05000390/LER-2001-001
| Event date: | 06-29-2001 |
|---|---|
| Report date: | 08-17-2001 |
| 3902001001R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 lul
I. PLANT
The time
II. DESCRIPTION
A.
B.
C.
CONDITION(S)
unit was in Mode was 588 degrees
Event:
OF EVENT
1 at 100 % power. � Plant operating temperature at the F with reactor coolant system pressure at 2235 psig 2001, at 1728, Watts Bar Unit 1 was in Mode 1 at 100% power.
the unit was manually tripped when the main condenser (Energy System (EIIS) code COND) back-pressure reached The rise in the back-pressure was caused by reduced condenser (CCW) (EIIS code NN) flow which was due to several sections (EIIS code CTW) PVC fill material obstructing the intake flume CCW pumps. All safety systems responded as required during the Feedwater (EIIS code BA) initiated due to reactor trip with low All control rods inserted properly.
Components, or Systems that Contributed to the Event:
On June 29, At this time Industry Identification procedure limits.
circulating water of cooling tower screens to the event. � Auxiliary Tavg, as required.
Inoperable Structures, None.
Dates and Approximate Times of Major Occurrences:
The following events occurred on June 29, 2001.
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- � ;:::;:- 1 000 + CCW pumps' suction pressure reported to be 3.5psig and level drop across trash rack to be -' 1 foot.
1230 Maintenance assigned task to clean cooling towers trash racks.
1235 Maintenance conducted prejob briefing with crew 1330 Crew notified Main Control Room (MCR) that they were going to clean the cooling towers trash racks.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Time Activity 1335 N . Crew noted that the CCW inlet flume was 3 feet lower than normal. N (Based on interviews this was not considered unusual based on previous experience) 1350 On lifting the U1 Rack, large pieces of the black plastic fill material were noted on the trash rack. Trash rack was lifted out of the flume and placed on the ground for cleaning. Note:
due to velocity and opacity of the water, visibility is less than one foot based on the interviews with the maintenance crew.
A 4-foot piece of plastic (fill) was observed by one of the crew to have passed into the Unit 1 flume.
1400 The MCR received call from the maintenance crew cleaning the cooling tower's racks that plastic tower fill material was being removed from U1 rack and communicated that an operator may want to look at the material being recovered.
1413 Maintenance notified the MCR that trash racks were cleaned.
1544 MCR received condenser vacuum low alarm. Alarm Response Instruction for low condenser vacuum and low hotwell level was entered and response taken. Hotwell makeup was taken out of automatic and put into manual due to dropping hotwell level (makeup flow increased to —1500 gpm). Dispatched Auxiliary Unit Operators (AUOs) to investigate cause of alarm.
1545 Started C Vacuum Pump (3rd pump).
1547 Low hotwell level and low condenser vacuum alarm cleared.
1548 AUOs found no water leaks or vacuum leaks; turbine seals, boot seal, feed pumps and condensate were acceptable, no problems found.
1600 - 1615 AUOs looking for Condenser Vacuum leaks — MFWPT condenser vacuum was at 18 inches of vacuum. N Normally it is at 20-21 inches of vacuum.
1630 - 1645 AUOs reported all CCW pumps at 245-250 amps on 6.9 Unit Board; changed from 220-230 amps for normal operation.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Time Activity 1648 1A CCW pump suction pressure started to fluctuate between 1.8 and 2.5 psig with amperage at 225 to 250. Other CCW pumps at 2.5 psig. � Based on interviews, this is approximately when the operating staff became aware that a boiler maker observed a four foot section of the fill enter the Unit 1 CCW flume.
1650 Delta P on water box low, found 11 degree increase on water box outlet temperature (122 to 133 degrees) and returned to normal.
CCW pump outlet pressure dropped from 43 to 28 psig then returned to 43 psig, 1A-CCW pump amps started to swing.
1655 MCR crew held briefing as contingency if rapid load reduction would be required for vacuum control. MCR supervisor discussed SOER-94-1 and the need for conservative decision making.
1709 Lowered Turbine control to get off Valve Position Limiter in preparation for load decrease and to remove 1A CCW Pump.
1710 System Engineer suggested shutting 1A-CCW Pump down while suction pressure continued to fluctuate every few seconds.
1725 CCW 1A discharge pressure swing of 15-50 psig occurred and continued for 45 seconds to 1 minute.
1727 Condenser back pressure increased to 6.5 inches Hg.
1727 Entered A01-39 and started a load reduction at 5%/minute to remove 1A CCW Pump to drop 10% load.
1728 Manual Reactor Trip and AFW started.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390
JI NUMBER
D. Other Systems or Secondary Functions Affected:
a number of sections of tower basin. T Several pieces while these screens were condenser circulating during this event starting at and no inappropriate between the clearer with respect to the and there should have Main Control Room transient using abnormal vacuum and rapid load 1) Emergency ES-0.1, Reactor Trip Operating Instruction and GO-6, Shutdown from Hot correctly followed, and the making process in response increasing condenser back- it is concluded that the to a normal parameter vacuum pump was manually tripping the reactor limit.
None.
E. Method of Discovery:
was determined that into the cooling flume screens resulted in reducing performance 2001, was professional However, communications should have been of the trash rack cause(s) was taken.
to the plant loss of condenser trip, Operations entered:
Safety Injection, 2) and 3) General to Hot Standby procedures were in Mode 3.
decision CCW flow and parameter trends, had been restored the third condenser action in back-pressure Subsequent to the plant event, it cooling tower fill material had fallen of this fill material got past the intake receiving their weekly cleaning which water (CCW) flow.
F. Operator Actions:
Maintenance and Operations personnel 0900 hrs the morning of June 29, personnel actions were identified.
Maintenance and Operations crews quantity of debris found during lifting been greater sensitivity to this occurrence.
Immediate response to identify the (MCR) personnel responded appropriately operating instructions which address reduction. T Upon turbine/reactor Procedure (E-0), Reactor Trip or Response, A01-1 7,Turbine Trip Response GO-5,Shutdown from 30% Power Standby. Emergency and abnormal plant was placed in a stable condition The operators demonstrated a conservative to the initial symptoms of decreased pressure. T In review of the pre-trip operating staff believed the plant values about 1 550 on June 29 (after started). T The operators took appropriate based on the reaching the condenser FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 G. � Safety System Responses:
required during the event. Auxiliary Feedwater low Tavg, as required. All control rods inserted Event and Assessment Of Safety further discussions.
the main condenser back-pressure reached rise in the back-pressure was caused by (CCW) flow which resulted from cooling intake flume screens to the CCW pumps. All during the event.
inadequate design output. Note 2 of Design issued in 1996, allowed support issued design document to be used as needed replacement. This output (note 2) was support installation following failure of a Supports installed at that time did not agree based on design output, no approval was needed details. The supports installed at that time which failed, leading to this event. The was a combination of deadweight loads to the water spray, loads from fallen drift to differential movement of the cooling tower anchorage points.
craft performing the screen cleaning activity and may have contributed to the delay in in condenser back-pressure.
All safety systems responded as initiated due to reactor trip with properly. See the Analysis Of The Consequences sections below for III. � CAUSE OF THE EVENT
A. Immediate Cause:
The unit was manually tripped when limits required by procedure. The reduced condenser circulating water tower fill material obstructing the safety systems responded as required
B. Root Cause:
The root cause of this event was Change Notice (DCN) R39027 (R-type), schemes other than those in the for cooling tower fill repairs and/or referenced in the work order for concrete beam in the Fall of 2000.
with existing details. � However, to deviate from the design output (U1C3 outage) are the same supports most probable cause for that failure from the fill, operating loads due eliminator panels, and loads due structures, leading to failure of the C. Contributing Factor:
The communication between the the operations personnel in the MCR recognizing the source of the rise FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3)
IV. ANALYSIS OF THE EVENT
The immediate post-trip response of the reactor and the associated safety systems were as expected.
The CCW pumps exhibited erratic motor amps, flow, head, and power prior to the manual reactor trip. The cause of this condition was low suction pressure due to PVC fill obstructing the intake flume screen. This condition was common to all 4 pumps and was most significant on 1A CCW pump due to suction conduit arrangement. Following the reactor trip, the 1A CCW pump was returned to service and operating parameters did not indicate damage to the pump.
The effect of elevated C Zone Condenser back pressure on C Low Pressure Turbine was evaluated and it was determined that the cumulative effect of the back pressure exceeding 6.2 inches Hg does not require immediate non destructive examination prior to returning the turbine to operation.
A post-trip inspection of the Turbine Building piping was performed by Civil Engineering following the event. No significant structural damage due to unanticipated transient events occurred.
An inspection of the Unit 1 Cooling Tower was performed by Civil Engineering and found that the tower was structurally sound apart from the failed fill supports.
V. ASSESSMENT OF SAFETY CONSEQUENCES
In response to plant status, the Operations personnel manually tripped the reactor, which initiated a turbine trip. Operators responded in accordance with Emergency Operating Instructions E-0, Reactor Trip; ES-0.1, Reactor Trip Response; A01-17, Turbine Trip. The action of the operators was consistent with plant protection and the proper control of plant cooldown.
There were no safety implications to the public related to the event. The only ESF equipment actuation was an AFW start on the reactor trip concurrent with low Tavg, as required which meant that immediate post-trip heat removal was accomplished via the normal method using auxiliary feedwater through the normal heat removal path which consists of the main condenser. Although not utilized during the post-trip recovery, the standby main feedwater pump was available following FWI reset.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 I N REVISION FSAR section 15.2.7 describes the LOSS OF EXTERNAL ELECTRICAL LOAD AND/OR TURBINE TRIP event. The plant trip on June 29, 2001, was less challenging than and bounded by the event described in the FSAR. The following plant conditions were bounded by the event described in the FSAR:
1. Reactor power was equal to or less than the analyzed value used in the FSAR.
2. Reactor control was in automatic versus manual as described in the FSAR.
3. Steam dumps operated as designed. The FSAR does not take credit for their use.
In summary, the reactor trip was manual. The reactor trip resulted in a turbine trip and station power was not lost during the event. The plant response remained within the FSAR boundary analysis. The pressurizer power operated relief valves and safeties were not required to limit Reactor Coolant System (RCS) (EllS code AB) pressure.
Similarly, the steam dumps and AFW operated as required so that steam generator power operated reliefs was not required. RCS pressure and loop average temperatures decreased during the transient rather than increasing as predicted by conservative FSAR assumptions. These differences between the FSAR and the plant event are associated with the conservatism of the FSAR analysis and the actual plant event which was quickly brought to a stable condition.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions:
These following actions are tracked under the TVA's corrective action program and therefore, are not considered to be regulatory commitments.
Operations responded to the plant transient in accordance with appropriate plant procedures.
Subsequent to the event, the Unit 1 Cooling Tower Basin and intake flume were inspected and the PVC fill removed. The Unit 1 Cooling Tower was inspected to identify other loose or poorly supported fill material. The identified material was removed. in addition, the west waterbox was drained and inspected for PVC fill material. N No significant accumulation was observed.
The 1 A CCW pump and motor were verified to be operating satisfactorily based upon observation of motor amps, suction pressure, flow, bearing and winding FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) temperatures.
Non-licensed assistant unit operator (NAUO) rounds for the outside routine were revised to define criteria for requiring the cooling tower trash rack to be cleaned.
In addition, a preventative maintenance instruction was issued to provide steps for cleaning of the trash racks and to define a criteria for level drop across the trash rack at which Operations must concur with the removal of the rack for cleaning.
A site bulletin was issued on sensitivity to communications in which abnormal parameter values or observations are being communicated.
A memorandum was issued to brief design engineering personnel which addresses lesson learned from this event with respect to design engineering practices.
The design control process no longer allows R type DCNs. However, a review of other R-DCNs issued during this time was completed with no other similar issues identified.
B. � Corrective Actions to Prevent Recurrence:
These following actions are tracked under the TVA's corrective action program and therefore, are not considered to be regulatory commitments.
A design change notice will be issued to clarify the design support requirements for fill material support and for trash rack extension modifications. At the appropriate time, the missing fill material will be restored to the Unit 1 Cooling Tower with adequate support features approved by TVA Engineering.
VII. � ADDITIONAL INFORMATION A. � Failed Components:
The root cause of the event was determined to be inadequate design output that led to cooling tower fill material supports installed in 1996 to fail, leading to FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) this event. The most probable cause for the failure was a combination of deadweight loads from the fill, operating loads due to the water spray, loads from fallen drift eliminator panels, and loads due to differential movement of the cooling tower structures, leading to failure of the anchorage points.
B. � Previous LERs on Similar Events:
A review of previous reportable events for the past three years was performed.
This manual plant trip was necessary due to low condenser vacuum caused by the presence of fill material obstructing the intake flume screens to the CCW pumps. Although the physical cause of low condenser vacuum was unique to this event, WBN has experienced low condenser vacuum resulting in reactor trip in two other LER instances. These two other events are summarized below:
1) Trip date March 13, 1996 - LER date April 11, 1996 Flow blockage through main condenser due to buildup of non- condensables because MFPT 1B sealing steam building up and being drawn into vacuum pumps.
2) Trip date February 19, 1996 - LER date March 21, 1996 Faulty hotwell indication and control resulting in insufficient makeup to hotwell.
Based on the review of the above LER, the failure mechanisms of this event and the previous events are not similar.
C. � Additional Information:
None D. � Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with NEI 99-02, Section 2.2.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 05000390 E. � Normal Heat Removal of normal heat removal in accordance with This event did not result in the loss NEI 99-02, Section 2.1.
VIII. � COMMITMENTS None.