05000461/LER-1995-001, :on 950105,FW Heater Train Operation During Removal of FW Heater from Svc Was Not Understood & Resulted in Decreased FW Temp,Loss of Heater Level Control & Manual Scram.Plugged 5B FW Heater Tubes
| ML20078M098 | |
| Person / Time | |
|---|---|
| Site: | Clinton  |
| Issue date: | 02/03/1995 |
| From: | Chear R, Jackie Cook ILLINOIS POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| JGC-049-95, JGC-49-95, L45-95(02-03)LP, L45-95(2-3)LP, LER-95-001, LER-95-1, U-602401, NUDOCS 9502130243 | |
| Download: ML20078M098 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(viii) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(x) |
| 4611995001R00 - NRC Website | |
text
ilknois Power Company Chnton Power Station P.O. Box 678 Chnton. IL 61727 Tel 217 935 5623 Fax 217 935 4632 l
John G. Cook i Vice President -
ILBJN9BS u-6o24oi P4tM R iss-95(o 2 -0 3 )te 2C.220 JGC-049-95 February 3, 1995 Docket No. 50-461 10CFR50.73 Document Control Desk Nuclear Regulatory Commission Washington, D.C. 20555
Subject:
Clinton Power Station - Unit 1 Licensee Event Reoort No. 95-001-00
Dear Si.:
Enclosed is Licensee Event Report No. 95-001-00: Lack of Understanding of Feedwater (1'W) lienter Train Ooeration During Remov. of FW lleater from Service Results in Decreasing FW Temnerature. Loss ofIleater Level Control. and Manual Scram This. report is being submitted in accordance with the reouirements of 10CFR50.73.
Sincerely yours, V
. Cook Vice President RSF/csm Enclosure ec:
NRC Clinton Licensing Project Manager NRC Resident Office, V-690 Regional Administrator, Region III, USNRC lilinois Department of Nuclear Safety INPO Records Center
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.NRC FO C 366 U.S. NUCLLAR REGULATORY COBBEISSION APPROVED BY OMB NO. 3150-0104 (5 92)
EXPIRES 5/31/95 ESTIMATED BURDEN PER RESPONSE TO COAOLY WITH THIS INF RMATION COLLECTION REQUEST:
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LICENSEE EVENT REPORT (LER)
FORWARD COMMENTS REGARDING BURDEN ESTIMTE TO THE INFORMATION AND RECORDS MANAGEMENT BRAh0W (MNBB (See reverse for required ruber of digits / characters for each block)
W 1 GT DC 2 555 0001 ND THE ER K REDUCTION PROJECT (3150-0104),
OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.
FACILITY NAME (1)
DOCKET NUMBER (2)
PAGE (3)
Clinton Power Station n5000461 1 OI: 6 l
ilTLE (4) Lack of Understandmg of Fecdwater (IV)llenter Train Operation Dunng Removal of I'W IIcater from semcc Results in Decreasing IV
)
i Temperature. loss ofIIcater I evel Control, and Manual Scram EVENT DATE (5)
LER IKSEER (6)
REP 01 T DATE (7)
OTHER FACILITIES INUOLVED (8) i MONTH DAY YEAR YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACILITY NAME DOCKET NUMBER NUMBER NUMBER None 05000 01 05 95 95 001 00 02 03 95 None OPERATING THIS REPORT IS SURMITTED PURSUuff TO THE RERJIREENTS OF 10 CFI 4: (Check one or more) C11)
MODE (9) 1 20.402(b) 20.405(c) y 50.73(a)(2)(iv) 73.71(b)
POWER 20.405f a)(1)f11 50.36feif1) 50.73fa)(2)(v) 73.71(c)
LEVEL (10) 79 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vil)
OTHER 20.405(a)(1) fill) 50.73(alt 2)ti) 50.73(alt 2)(viii)(A)
(specifyin Abstract 20.405(a)(1)(iv) 50.73(a)(2)(li) 50.73(a)(2)(viii)(B>
below and in Text.
20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x)LICENSEE CONTACT FOR THIS LER (12)
NAME TELEPHONE MUMBER (include Area Code)
R. T. Chear, Plant Engineer (217) 935-8881, Extension 3664 COMPL !TE ONE LINE FOR EACH CCBIPollENT FAILURE DESCRIBED IN THIS REP (EtT (ii)
CAUSE
SYSTEM COMPONENT M.*NUFACTURER REPORTABLE
CAUSE
SYSTEM COMPOWENT MANUFACTURER REPORTABLE TO NPRDS TO NPRDS l
X EL RLY G082 Y
SUPPLEMENTAL REPORT EXPECTED (141 EXPECTED MONTH DAY YEAR YES y
NO SUBMISSION 1
(If yes, complete EXPECTED SUBMISSION DATE).
DATE (15)
ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single spaced typewritten lines) (16)
With the plant at 100 percent reactor power, operators decided to reduce power to 53 percent of rated thermal power (RTP) and isolate the B low pressure (LP) feedwater heater string to correct degradation related to suspected tube leaks, in the 5B LP heater. With reactor power at 70 percent RTP and extraction steam to the B heater string secured, operators began to slowly open the (LP) heater string bypass valve. While opening the valve, water levels in the 6A high pressure heater and the A side reheater drain tank began to oscillate to oft-scale high causing the 6A heater to isolate from extraction steam. Feedwater temperature decreased 50 degrees and
)
reactor power increased to 79 percent RTP. In response to these conditions, the shift supervisor ordered the initiation of a manual scram. The cause of this event is attributed to insufTicient understanding of heater train operation in an abnormal condition. Personnel uid not fully understand the impact that opening the bypass valve would have on the 6A heater operation. Corrective action for this event includes plugging 5B feedwater heater tubes, revising the system operating procedure to incorporate additional guidance for removing a heater string from service and restoring it, and inspecting tubes in the next refueling outage.
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FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO THE INF RMATI N AND RECORDS MANAGEMENT BRANCH (MNBB TEXT CONTINUATION 7714),
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FACILITY NAME (1)
DOCKET MlMBER (2)
LER NUMBER (61 PAGE (3)
YEAR SEQUENTIAL REVISION NUMBER NUMBER Clinton Power Station 05000461 95 001 00 2OF6 TEXT (if more space is required, use additional copies of NRC Form 366A) (17)
DESCRIPTION OF EVENT
Operations and Plant Engineering had been monitoring the 5B feedwater [SD] low pressure (LP) heater [HX]
since December 12,1994, when tube [TBG) leaks were first suspected. Condition Report 1-94-12-016 and maintenance work request D50131 were initiated to investigate and corret t the condition.
On January 5,1995, the plant was in Mode 1 (POWER OPERATION) at about 100 percent reactor [RCT] power.
The water level on the SB heater began to increase rapidly and Operations decided to remove it from service. A cominon feedwater bypass line is installed between the two strings ("A" an i "B") of LP heaters to enable isolation of the 2,3,4, and 5 heaters on either string.
At 2003 hours0.0232 days <br />0.556 hours <br />0.00331 weeks <br />7.621415e-4 months <br />, in preparation for removing the 2B, 3B,4B, and SL feedwater low pressure heater string ("B")
from service, operators began reducing reactor power by reducing reactor recirculation (RR) system [AD] flow.
Operators planned to reduce reactor power to about 53 percent of rated thermal power (RTP) and then isolate the "B" feedwater low pressure heater string for repair.
At 2l00 hours, with reactor power at about 72 percent of RTP, operators stopped the reactor power reduction using RR flow. At 2122 hours0.0246 days <br />0.589 hours <br />0.00351 weeks <br />8.07421e-4 months <br />, operators began reducing reactor power by inserting control rods. At 2135 hours0.0247 days <br />0.593 hours <br />0.00353 weeks <br />8.123675e-4 months <br />, operators stopped the reactor power reduction using control rods at 70 percent of RTP because of significant oscillations in the water level of the 5B heater.
At 2140 hours0.0248 days <br />0.594 hours <br />0.00354 weeks <br />8.1427e-4 months <br />, operators began removing low pressure heaters 2B,3B,4B, and 5B from service, in accordance with system operating procedure CPS 3102.01, " Extraction Steam /HTR Vent & Drains (ES, HD)," by isolating the extraction steam [SE) supply to the 5B heater.
By 2208 hours0.0256 days <br />0.613 hours <br />0.00365 weeks <br />8.40144e-4 months <br />, operators had successfully secured the extraction steam supply, normal drains [DRN] and other heat sources to the 2B,3B,4B and 5B heaters. Since isolating the 'B' heater string would cause LP heater string bypass valve [V) ICB007 to open fully, operators were concerned that feedwater temperature would decrease more than 50 degrees Fahrenheit (F). Therefore, at 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br />, operators began to open the LP heater string bypass valve slowly, in short increments.
By 2228 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.47754e-4 months <br />, the removal of heat sources from the "B" LP heater string and the partial opening of the LP heater string bypass valve had caused the feedwater temperature to decrease by about 50 degrees F.-
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FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO THE TEXT CONTINUATION INFORMATION AND RECORDS MANAGEMENT BRANCH (MNBB 7714),
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FACILITY NAME (1)
DOCKET NLMBER (2)
LER NUMBER (61 PAGE (3)
YEAR SEQUENTIAL REVISION OF NUMBER NUMBER Clinton Power Station 05000461 95 001 00 3 of 6 TEXT (if more space is required, use additional copies of NRC form 366A) (17)
At 2230 hours0.0258 days <br />0.619 hours <br />0.00369 weeks <br />8.48515e-4 months <br />, while operators continued to open the LP heater string bypass valve, oscillations began to occur in the water levels of the 6A feedwater [SJ) high pressure heater and the "A" side reheater drain tank [TK]. The level in the 6A heater increased to off-scale high, causing automatic isolations of extraction steam to the 6A heater and drain flow from the "A" reheater drain tank. These isolations caused the water level in the "A" side reheater drain tank to increase to oft-scale high and isolations of both moisture separator reheaters from main steam. The level increase in the 6A heater was due to a drop in its shell pressure (caused by colder feedwater temperatures) to less than the 5A heater shell pressure. Thus, the normal heater drain flow (from the 6A heater to the SA heater) reversed direction (from the 5A heater to the 6A heates).
At 2233 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.496565e-4 months <br />, with the plant at about 79 percent of RTP, operatore initiated a manual scram of the reactor as directed by the Operations shift supervisor (SS) by placing the reactor mode switch [HS] into the shutdown position. The SS decided to initiate the scram on the basis that the "B" LP heater string was out of service and feedwater temperature already had decreased more than 50 degrees F, and the 6A heater had unexpectedly isolated due to high water level, causing further loss of feedwater heating.
Reactor water level decreased to the low reactor water level (Level 3), initiating close signals to containment isolation valves [ISV] in Groups 2 (residual heat removal system (RHR) [BO] to upper containment pools), 3 (RIIR shutdown cooling) and 20 (miscellaneous valves).
At 2234 hours0.0259 days <br />0.621 hours <br />0.00369 weeks <br />8.50037e-4 months <br />, reactor water level increased to the high reactor water level (Level 8), initiating a trip of the main turbine.
At 2236 hours0.0259 days <br />0.621 hours <br />0.0037 weeks <br />8.50798e-4 months <br />, operators manually transferred all electrical busses [BU) from the in-house power supply to the ofr-site power supply and manually opened the main generator (GEN] [TB] circuit breakers [52] [FK] to disconnect the generator from the grid. The generator breakers were opened manually because they did not trip on reverse power.
At 2255 hours0.0261 days <br />0.626 hours <br />0.00373 weeks <br />8.580275e-4 months <br />, operators confirmed that containment isolation valves in Groups 2,3, and 20 closed as appropriate in response to the low reactor water level signal by completing oft-normal procedure checklist CPS 4001.02C001,
" Automatic Isolation Checklist."
By about 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br />, the plant was in a stable condition it. Mode 3 (HOT SHUTDOWN).
Maintenance work request (MWR) D58734 was initiated to investigate the failure of the main generator to trip on reverse power..
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FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO THE TEXT CONTINUATION INFORMATION AND RECORDS MANAGEMENT BRANCH (MN88 T714),
U.S.
NUCLEAR REGULATORY COMMIS$10N, WASHINGTON, DC 20555-0001, AND To THE PAPERWORK REDUCTION PkOJECT (3150-0104),
OFFICE OF MANAGEMENT AND BUDGEY, WASHINGTON, DC 20503.
FACILliY NAM (1)
DOCKET IRBEER (2)
LER 9_"R f 61 PAGE (3)
YEAR
$EQUENTIAL REVISION OF NUM8ER NUMBER Clinton Power Station 05000461 95 001 00 4 of 6 Condition report 1 95-01-003 was initiated to track the root cause evaluation and corrective action determination for this event.
The investigation of the main generator reverse power trip failure identified that one of two reverse power relays (RLY][EL] was degraded and incapable of performing its function. The other reverse power relay was out of calibration (on one point, but passed its operational, that is, overall test) and was capable of performing its function. The investigation determined that the reverse power relay scheme operated as designed; the magnitude of the reverse power condition was not sufficient for an adequate time period to automatically cause the generator to trip off.
No other automatic or manually initiated safety system responses were necessary to place the plant in a safe and stable condition. No other equipment or components were inoperable at the start of this event to the extent that their inoperable condition contributed to this event.
CAUSE OF EVENT
The cause of this event is attributed to insufficient understanding of heater train operation in an abnormal condition. Operations and Plant Engineering personnel did not fully understand the !mpact that opening the LP heater string bypass valve has on the 6A feedwater heater operation. The isolation valves for each string of heaters are interlocked with the bypass valve so the bypass valve opens anytime one of the isolation valves is moved more than 10 percent from the full-open position. During this event, operators were fully aware that feedwater ternperature should not be allowed to decrease more than 50 degrees F. Therefore, operators decided to open the bypass valve slowly, in short increments because isolating the "B" heater string would cause the bypass valve to automatically open fully.
Beginning with the 6A (or 6B) heater, each consecutive heater has a lower shell-side internal pressure so drains will flow from the 6A (or 68) heater through the heater string to the 2A (or 28) heater and then to the condenser.
With the LP heater string bypass valve open any significant amount, the temperature of the feedwater that bypasses the low pressure heaters and enters the 6A (or 6B) high pressure heater is much cooler than the temperature at the outlet of the 5A (or 5B) low pressure heater.
In this event, the cooler feedwater entering the 6A heater caused its shell-side pressure to decrease to below the pressure of the SA heater. As a result of this decrease in 6A heater shell pressure, drain flow from the 6A heater to the SA heater reversed direction. The reverse drain flow from the SA heater caused the water level in the 6A heater to increase faster than the emergency drains could respond, causing the water level in the 6A heater and then the water level in the "A" side reheater drain tank to increase to off-scale high. Because of the loss of heater level control, along with the decreased feedwater temperature, the shift supervisor directed operators to initiate a manual scram.
NRC 70RM 366A (5-92)
NRC' FORM 366A U.S. NUCLEAR REGULATORY C0pmISSION APPROVED BY OMB No. 3150-0104 (5-92):
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FORWARD COMMENTS REGARDING BURDEN ESilMATE TO THE TEXT CONTINUATION INFORMATION AND RECORDS MANAGEMENT BRANCH (MhBB 7714),
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NUCLEAR REGULATORY COMMIS$10N, WASHINGTON, DC 20555 0001, AND TO THE PAPERWORK REDUCTION PROJECT (3150-0104),
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FACILITY NAME (1)
DOCKET NUMBER (2)
LER NUMBER f61 PAGE (3)
YEAR SEQUENTIAL REVISION OF NUMBER NUMBER Clinton Power Station 05000461 95 001 00 5 of 6 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)
The cause of the 5B feedwater low pressure heater tube failures is postulated to be vibration-induced tube fretting.
A total of 27 U-type tubes were found damaged in the 5B feedwater low pressure heater,14 tubes were completely severed, seven tubes had substantial openings, two tubes had small fractures without loss of metal, and four tubes had minor leaks but no visible damage.
Plant Engineering and two independent consultants reviewed the 5B heater tube damage and the heater level transient that occurred during this event. The review postulated that one or more tubes had failed due to vibration induced fretting as a result of possible loose fit with the drain cooler end plate holes. The fretting progressed to tube failure at or near the drain cooler end plate and allowed the relatively high pressure feedwater to flow into the bottom of the heater near the level sensor tap. The high pressure feedwater flow on the level sensor caused the level instrument to sense a false high water level condition and the heater level controller to open the drain valve to lower the water level in the heater. The lower level allowed steam to penetrate the drain cooler section of the heater. The presence of steam in the drain cooler increased vibration of other tubes in the vicinity and caused additional tube failures.
CORRECTIVE ACTION
Based on the knowledge gained from removing the "B" string heaters from service during this event and the ensuing plant shutdown, system operating procedure CPS 3102.01 was revised to incorporate additional guidance for removing a heater string from service and restoring it. The revision includes a provision for installing a temporary modification to remove the interlock between the LP heater string bypass valve and the heater string isolation valves when preparing to rernove a heater from service while the plant is at power. This procedure change will reduce the amount of feedwater flow bypassing the operating train and thus will minimize the total temperature reduction of the feedwater.
The 27 damaged heater tubes and 29 adjacent, non-damaged tubes were plugged in the 5B heater in accordance with MWR D50131.
Additional tubes in the 5B heater will be tested for wall thinning during the fifth refueling outage.
ANALYSIS OF EVENT
This event is reportable under the provisions of 10CFR50.73(a)(2)(iv) due to the manual initiation of the reactor protection system [JC] (scram).
)
NRCcFORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMc NO. 3150-0104 (5-9/)r EXPIRES 5/31/95 ESilMATED BURDEN PER RESPONSE TO COMPLY WITH THl3 INFORMATION COLLECTION REQUEST:
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LICENSEE EVENT REPORT (LER)
FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO THE TEXT CONTINUATION INFORMATION AND RECORDS MANAGEMENT BRANCH (MNBB 7714),
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NUCLEAR REGULATORY COMMIS$10N, WASHINGTON, DC 20555 0D01, AND TO THE PAPERWORK REDUCTION PROJECT (3150-0104),
OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20503.
FACILITY NAME (1)
DOCKET NLMBER (2)
LER NUMBER (61 PACE (3)
YEAR SEQUENTIAL REVISION OF NUMBER NUM8rR Clinton Power Station 05000461 95 001 00 6 of 6 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)
Assessment of the safety consequences and implications of this event identified that this event was not nuclear safety significant. The event was compared to the Loss of Feedwater Heating and Turbine Trip transients discussed in Chapter 15 of the Updated Safety Analysis Report (USAR) and was determined to be within the design basis of the plant.
ADDITIONAL INFORMATION
The relay that was found degraded during the investigation of this event was a 120-volt,5-amp,60-hertz reverse power relay, part number 12GGP53Cl A, manufactured by General Electric Corporation.
The 5B feedwater low pressure heater is a horizontal shell/ tube heat exchanger manufactured by Westinghouse Electric Corporation. The heat exchanger contains 20 gauge,0.75-inch outside diameter, SA688, type 304 stainless steel U-type tubes and a 9.125-inch thick,64-inch diameter, SA105 tube sheet.
Clinton Power Station has not previously reported an event similar to the event described in this LER.
For further information regarding this event, contact R. T. Chear at (217) 935-8881, extension 3664.