05000237/LER-2003-004
| Event date: | 10-06-2003 |
|---|---|
| Report date: | 12-05-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2372003004R00 - NRC Website | |
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LICENSEE EVENT
_ � _ FACILITY NAME (1) DOCKET NUMBER 12) LER NUMBER (6 _ Dresden Nuclear Power Station Unit 2 05000237 (If more space Is required, use additional copies of NRC Form 366AX17) Dresden Nuclear Power Station Unit 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 02 � Event Date: 10-06-2003 � Event Time: 2215 CDT Reactor Mode: 1 � Mode Name: Power Operation � Power Level: 86 percent Reactor Coolant System Pressure: 1000 psig
B. Description of Event:
On October 6, 2003, during the performance of procedure DIS 2300-08, 'Unit 2 Contaminated Condensate Storage Tank Level Switches Functional Test and Unit 2 Torus Level Switches Functional Test," the Unit 2 High Pressure Coolant Injection System (HPCI) [13J] Torus High Level switch 2-2351B [LS] was determined to be Inoperable. Technical Specification 3.3.5.1, 'Emergency Core Cooling System (ECCS) Instrumentation,' Required Action D.2.2 allows HPCI to remain operable with an inoperable Torus High Level switch provided the HPCI pump suction is aligned to the suppression pool within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. HPCI pump suction is normally aligned to the Condensate Storage Tank (CST) rnq to minimize injection of suppression pool water into the reactor pressure vessel [RPV). A simplified drawing of this alignment is contained in Figurel on page 5. When the Unit 2 HPCI pump suction was aligned to the suppression pool in accordance with procedure DOP-2300-01, `High Pressure Coolant Injection (HPC1) System Standby Operation,' the water level in the HPCI Gland Seal Leak Off System (GSLO) condenser increased causing the HPCI GSLO pump IP) to operate. In this configuration, the GSLO pump has no discharge flow path unless HPCI is operating. The increasing water !eye! in the GSLO condenser was unexpected in this HPCI system alignment since there is normally not a source of steam or water to the HPCI GSLO condenser that would cause the HPCI GSLO pump to operate. The Unit 2 HPCI pump suction had to be realigned to the CST to allow the pumping of the GSLO condenser. On October 6, 2003, at 2215 (CDT), the Unit 2 HPCI was declared inoperable.
An Emergency Notification System (ENS) call was made on October 7, 2003, at 0520 hours0.00602 days <br />0.144 hours <br />8.597884e-4 weeks <br />1.9786e-4 months <br /> (CDT), for the above- described event. The assigned ENS event number was 40227.
This event is being reported in accordance with 10 CFR 50.73(aX2)(vXD), `Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident." The Unit 2 HPCI is a single train system.
The cause of the inoperability of the Unit 2 Torus High Level switch 2-2351B was indeterminate. The Torus High Level switch is a Magnetrol Model Number 291-S-13-512 level switch that uses a float and a magnet assembly to indicate water level. The potential failure mechanism was anticipated to be corrosion or foreign material in the switch. No foreign material was discovered when the float chamber was disassembled and no deficiencies were identified during an inspection of the magnet assembly. However, it was conservatively decided to replace the float assembly. Subsequent testing found that the switch functioned satisfactorily.
133), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, LICENSEE EVENT REPORT (LER) and to be Paperwork Reduction Project (3150-0104), Office Of Management 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 coaection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17) HPCI was declared operable on October 7, 2003.
C. Cause of Event:
The unexpected operation of the HPCI GSLO pump was due to a high level in the HPCI OSLO condenser that was caused by leaking OSLO pump discharge check valve 2-2301-76 [V] and regulating valve 2-2399-79 [V]. The root cause of the leaking valves was the preventive maintenance performed on the valves did not adequately assess or test the leak tight condition of the valves. The inability of the OSLO pump discharge check and regulating valves to prevent leakage became evident when HPCI was aligned to the suppression pool. By design, the back pressure on the valves increased to approximately 50 pounds per square inch due to alignment of the Emergency Core Cooling System keepfill-system to the HPCI piping in this configuration. The GSLO pump check valve was replaced with a check valve that has softer seats and better leak tight characteristics.
D. Safety Analysis:
The function of the HPCI is to provide water from either the CST or the suppression pool to the reactor pressure vessel. HPCI pump suction is normally aligned to the CST to minimize injection of suppression pool water into the reactor pressure vessel. However, if the CST water supply is low, or if the suppression pool water level is high, an automatic transfer of HPCI pump suction to the suppression pool water source ensures a water supply for the continuous operation of HPCI. The HPCI system can be initiated automatically or manually from the control room.
The manual initiation of HPCI is not credited in any Update Final Safety Analysis Report accident or transient analyses.
The function of the Torus High Level signal is to transfer the suction source of the HPCI pump from the CST to the suppression pool to eliminate the possibility of the HPCI providing additional water to the suppression pool from an outside source when the water level in the suppression pool is high. Excessively high suppression pool water level could result in the loads on the Torus exceeding design values during a postulated design basis accident or transient. The Torus High Level signal can be generated by either of the two Unit 2 Torus High Level switches, 2- 2351A and 2-2351B. The failure of the Torus High Level switch 2-2351B would not have prevented the operable Torus High Level switch 2-2351A from generating a Torus High Level signal and would not have prevented manual operation of the HPCI system.
The function of the HPCI OSLO system is to condense leakage from the HPCI steam turbine and the turbine's stop and control valves. The major components of the GSLO are a condenser and pump. The GSLO pump automatically operates to control the water level in the GSLO condenser and a discharge flow path must be available for this to operate correctly. The pump discharges the condensate to the CST when the HPCI pump suction is aligned to the CST or to the suction side of the HPCI pump when the HPCI pump is operating. The OSLO pump is equipped with discharge check and regulating valves, which prevent back leakage into the GSLO condenser. The failure of the OSLO pump discharge check and regulating valves to prevent back leakage would not have prevented the successful operation of the OSLO system during HPCI pump operation.
In summary, the event did not prevent the generation of a Torus High Level signal from switch 2-2351A and manual operation of HPCI from the control room. Additionally, the Isolation Condenser System [BL] was operable 133). US. Nuclear Regulatory Commissicc, Wastoktglon, DC 20555-0001, LICENSEE EVENT REPORT (LER) aixi to the Paperwork Reduction Propct (31500104), Orce Of AAanagement &splay a currently valid 01,413 control number. the NRC may not conduct or sponsor, and a person is not regtired to respond to, Itie Information correction.
FACILITY NAME (1) ' � DOCKET NUMBER (2) LER NUMBER (6) '-- � PAGE (3_, Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER (II more space is required, use additional copies of NRC Form 366AX17) during this event, Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
The Unit 2 HPCI GSLO pump discharge check valve 2-2301-76 was replaced.
A new procedural requirement to perform periodic seat leakage testing for the Unit 2 and 3 HPCI GSLO pump discharge check and regulating valves will be prepared and is being tracked in our Corrective Action Program by AIR 179654-30.
Initial seat leakage testing in accordance with the new procedural requirement will be performed for the Unit 2 and 3 HPCI GSLO pump discharge check and regulating valves and Is being tracked in our Corrective Action Program by AIR 179654-31.
Operating procedure DOP-2300-01 will be revised to include additional guidance on the affect to the HPCI GSLO of aligning the HPC1 to the Torus and is being tracked in our Corrective Action Program by NR 179654-36.
The Torus High Level switch float was replaced.
F. Previous Occurrences:
A review of recent Dresden Nuclear Power Station Licensee Event Reports and operating experience identified no previous events that were associated with HPCI inoperability due to back leakage Into the GSLO condenser.
G. Component Failure Data:
N/A FIGURE 1 SIMPLIFIED DRAWING of HPCI ALIGNED to the CST Opens when HPCI aligned to Suppression Pool.
Closes when � -HPCI _ HPCI aligned to Suppression Pool.
Frain ECCS ", 'Keepfill SyS., RPV; GSLO Check and Regulating Valves
GSLO
�
GSLO
',Pump . 4.
t33). U.S. Nudear Regulatory Commission. Washington. DC 20555-0001, LICENSEE EVENT REPORT (LER) and to the Paperwork Reduction Project (3150.0104), Okla Of Management display a amenity va5d OMB control number, the NRC may not conduct or _ � __ sponsor. end a person is not required to respond to, the kicenatico collectica.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 _
- YEAR SEQUENTIAL REVISION Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER (If more space Is required, use additional copies of NRC Form 366A)(17)