ML20197A740
| ML20197A740 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, 05000000 |
| Issue date: | 04/29/1986 |
| From: | Ainger K COMMONWEALTH EDISON CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 1516K, NUDOCS 8605120377 | |
| Download: ML20197A740 (10) | |
Text
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[N Commonwealth Edison j
~) One First National Plaza. Chicago. Ilhnois Address Reply to Post Office Box 767 (q.s Chicago, Ilknois 60690 April 29, 1986 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Braidwood Station Units 1 and 2 Environmental Effects of High Energy Line Breaks WRC Docket Nos. 50-454, 50-455, 50-456 and 50-45'l Reference (a): December 11, 1985 letter from K. A. Ainger to H. R. Denton
Dear Mr. Denton:
Reference (a) provided additional information in response to NRC questions regarding plant modifications to automatically detect and isolate high energy line breaks in the Steam Generator Blowdown and Auxiliary Steam systems. As discussed in reference (a), additional modifications were initiated to obviate the need for manual operator action due to postulated single active failures. Attachment A to this letter provides design details of the additional modifications which provide double, series, automatic isolation.
Attachment B to this letter contains information concerning operation of the Unit 1 to Unit 2 steam generator blowdown condenser crosstie. For reasons discussed in the attachment, we believe sufficient time is available to isolate steam generator blowdown from the unaffected unit by operator action from the control room.
Attachment C describes the relocation of temperature sensors in the steam generator blowdown condenser room. Operational experience at Byron Unit I revealed unexpected hot spots in the blowdown condenser room which caused inadvertent actuation of the system.
8605120377 860429 PDR ADOCK 0500 4
i
H. R. Denton April 29, 1986 please direct any further questions regarding this matter to this office.
One signed original and fifteen copies of this letter and the attachments are provided for NRC review.
Very truly yours, 7
K. A. Ainger Nuclear Licensing Administrator la Attachments cc: Byron Resident Inspector Braidwood Resident Inspector 1516K i
ATTAQWWIT A FOLLOW-UP DESCRIPTION OF IbSTALLED MODIFICATIONS ON STEAM GENERATOR BLOWDOWN AND AUXILIARY STEAM SYSTEMS As a result of additional calculations which indicated that insufficient time is available to take manual operator action to close the SD001 and AS012 valves, modifications (described below) have been completed on Byron Unit I which provide double, series, automatic isolation:
Steam Generator Blowdown (SD) System (See revised Figures lA and IB, attached).- Additional automatic isolation valves have been added upstream in series with the existing valves on the normal blowdown lines (B, D, F and H).
These valves (ISD054B,.D, F, and H) are of the same design as the existing valves (ISD0028, D, F, and H) with plant instrument air normally supplied to maintain the open position and valve closure occurring upon interruption of air supply. Two sets of temperature switches (each set powered from a separate electrical division) in each of the two HELB areas (Fig.'lB) provide electrical and mechanical redundancy.
Temperature signals from switches powered from Division 11 close the upstream F and H valves (SD054) concurrently with the downstream B and D valves (SD002) while the Division 12 signals close the upstream B and D valves (SD054) and the downstream F and H valves (SD002). This design assures automatic isolation by at least one valve in the event of a power division failure or mechanical temperature sensor failure.
Auxiliary Steam (AS) System (See revised Figures 2A and 28, attached) An additional automatic isolation valve (OAS286) on the 20" supply header has been added downstream of the parallel-flow valves (OAS167 and 0AS013) to provide mechanical redundancy in the event either 0AS167 or 0AS013 fails to close. Two temperature switches in each of the six areas provide simultane-ous signal input to their respective solenoids to close the upstream and downstream valves. This design provides electrical and mechanical redundancy in the event of either a temperature switch, valve or power division failure.
1516K
I FIGURE 1A STEAM GENERATOR BLOWDOWN CONFIGURATION (UNIT 1)
SDOO18.D, F orH
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SDO54 B,D, F orH SD 002 B D,F orH 3
-A
_A T_
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_L BLOWDOWN CONDENSER f;
SA SC_
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.I.29 STEAM ISOLATION VALVE ROOM STEAM TUNNEL AUXILIARY BUILDING
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p_
CONTAINMENT WALL REV.02-28-86 ADDITION OF SDO54 AUTOMATIC VA LVE a
REV. 02-28-86 ADDITION OF.
SDO54 VALVES FIGU RE 1B S.G. BLDN ISOL VALVES TEMPER ATURE CONTROL
( BYRON -1 )
AREA 2 AREA 1
SD PIPE PENETRATION CORNER CONDENSER ROOM E11 E11 E12 E12 E11 E11 E12 E12 TEMP.
TEMR TEMP.
TEMR TEMP.
TEMP.
TEMP.
TEMP.
SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH V
U v
if e
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U V
f y E12
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E 11 RESET RESET HAND SWITCH HAND SWITCH DESCRIPTION OF OPERATION SOL WHEN TEMPERATURE IN EITHER AREA REACHES-l SOL A
C THE SETPOINT OF 134
- F t 6*,THE CORRESPONDING 1SDO54B 1SDOO2B TEMPERATURE SWITCHES WILL ACTUATE TO SOL SOL C LOSE ALL EIGHT(ONE SET OA EACH OF 4 BLOWNDOWN LINES) ISOLATION VALVES.
1SDOO2 D ELECTRICAL REDUNDANCY ASSURES UPSTREAM 15 DO54 D SOL
~
SOL AND /OR DOWNSTREAM ISOLATION.
C A
DOUBLE SWITCHES ON EACH POWER DIVISION 1SDOO2F 1SDO54 F PROVIDE MECHANICAL REDUNDANCY.
VALVES REMAIN CLOSED UNTIL THEY ARE SOL SOL C
A MANUALY RESET.
m ISDOO2H 1SDO54 H D
FIGURE 2 A AUXILIARY STEAM CONFIGURATION
( COMMON UNITS 1&2 )
-q-h O AS 012 C)9
^
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=
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O AS 013 TURBINE AUXILIARY BUILDING BUILDING EXTRACTION ES STEAM R EV. 0 2-2 8-8 6 ADDITION OF OA5286 AU TOM A TIC VA LV E
AS286 FIGURE 2B AUX. STEAM ISOLATION VALVE TEMPERATURE CONTROL RECYCLE EVAP.
RECYCLE EVAP. RADW. EVAP.OA RADW.EVAP.OB RADW EVAP OC AUX STM TUNNEL OA OB E 21 E22 E 21 E 22 E21 E22 E 21 E22 E 21 E22 E21 E22 TEM P.
TEMR TEMR TEMP.
TEMP.
TEMP.
TEMP.
TEMP.
TEMP.
TEM P.
TEMP.
TEMP.
SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH SWITCH U
U U
V f
y U
m f
U m
s DESCRIPTION OF OPERATION E 21 E22 y
y WHEN TEMPERATURE IN ANY OF SIX AREAS RESET RESET RE ACHES THE SETPOINT OF 134
- F 2 6*'
HAND SWITCH HAND SWITCH THE CORESPONDING TEMPERATURE SWITCHES WILL ACTUATE TO CLOSE AUX.ST M.lSOL. VALVES (OAS013, OAS167 AND OAS286). TWO SOLENOIDS UPSTREAM DOWNSTREAM POWERED FROM SEPARATE ELECTRICAL DIVISIONS, PROVIDE ELECTRIC AL REDUNDANCY AND DOUBLE SWITCH ARRANGEMENT PROVIDES MECHANICAL SOL SOL REDUNDANCY.
A B
VALVE REM AINS CLOSED UNTIL IT IS OASO13 &
OAS 286 M ANU ALY RESET.
OAS 167 s
3 ATTAOSSNT B STEAM GENERATOR BLOWDOWN CROSSTIE The design of the steam generator blowdown system includes a piping crosstie between the Unit 1 and Unit 2 blowdown condensers. This crosstie is normally isolated by manual isolation valves. The purpose of the crosstie is to permit continued, but limited, 2 unit blowdown and plant operation even if a blowdown condenser must be temporarily removed from service for maintenance.
Breaks or cracks are not postulated in the crosstie piping itself because the piping is not considered high energy piping under the guidelines of the Standard Review Plan. However, in the event the crosstie line is in service concurrently with a failure in the normally operating high energy portion of the system, high energy steam from the unaffected unit will continue to be released through the non-isolated crosstie until manual action is taken from the control room to close the SD002/054 valves.
The unit-separation design basis of the station makes it impractical to isolate the crosstied unit automatically in this event. A review of the necessary condition for this event to occur demonstrates that its probability of occurring is extremely low and that the complication of providing opposite unit isolation with the undesirable potential addition of Unit 1/ Unit 2 safety system interactions is not justified.
If this event were postulated with both units operating and only one blowdown condenser operable, the units would each be blowing down at 50%
or less of the maximum rate because of limitations in condenser size.
Therefore, when one unit is automatically isolated, the blowdown to the auxiliary building will be considerably less than postulated for the design basis case which extends the time available to take manual operator action.
In the case where only one unit is operating there would not be uncertainty i
about the source of the break. A failure in a high energy line is, of course, a very low probability event. A full break, as contrasted with a leakage crack, is even less probable. For this to occur in the limited time that one could expect a blowdown condenser (a passive mechanical component) to be out of service is of such low probability that additional hardware modifications cannot be justified to respond to the event.
In the event the blowdown crosatie is in service the plant operators will be aware of the interconnection and, if one unit is automatically isolated because of high temperature in the blowdown condenser area, the other unit will be isolated by operator action from the control room. Operating procedures are being modified to require this action.
1516K
F l
ATTACHMENT C INSTRUMhaT RELOCATION IN STEAM CENERATOR BLOWDOWN (SD) CONDENSER ROOM As shown on the revised Figure IC, temperature sensors have been relocated from the south wall to the east wall. The relocation resolved an operational problem which occurred due to unexpected hot spots near the original location which caused inadvertant actuations.
Operating environmental temperatures were re-examined to determine a location which would not be affected by localized heating. The new sensor locations are rotated counter-clockwise from the south wall to the east wall (approximately 10 ft.) at the same previous elevation (approximately 375 ft.).
The same change is being made on Byron 2 and Braidwood 1 and 2 in order to avoid a possible recurrence of the same problem when these plants start-up.
1516K
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FIGURE 1C STEAM GENERATOR BLOWDOWN SYSTEM HELB MONITORING h
h (UNIT 1) 73-6 e
vi o
(1 SD01EB 3
(1SDO1EA"3)
(1 SDO1 ED 3
1SDO1EC 3)
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TS TS TS TS ETS
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TYPIC AL FOR FIGURES i
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2C, 2 D, 2 E L 2 F q BLOWDOWN CONDENSER TEMPERATURE SENSOR 1SDO1A n{ -
TUNNEL _ _ AUX.
EL 364'-O~
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