable (second of four recent trips, failure one of two for CS Pump 1). Corrective action for the September 5, 2003, failure of CS Pump 1 was to increase the circuit breaker instantaneous trip set point again (this action was implemented on both CS Pump 1 and CS Pump 2 breakers). Because 480 volt bus voltages had been increased due to transformer tap changes (reference Davis-Besse LER 2003-007-00) on August 30, 2003, it was believed that the higher bus voltage may have contributed toward a higher initial motor starting current causing the instantaneous trip function actuation.

Following the trip on September 5, 2003, CS Pump 1 was returned to service and operated successfully (at least five starts) before tripping again on September 17, 2003.

On September 11, 2003, with the plant in Mode 5. an attempt to start CS Pump 2 resulted in a trip of the CS Pump 2 breaker (third of four recent trips, failure two of two for CS Pump 2). CS Pump 2 was declared inoperable. A problem solving team was assembled for the failure of CS Pump 2 breaker. Because the two instantaneous trip set point changes had not prevented recurrence of the actuations, the problem solving team believed that the condition was confined to the Solid State Trip (SST) system hardware through the process of elimination.

Corrective action for the September 11, 2003, failure of CS Pump 2 was to replace the circuit breaker (the replacement breaker has not failed since it was installed on September 12, 2003). The Problem Solving Team determined that CS Pump 2 could be placed back into service because 1) the replacement breaker contained a different SST system (different in the context that the SST was replaced with the same type of SST that originally existed in the previous breaker) and 2) de- energized testing was performed on the motor and cable from the cubicle and proper insulation values for the CS Pump 2 motor and associated cables was confirmed.

During the trip of CS Pump 2 breaker on September 11, 2003, a 3-phase trace of motor starting current was obtained from monitoring equipment connected to CS Pump 2 when it failed to start. The data obtained indicated that motor starting current appeared to be well within the SST system's instantaneous current set point throughout the starting period. Upon recognizing that the previous CS Pump breaker trips may not have been caused by the SST system's instantaneous trip function, the investigation shifted to the SST ground fault function.

The evaluation performed for these four CS Pump breaker failures determined the most likely cause to be a "false trip" of the ground fault function of the circuit breaker overcurrent device (gee Apparent Cause Section). The overcurrent device is a component of the breaker's SST system consisting of current sensors, programmer unit, wiring harness, and flux shift trip device (spring release plunger).

DESCRIPTION OF OCCURRENCE (continued):

As described above, the September 5, 2003, breaker trip identified the failure of CS Pump 1 to start due to an instantaneous trip. Based on subsequent evaluation of the CS Pump 2 September 11 trip, the issue was determined to be the SST ground fault. Therefore, it appears that the SST False Ground Fault Trip susceptibility most likely existed during the transition from Mode 5 to Mode 3 for Containment Spray Pump 1.

Technical Specification 3.0.4 states that entry into an operational mode or other specified applicability condition shall not be made unless the conditions of the Limiting Condition for Operations are met without reliance on provisions contained in the ACTION statements unless otherwise excepted.

Transitioning from Mode 5 to Mode 3 with an inoperable Containment Spray Pump is a violation of Technical Specification 3.0.4 and is reportable in accordance with 10 CFR 50.73(a)(2)(1)(B). 10 CFR 50.73(a)(2)(i)(B) requires reporting any operation or condition prohibited by the plant's Technical Specifications. Because the corrective action for CS Pump 2 (replacement of the circuit breaker) appears to have been effective as evidenced by the 8 ouccessful starts without failure, CS Pump 2 failures are not being reported in accordance with 10 CFR 50.73.

APPARENT CAUSE OF OCCURRENCE:

The cause of the CS Pump 1 failure to start on September 17, 2003, based on investigation and industry experience, has been determined to be the false operation of the circuit breaker SST system. The SST system is believed to have inaccurately sensed and/or processed motor starting current, resulting in a false ground fault trip of the circuit breaker.

A false trip is an unexplained or unwarranted trip of circuit protective device.

The SST system on the breaker does not directly cerise ground faults. Rather the device calculates ground fault current from vector addition of the three individual phase currents. As stated above, the SST for the CS Pumps had the following trip functions (prior to breaker replacement for CS Pump 1): Long-Time, Instantaneous, and Ground Fault. The Ground Fault trip protects against low-level short circuits to ground. Arcing ground faults can occur that are below the level of the instantaneous setting. The ground fault trip was set at approximately 300 amps with trip time of approximately 0.065 to 0.15 seconds (3.9 to 9 cycles). Because Essential 480 Volt AC busses (ED) El and Fl are resistance-grounded with the neutral of the 4160/480 volt delta-Y transformer connected to ground through a 76 ohm resistor, ground fault current is limited to less than 5 amps through a single fault. The Problem Solving Team determined that the ground fault portion of the SST system provided no substantial protection beyond that of the resistance ground and therefore, it was removed in accordance with the modification process.

During the trip of CS Pump 2 on September 11, 2003 monitoring of the motor starting current data revealed that the breaker opened after about 7 cycles. As stated above, the problem solving team recognized that the breakers tripped within the expected time band for a ground fault trip. It appeared the previous CS Pump breaker trips may not have been caused by the SST instantaneous function, and therefore, the investigation shifted to the SST ground fault trip.

APPARENT CAUSE OF OCCURRENCE (continued):

Although testing was performed, the specific cause of a false ground fault trip is inconclusive. It is believed that the SST overcurrent system may be susceptible to false ground fault trips and that this susceptibility may have been increased when 480 Volt Bus voltage was increased. Other possible contributing factors include:

• Variation may exist between the 3 current sensor input-output characteristics causing the SST to calculate a ground fault current when none exists.

• An anomaly in the programmer may result in inaccurate SST vector addition of the 3 phase currents (calculation of ground fault current).

• Electronic noise may produce or amplify an erroneous SST calculation of ground fault current.

• DC offset of the AC starting current may amplify erroneous SST calculation of ground fault current (this factor is random and depends on voltage wave at time of motor start).

ANALYSIS OF OCCURRENCE:

The Containment Spray System is an engineered safety feature and is provided, in conjunction with the Containment Air Cooling System (CACS) IBK), to perform the safety function to reduce rapidly, consistent with the functioning of other associated systems, the containment pressure and temperature following a loss-of- coolant accident and maintain them at acceptably low levels. The CS System also provides an additional safety function to reduce the concentration of fission products released to the environment following postulated accidents.

The Technical Specification required number of operable Containment Air Coolers (CAW was met during this occurrence and throughout the time the plant was in Mode 3 and Mode 4 for the NOP/NOT Test. Therefore, the CAC system would have provided a means of heat removal and Containment atmosphere cleanup post-accident while in Mode 3. Also, as stated above, because CS Pump 2 remained operable, the CS System was still capable of performing its safety function with operation of CS Pump 2.

The Corrective Action taken (replacing the breaker with a breaker with a different SST system) was effective as verified by at least eight successive starts without failure of CS Pump 2.

The most significant consequence of this false trip is that CS Pump 1 did not start and run upon demand (as occurred on September 5, 2003, and September 17, 2003).

Between July 10, 2003 and September 17, 2003, prior to the breaker being replaced without the ground fault function (eliminating the ground fault trip function), CS Pump 1 successfully started at least six out of eight start attempts. This shows that there were successful starts of the CS Pump during this time period.

The unavailability of a CS pump has no effect on the Core Damage Frequency (CDF) because CS is not a system credited for mitigating Core Damage. CS is however, credited in the Probabilistic Safety Assessment (PSA) for the mitigation of releases from containment and consequently the unavailability of a CS pump will increase Large Early Release Frequency (LERF). The increase in LERF is negligible with a single CS pump unavailable. Results of the Level 2 PSA have shown the CS ANALYSIS OF OCCURRENCE (continued):

system is not significant with regards to either Risk Achievement Work (RAW) or Fussell-Vessely (FV) importance measures. This result is a reflection of the amount of redundancy in containment cooling systems. There are two systems, CS and CACS that provide diverse means of containment heat removal. As a consequence, the LERF importance measures tend to be most significant for support systems, such as Service Water IBI], because loss of these systems can both lead to core damage and loss of all containment cooling. The contribution to LERF caused by individual component failures following a core damage sequence with an independent initiating event, is very low. Therefore, the unavailability of a CS pump for the length of time described in this report will have a negligible effect on Large Early Release Probability (LERP).

The extent of condition for the breaker problem involves General Electric 480 volt model AK-25 metal clad circuit breakers installed in the essential and non- essential 400 volt substations. These circuit breakers (with the exception of the breaker for CS Pump 1) have SST systems installed that include a ground fault function. Based on events and test data, the probability of a false ground trip increases with magnitude and extended duration of inrush current, such as caused by large motors.

The 200 hp CS Pump motor starting current is approximately 1500 amps-rms and is the only load that has experienced false trips. The CS Pump motor is the largest single motor load on either 480 volt bus El or Fl. The next largest load is the Containment Air Cooler (CAC). These motors are rated 150/40 hp (fast/slow speed). The CACs receive a Safety Features Actuation System (SFAS) signal to start only in the slow speed (40 hp) mode. There have been no known false trips of the CACs in high speed (150 hp) mode, and it is further unlikely that a false trip will occur during an automatic (SFAS) start in slow speed (40 hp) mode. Additionally, four circuit breakers that feed Motor Control Centers (MCCs) were originally identified as being susceptible.

However, based on an evaluation performed to determine the extent of condition of CS Pump false trip, the four MCCs were concluded to not be susceptible based on differences in application, event history and electrical load characteristics.

CORRECTIVE ACTIONS:

Corrective action for CS Pump 1 was to replace the breaker that contained the SST system with a breaker without the ground fault function, eliminating the ground fault trip function.

The corrective action performed for CS Pump 2 for the failure on September 11, 2003, was to replace the circuit breaker. This action effectively placed a different SST system into service (different in this context means that the SST was replaced with the same type of SST which originally existed in the previous breaker). This action is considered remedial because present operating practice permits interchanging circuit breakers between cubicles. Although the breaker installed following the September 11, 2003, failure for CS Pump 2 has been tested several times to gain confidence that it would start when necessary, there was no process to ensure that this circuit breaker would not be replaced with a different NRC FORM366A (T-2001) FACILITY NAME (1) Davis-Besse Unit Number 1 DOCKET (2) � LER NUMBER (6) 05000346 � � CORRECTIVE ACTIONS (continued):

breaker/SST combination. Therefore, a preventative action was taken to replace the programmer unit of the SST system on the circuit breaker for the CS Pump 2 with a programmer unit that does not include a ground fault function.

The problem solving team concluded that although the cause of a false ground trip could not be definitively identified, eliminating the ground fault function from the SST would prevent recurrence of the problem. As discussed above, the essential 480 volt AC busses El and Fl are resistance-grounded, and fault current is limited to less than 5 amps. Because the ground fault trip is set at around 300 amps it was determined this portion of the SST provides no substantive protection beyond that of resistance ground. The problem solving team recommended that an engineering change be made to eliminate the ground fault function of the SST for CS Pump 1 in conjunction with replacing the breaker (and later, as stated above it was decided to generate a corrective action to replace the SST programmer unit on the circuit breaker for the CS Pump 2 with one that does not contain a ground fault unit).

FAILURE DATA:

There have been no License Event Reports in the past three years submitted from Davis-Besse Nuclear Power Station, reporting an event due to the inoperability of a Safety related piece of equipment due to a "false trip.

No other previous similar events are known to have occurred in the last three years for which corrective action could have been expected to prevent this occurrence.

Energy Industry Identification System (EIIS) codes are identified in the text as [100.

NP-33-03-011-00 � CRs 03-07794, 03-05464, 03-07351, 03-07608, 03-10376