ML20156A244
| ML20156A244 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1980-029-00 | |
| Download: ML20156A244 (5) | |
Text
PRECURSOR DESCRIPTION AND DATA NSIC Accession Number:
158233 Date:
June 25, 1980
Title:
Loss of Reactor Coolant Pumps and Top Head Bubble Incident at St. Lucie I The failure sequence was:
- 1. With the reactor at 100% power, moisture in the vicinity of the solenoid for valve HCV-14-6 resulted in a short circuit across the solenoid valve terminal board.
This caused one of the two in-series containment isolation valves in the component cooling water (CCW) common return line from all reactor coolant pumps (RCPs) to fail closed.
This resulted in unavailability of component cooling water to all RCPs.
- 2.
The operators manually tripped the plant and RCPs seven minutes later.
- 3.
Natural circulation flow using the steam generators (SGs) was initiated and plant cooldown was began.
- 4.
The CCW flow was restored by jumping the failed valve at 1-1/2 hours after its failure but plant cooldown was continued.
- 5.
The cooldown rate did not exceed Tech.
Spec. limits, but unknown to the operator, the top head was not cooling down as fast as the primary loops.
- 6.
While reducing pressurizer pressure (using auxiliary spray from the charging system) to initiate the shutdown cooling system (SCS),
the top head water flashed to steam and formed a large steam bubble.
The bubble was not initially detected by the operators.
- 7.
Pressure and level oscillations occurred in the reactor coolant system (RCS) as a result of the steam bubble.
Pressurizer level fluctuations continued for approximately 6 h.
Each time charging was shifted to the cold leg, the steam bubble slowly condensed and shrank.
- 8.
The plant finally cooled down to the SCS entry condition and entered SCS at 235 psi.
At this time the SCS relief valves opened, slowly discharging RCS water to the refueling water tank, a result of a partially opened valve in the recirculation line from LPSI pump 1B.
(The LPSI pumps double as SCS pumps).
The second LPSI was subsequently initiated and operated in the injection mode to restore RCS pressure until the shutdown cooling loops were isolated from the RWST (approximately 1-1/2 h).
- 9.
Conduction cooling of the top head eventually collapsed the bubble.
Corrective action:
- 1. The solenoid and terminal block were replaced.
- 2.
The RCP seals required replacement.
- 3.
The blowdown line which was the source of steam that caused the short was modified by replacing flanged (leaking) joints with welded joints.
B-40
- 4.
A backup nitrogen supply was provided to the RCP CCW isolation valves.
- 5.
A manual jacking device was made more accessible.
Design purpose of failed system or component:
- 1.
The CCW RCP discharge line is common to all RCPs and is part of the RCP seal cooling system.
- 2.
The LPSI recirculation line is used to prevent deadheading of the pump.
Reactor at Manual Natural Due to slower RCS cooldown Leakage from Second LPSI power and RCP Reactor/
circulation top head cool-depressurization shutdown cool-pump used to CCW return RCP trip cooldown ing than RCS, to SCS entry Ing loops to RWST re-establish valve fails initiated steam bubble point effected occurs due to RCS inventory shut due to forms in top and SCS partially open shorted head when initiated recirculation solenoid auxiliary valve spray was used to reduce pres-surizer level (Anomalous pressurizer response noted)
Potential Severe Core Damage No No-HPSI pumps avail-able for RCS inventory control No No -
natural circula-tion cooling was providing adequate heat removal No No -
(potential RCP seal failure if pumps remain in operation)
NSIC 158233 - Actual Occurrence for Loss of Reactor Coolant Pumps and Top Head Bubble Incident at St. Lucie 1
Reactor at Manual RCP seal Due to slower Steam bubble Lack of natural High power and reactor/
failure due top head cool-prevents natural circulation pressure RCP CCW RCP trip to loss of down than RCS, circulation detected by injection return valve CCW steam bubble by operator and fails shut forms in top RCP started for due to shorted head core cooling solenoid Potential Severe Core Damage Sequence No.
No Yes No No Yes No No No Yes No NSIC 158233 - Sequence of Interest for Loss of Reactor Coolant Pumps and Top Head Bubble Incident at St. Lucie I
CATEGORIZATION OF ACCIDENT SEQUENCE PRECURSORS NSIC ACCESSION NUMBER:
158233 LER NO.:
80-029 DATE OF LER:
June 25, 1980 DATE OF EVENT:
June 11, 1980 SYSTEM INVOLVED:
Component cooling water, reactor coolant COMPONENT INVOLVED:
Solenoid valve CAUSE:
Short circuit resulted in a failed-closed solenoid valve SEQUENCE OF INTEREST:
Loss of RCP seal cooling ACTUAL OCCURRENCE:
Loss of RCP seal cooling REACTOR NAME:
St. Lucie 1 DOCKET NUMBER:
50-335 REACTOR TYPE:
PWR DESIGN ELECTRICAL RATING:
802 MWe REACTOR AGE:
4.1 years VENDOR:
Combustion Engineering ARCHITECT-ENGINEERS:
Ebasco OPERATORS:
Florida Power and Light LOCATION:
12 miles SE of Ft. Pierce, Florida DURATION:
N/A PLANT OPERATING CONDITION:
Full power TYPE OF FAILURE:
Inadequate performance; failed to start DISCOVERY METHOD:
Operational event COMMENT: