ML20168A742
| ML20168A742 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 06/16/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1974-LTR-00 | |
| Download: ML20168A742 (5) | |
Text
PRECURSOR DESCRIPTION AN~D DATA N'SIC Accession Number:
93702 Date:
May 31, 1974
Title:
Two-Unit Trip at Turkey Point 3-and 4, Loss of Offsite Power at Turkey Point 4 The failure sequence was:
- 1. Units 3 & 4 were at full power, with unit 4 startup transformer electrically isolated for functional testing of protection system relays and for periodic maintenance on associated equipment.
- 2. After the transformer protective relays were tested, the transformer lockout relay was reset and current flowed through an unanticipated path (a "sneak circuit"t) and energized unit 4 generator breaker protection system relays, which opened the generator breakers for units 3 and *4.
- 3. Both unit 3 & 4 reactors automatically tripped when their turbine-generators tripped.
The instantaneous grid loss was 1443 MWe.
- 4. Since the unit 4 startup transformer was isolated, off-site power was lost to unit 4.(seadtoapg)
Corrective action;(seadtolpg)
- 1. Modifications were recommended to the startup transformer protective system to provide more complete isolation of the protection system relays during relay tests.
- 2. The stuck-open safety valve was closed by removing its manual lifting-device.
Design purpose of failed system or component:
- 1. Offsite electric power provides an alternative source of power to plant auxiliaries in the event the unit generator is inoperable.
- 2. The main steam safety valves provide over-pressure protection for the steam generators and main-steam piping.
Unavailability of system per WASH 140G:
- Offsite power:
10-3 following trip Unavailability of component per WASH 1400:
- Relief valves, failure to close:
10-2 /D Unavailabilities are in units of per demand D-1.
Failure rates are in units of per hour HR The failure sequence was:
(Continued)
- 5. Diesel-generators A & B started and assumed safety-related loads for unit 4.
- 6. Five minutes after reactor trip on unit 4, a main steam safety valve lifted and did not reseat.
- 7. Additional boron was added to the reactor coolant system to provide shut-down margin for the additional cooldown, resulting from the stuck-open safety valve.
Units 3 and 4 Transformer lockout Current flow through Diesel-generators Main steam Unit 4 boratlon at full power relay reset after a "sneak circuit" path A and B start to safety valve to compensate with unit 4 tesring energizes unit 4 genera-provide power to sticks open for cooldown startup trans-tor breaker protection unit 4 (unit 3 on unit 4 due to stuck former isolated system relays, opening offsite power open safety for functional the generator breakers available) valve testing for units 3 and 4 Potential Severe Core Damage 4
No -
additional boron injection available via safety injec-tion system if required
______________________________No J
No -
turbine driven ANW pumps available for R. cooling No No NSIC 93702 -
Actual Occurrence for Two-Unit Trip at Turkey Point 3 & 4
Loss of Turbine Erner-Auxiliary PORV PORV or High Long Offsite Generator gency Feedwater and Demanded PORV Isola-Pressure Term Power Runs Back Power Secondary tion Valve Injection Core and Assumes Heat Removal Closure Cooling House Loads Potenti Severe Core Damage
-No No Yes Yes Nes No Yeo Yes NSIC 93702 -
Sequence of Interest for Two Unit Trip at Turkey Point 3 & 4
al Sequence No.
2 3
4 5
6 7
8 9
10 11 12 13
CATEGORIZATION OF ACCIDENT SEQUENCE PRECURSORS NSIC ACCESSION NUMBER:
93702 DATE OF LER:
May 31, 1974 DATE OF EVENT:
April 25, 1974 SYSTEM INVOLVED: Electric power system, main steam COMPONENT INVOLVED:
Startup transformer protective relays,- main steam safety valve CAUSE:
Current flow in a sneak circuit during protective relay testing resulted in actuation of the Unit 3 & 4 generator breakers.
After reactor trip, a Unit 4 main of offsite power ACTUAL OCCURRENCE:
Reactor trip with loss of of~fsite power (Unit 4 only)
REACTOR NAME:
Turkey Point 4 DOCKET NUMBER:
50-251 REACTOR TYPE:
PWR DESIGN ELECTRICAL RATING:
693 INe REACTOR AGE:
.9 yr VENDOR:
Westinghouse ARCHITECT-ENLGINEERS:
Bechtel OPERATORS:
Florida Power & Light Co.
LOCATION:
.25.miles-south.of Miami, Fla.
DURATION:
N/A PLANT OPERATING CONDITION:
Both units at full power.
SAFETY FEATURE TYPE OF FAILURE:
(a) inadequate performance; (b) failed to start; (c) made inoperable; (0inadvertant-opprat-inn DISCOVERY METHOD:
During operation COMMENT:
This event and a previous event on March 1, 1974 resulted in the separation of both Turkey Point Units from the grid. However, only Unit #4 and only during this event lost of fsite power.