ML13308B503
| ML13308B503 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/31/1975 |
| From: | Fogarty D Southern California Edison Co |
| To: | Anthony Giambusso Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8103060644 | |
| Download: ML13308B503 (5) | |
Text
Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE DAVID J. FOGARTY ROSEMEAD, CALIFORNIA 91770 TELEPHONE vcE pR
.sIE.T 213-572-2796 March.31, 1975 Mr. Angelo Giambusso, Director Division of Reactor Licensing
- APR, Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Mail Code:
BETH:042 Washington, D.C.
20545
Dear Sir:
Docket Number 50-206 San Onofre Unit 1 This report describes an.Unusual Event concerning the San Onofre Nuclear Generating Station Unit 1 Safety Injection System.
The report is submitted in accordance with the requirements of Technical Specification 6.5.B. (2).
INTRODUCTION The Southern California Edison Company and the Westinghouse Electric Corporation have recently been engaged in an extensive design review of the existing Unit 1 Safety Injection System.
During the course of this review, Westinghouse determined and notified SCE on February 28, 1975 that there existed a possible second order mechanical failure which could prevent the Safety Injection System from fulfilling its design requirements under certain accident conditions.
Specifically, failure of one of the two feedwater pump discharge valves (MOV 852A or B) to close may result in a portion of the safety injection water intended for injection into the reactor vessel being diverted to the steam generators.
This would be particularly applicable to accident conditions under which steam generator pressure is less than reactor coolant pressure (e.g. steam line break accidents).
DISCUSSION The Safety Injection System has two functions:
a) To limit core damage due to overheating, following a loss-of-coolant accident, by prompt delivery of borated water to the core and by subsequent recirculation of the spilled borated water through the core for long-term cooling, and 3805
-2 b) To insert negative reactivity in the form of borated water during rapid cooldown of the reactor caused by a main steam line rupture.
The Final Safety Analysis requires that the system operate satisfactorily with a second-order mechanical failure. The loss of coolant or steam line break is the hypothesized first-order condition. The failure of any component to actively respond in the prescribed manner would constitute the second order malfunction and is to be explicitly tolerated without loss of the ability to provide the necessary protection.
Although numerous third-order failures may be tolerated, the design is not intended to cover all such cases.
To meet the criteria stipulated in the FSA, the system has been arranged so that there are two pumping trains for delivery of water, borated at refueling concentration, into the reactor coolant system. Each injection train utilizes one of the two station feed water pumps which deliver feedwater to the steam generators during power operation. The Injection System'begins operation when an actuation signal is sent to the appropriate pumps and valves (Attachment 1).
The valves automatically align the injection flow path from the refueling water storage tank to the reactor coolant loops and isolate the feedwater-condensate system.
For safety injection service, the feedwater pumps are each supplied with borated refueling water by one of the two safety injection pumps.
Each pumping train takes suction from the 240,000 gallon refueling water storage tank (approximately 11 times the reactor vessel volume) and discharges into a common injection header, from which three separate injection lines run into the containment sphere and discharge into each of the three reactor coolant loops.
The injection lines between the feedwater pump discharge isolation valves and the reactor coolant system isolation valves are always kept filled with borated water. Similarly, the safety injection line to the suction of these pumps is maintained filled with borated water. The safety injection system will protect the core even if one complete injection pumping train fails to function.
The Westinghouse design review of the San Onofre Nuclear Gen erating Station Unit 1 Safety Injection System revealed the possible existence of a second-order mechanical failure, which could result in a portion of the borated water intended for injection into the reactor vessel being diverted to the steam generators. The east feed pump discharge valve MOV 852A aligns the east train feedwater pump to a flow path into the steam generators and MOV 852B similarly aligns the west train.
Since the feed water pumps supply a common safety injection header, failure of one of the two MOV's 852A or B to close may result in a portion of the safety injection water from both trains intended for injection into the reactor vessel being diverted to the steam generators.
-3 The occurrence of a second-order failure of either MOV 852A or B could, therefore, result in the loss of the capability of the Safety Injection System to perform its safety function.
CORRECTIVE ACTION Following identification of the possible problem on February 28, 1975, meetings of the safety committees were convened.
The ramifications of the postulated failure were discussed. The safety committees concluded that reason for concern did exist.
Administrative controls for immediate use were approved and utilized until the unit was removed from service March 14, 1975 for refueling.
The interim administrative controls approved by the safety committees are designed to mitigate the consequences of having safety injection water diverted to the steam generators during postulated accident conditions.
These controls include an attempt to close the affected MOV 852 A or B using the control switches and if this is unsuccessful, the feed pump discharge valve to the Safety Injection System (MOV 851A or B) in the affected train.
In conjunction with this, all feedwater regu lator valves are closed. The resulting configuration assures that borated water is delivered to the core should either MOV 852 A or B fail to close following safety injection initiation.
A permanent physical modification is being considered which will alter system logic such that all feedwater regulator valves will shut upon safety injection initiation.
This would assure that safety injection water is delivered to the core even should the postulated valve failure occur.
Alternative modifi cations are being evaluated.
It is anticipated that system modifications will be completed during the current refueling outage.
Sincerely, cc:
Director, Region V Regulatory Operations Office
ATTACHMENT I SAFETY IN JECTIN/FEED WATER SYSTEM CONFIGU TION SAN ONOFRE NUCLEA GN RATING STATION UNIT 1:
TO EAST FIRST POINT HEATER oVa1A MOV 852 A REFUELING Mov 850 C OOV 853 A WATER TO COLD LEG STORAGE LOOP C L
~
oOP C
)(
I L
TANK MOV 850 B Mov 854 A TO COLD LEG FEEDWATER SAFETY INJECTION LOOP B FROM EAST 2nd POINT PUMPS PUMPS FRMPUMPS HEATER (CONDENSATE DISCHARGE)
MOV 850 A MOV 853 B LO MOV 851 8 TO COLD LEG LOOP A MOV 854 B MOV 852 8 FROM WEST 2nd POINT ELECTRIC DRIVEN HEATER kCONDENSATE DISCHARGE)
AUXILIARY FEED PUMP 1st POINT..
HEATER
,LOOP A
FWRVs STEAM ORIVEN AUXILIARY FEE_
PUMP MOV 20 FCV 457 TYPICAL STEAM GENERATOR "B
CV f43 FROM EAST FIRST POINT LOOP C HEATER.F
.F WR Vs
ATTACHMENT I SAFETY INJECTION/FEEDWATER SYSTEM ONFIGURATI0N SAN O MOFRE NUC AR GENERATING STATION - UNIT 1 TO EAST FIRST POINT HEATER MOV 851 A MOV 852 A REFUELING ov aso C MOV 853 A WATER DTo COLD LEG STORAGE LOOP C
-7 MOV 850 B MOV 854 A TO COLD LEG FEEDWATER SAFETY INJECTION LOOP a 1
FROM EAST 2nd POINTP S
HEATER (CONDENSATE DISCHARGE)
MOV 850 A MOV 853 B LO MOV 851 8, TO COLD LEG LOOP A MOV 854 B' MOV 852.B,'
FROM WEST 2nd POINT ELECTRIC DRIVEN HEATER (CONDENSATE DISCHARGE)
AUXILIARY FEED ---
x PUMP 1st POINT I
HEATER LOOP A FWRVs STEAM DRIVEN AUXILIARY FFFD-_--"
PUMP MOV 20 FCV 457 TYPICAL STEAM GENERATOR "B'