ML19338G427
| ML19338G427 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 09/30/1980 |
| From: | Lanik G NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML19338G381 | List: |
| References | |
| FOIA-80-514 NUDOCS 8010290317 | |
| Download: ML19338G427 (40) | |
Text
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REPORT ON THE INTERIM EQUIPMENT AND PROCEDURES AT BROWNS FERRY TO DETECT WATER IN THE SCRAM CISCHARGE VOLUME by the CFFICE FOR ANALYSIS AND EVALUATCN OF OPERATIONAL DATA September 1980 h
Prepared by:
George Lanik 8010290 31 7
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i EXECUTIVE
SUMMARY
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l On June 28, 1980, the Browns Ferry Unit 3 reactor experienced a partial failure to scram while shutting down for a scheduled outage.
As recorted by the Office for Analysis and Evaluation of Operational Data (AECD) on July 30, j
1980, the apparent cause of this event was found to be water accumulation in the Scram Discharge Volume (SDV) prior to the attempted scram.
The AEOD study identified possible fundamental deficiencies in the SDV which cast doubt on the ability of the Scram Discharge Volume / Scram Instrument Volume (SIV) to j
adequately perform their intended functions.
In view of these deficiencies, i
AEOD recommenced design changes to improve the performance of the scram system for the long term.
Following the event, the Office of Inspection and Enforcement (IE) issued Bulletin 80-17 and Supplement Nos. 1, 2, and 3.
Supplement 3 was issued in response to the concerns raised by the AE00 memorandum of August 18, 1980 which identified degraded air pressure in the control air system as a mechanism i
which could rapidly fill the SDV.
The equipment and procedural changes required cy Bulletin 80-17 and its Supplements are intended to provide the basis for I
continued operation of SWR's during the period prior to completion of design changes to the scram system.
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AE00 has evaluated the procedures and equipment at the Browns Ferry Units 1, 2 and 3 to determine their adequacy with respect to providing assurance that the SDV will not fill with water and interfere with a successful scram.
This I
evaluation applies specifically to the Browns Ferry units.
However, the findings and recommendations should be considered in the review process for all applicable SWR's.
The principal findings of the study are summarized below:
The present system, which uses recently installed ultrasonic water detec-e tion equipment and special procedures, in conjunction with previcusly i
installed instrumentation and procedures, does not restore the level of i
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scram protection capability thought to be assured in the original design.
However, except for degraded control air pressure events, it does provice l
adequate assurance for the interim that, accumulation of water in the i
Scram Discharge Volume (from currently identified sources),which could result in a loss of scram capability,will be reliably detected and adequately responded to by the operator.
e Degraded HCU control air pressure could result in scram outlet valve leakage to the SDV which would require operator action to manually scram f
the reactor within a few minutes before scram capability would be com-pletely lost.
Control air related disruptions in the plant would likely also initiate a plant disturoance which would require a scram.
Such an event would be accompanied by numerous control rocm alarms and indications which could distract the operator from a prompt manual scram actuation.
The current system does not adequately assure sufficient time for operator diagncsis and actions for this event.
Operating experience indicates that a significant number of reactoi e
scrams attributed to loss of HCU control air pressure have occurred.
2 These provide evidence that rapid filling of the SDV is a credible event.
The principal recommendations of the study are as follows:
a e An immediate manual scram should be required based on control room indi-cation of degraded HCU control air pressure.
Review of licensee proposals I
should include consideration of the available pressure indications and procedures to assure that other alarms and indications do not divert operator attention from this priority action, Redundant HCU air header pressure instrumentation should be provided in e
the control room.
To aid the operator in quickly focusing his attention on the need for protective action, a distinctive alarm for degraced air i
pressure should be provided.
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- Because of the possibility that a currently unidentified water source could result in water accumulation in the 50V, it would be prudent to monitor the ultrasonic system alarm output in the control room and require an immediate verification of a sustained alarm by operator dispatch to the equipment.
Operacility and calibration checks of the system should be continued on a schedule of once per shift.
The conclusions of the study are summarized below:
AECD has reviewed the interim surveillance system at Browns Ferry used to detect the presence of water in the SDV.
The AEOD assessment considers the procedures and equipment changes initiated in response to IE Bulletin 80-17 with Supplements, 1, 2, and 3 to be adequate for continued interim operation of the Browns Ferry Nuclear Plant, if the recommmendations of this report relating to degraded centrol air pressure are implemented.
4 As of the date of this report, the instrumentation and procedures in place to respond to the loss of control air scenario at Browns Ferry are judged to be inadequate.
For this event the cperator must respond promptly to a single indistinctive alarm for loss of control air pressure during a period when numerous alarms may be occurring.
Additionally, the operator must take actions outside the control room in a very limited time frame because of the absence of a pressure readout in the control room.
IE is currently taking steps to upgrade the procedure fcr response to the degraded control air pressure event.
In the past, coerator action to perform a vital safety function within less than 10 minutes has not been considered acceptable by the NRC.
- However, providing the operator with both a distinctive low pressure alarm and reliable air pressure instrumentation in the control room would help assure adequate operator response within the required time period.
Such an arrangement snould be acceptacle for the interim.
A dedi W.a.'. crerator with adequate alarms and instrumentation in the control room could provice even greater assurance of a timely manual scram.
If the provisions made to accomplish a manual scram are iii
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found to be untimely or inadequate, provisions should be made for an automatic scram on low HCU control air pressure.
For the long-term, the scram system should be upgraded according to the recom-mendations of the AE00 report of July 30, 1980.
However, the consequences of degraded air pressure in the HCU control air headers were not fully recognized at the time of that report and were not directly addressed.
Although the recommended scram system modifications may be sufficient to enable the scram I
system to respond to rapid inflows of water from the scram outlet valves due to degraded HCU control air pressure, design review of the long-term modifications snould include specific consideration of the effects of degraced air pressure.
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.l TABLE OF CONTENTS PAGE
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EXECUTIVE
SUMMARY
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1.
INTR 0.<CTION........................................................
1 t
2.
SYSTEM DESCRIPTION............................................
3 2.1. Calibration and Cperation......................................
6
- 2. 2 Operating Procedures...........................................
8 2.3 Other Leakage Detection Capabili ties...........................
8 2.4 Procedures for loss of Control Air..............
9 2.5 Procedures for Stancby Liquid Control Initiation...............
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ANALYSIS AND EVALUATION..................................
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1,.
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3.1 Water From the Previous Scram..................................
1;2,
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- 3. 2 Purge Li ne Infl ow.....................
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e 3.3 Water or Steam from the Drain System.........
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3.4 Single Scram Outlet Valve Leakage..............................
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- 3. 5 Multiple Scram Outlet Valve Leakage frcm a Common Cause.......
16.
i 3.6 Degraded Control Air...........................................
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3.7 Operating Experience with Degraded Control' Air Supply..........
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4 FINDINGS............................................................
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5.
RECCMMENDATIONS...........................
25 6.
CONCLUSIONS.................................................
26 REFERENCES......................................
28 TA5LE............
29 FIG'.'RES................................................................
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... - -, - ~ - - - - -
1.
INTRCDUCTION i
On June 28, 1980, the Browns Ferry 3 reactor experienced a partial failure of the scram system while shutting down for a scheduled outage.
The operators were able to completely insert all control rods within 14 minutes of the initial scram attempt.
Because of the initial partial success of inserting rocs on the f.irst scram and because no unplanned transient requiring a scram
'was in progress, no immediate challenge to reactor safety and integrity developed.
c.i5 As dg.cumented in the AE00 report dated July 30, 1980, (I) the cause of this levent$ asI.found to be water accumulation in the East Bank Scram Discharge n
's*
Tbmle..(;S[DV) prior'tothefirstattemptedscram.
Following the event, IE iul'letJn80-17andSupplementNos.1,2,and3wereissued.
These directed
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I5WR li@erisees to.b@in surveillance of the SDV to detect the presence of 5
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wate,r./. v.r'equirement for continuous monitoring of the SDV water level in the 4
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' control (rooni was stated in Supplement No.1.
Scram system problems revealed
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1 b'y testing subsequent to the Browns Ferry event were reported in Supolement 3
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No. 2. ' Supplement No. 3 was issued in response to the concerns raised by the
. 1-AE'CO me.morandum of August 18,1980(2)
This supplement required operator
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ac,tiens for a loss of control rir to the Hydraulic Control Units (HCUs).
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I' The following report is an evaluation of the current measures ceing taken at
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Browns Ferry in response to the IE bulletin and supplements to prevent events
,E of-the type that occurred en June 2C,1950. This assessment was undertaken by
. A$0D Decause of its concern abcut the adequaty of the interim system which will be used during the period preceding the implementation of icng-term corrective measures. The scope of this recort is purposely limited to:
a). Browns Ferry Units 1, 2, and 3: b) the interim measures; c) selected bulletin requirements; and d) procedures and ecuipment in olace en tne date j
of this report.
The findings, recommendations, and cccclusicns are based en informaticn gathared through informal enannels between AE00 and the Tennessee Valley Authority, tne Generai Electric Ccmpany, and the U.S. NRC headquarters and regional offices.
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Section 2 of this report contains a description of the present equipment and procedures at Browns Ferry used to prevent a recurrence of the Iailure to scram event.
Section 3 provides an AE00 evaluation of tne effectiveness of the present system (equipment plus procedures) for providing a timely response to a range of postulated scenarios.
Sections 4 and 5 present, respectively, the findings and recomendations. The conclusions E.re given in Secticn 5.
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SYSTEM DESCRIPTION To comoensate for the identified deficiencies (1) associated with the protection system instrumentation installed at Browns Ferry prior to the June 28, 1980 event, additonal hardware and operating procedures have been put in place.
The additional equipment installed at Browns Ferry for monitoring the SDV for the presence of water is an ultrasonic (UT) system.
Ultrasonic transducers are mounted on the East and West SOV header low points.
The transducer is criven by a signal generating and processing device wh'ch incorporates a-cathode ray tube (CRT) display and provides an output to a strip chart recorder.
Unit 3 has eleven transducers located as shown in Figure 1.
Units 1 and 2 eacn have four transducers located as shown in Figures 2 and 3. Unit 3 was instrumented to a greater extent to attempt to find the cause of the June 28, 1980, partial scram event.
Since completing the testing of SDV drainage, long-term monitoring has been limited to use of transducers #2 and #7 on Unit 3; transducers #12 and #13 on Unit 2; and transducers #14 and #15 on Unit 1.
In the case of failure of these transducers, a backup transducer is available i
on each header.
The transducers are bonded to the headers with a high tempera-ture achesive.
The pulse-echo technique of depth measurement is used and is illustrated in Figure 4 The top illustration shows a cross section of the SDV pipe on which the transducers are mounted.
The bottom illustration shows the CRT display arising from this situation.
Since sound travels one-fourth the speed in water as in steel, the reflection from the inner tube wall is received very quickly following the initial pulse.
This is shown on the left hand side of the CRT display in Figure 4.
Multiple reflections are seen on the CRT because of sound reflections between the inner and outer diameter of the pipe.
These show a decreasing amplitude and die out rapidly.
The samole illustration is shown containing 5.2 inches of water.
A second series of eenoes is received at a later time on the CRT indicating the 3
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reflection from the water-air surface at a distance of 5.2 inches.
The instru-ment has been previously calibrated on a pipe with a known water level.
The numoers shown on the horizontal axis of the CRT display correspond to the depth in inches of water in the SDV acove the transducer location.
A continuous recorder is provided.
By use of gating devices, it is possible to pick the signal of interest to look at which is the water-air interface and not the pipe inside diameter (i.d.).
The gating device is set to gate signals I
which come in at a time later than those corresponding to one inch of water.
Tnis eliminates the reflection from the i.d. of the SDV pipe and the associated multiples.
The gating device is also set to gate only those signals with an amplitude greater than approximately 20% of full-scale amplitude.
The first signal associated with a given pulse to pass the gate is transmitted to the recorder.
The recorder is a two channel recorder; one channel recorcs the amplitude of the gated signal, and the other records the calibrated cepth of water associated with the gated signal.
A local alarm is p.ovided.
Any echo signal which passes the gate will generate an audible and visual alarm.
The alarm is generated when the water level is greater than one inch and self-clears when the level is less than one inch.
i Two characteristics of the gating method used are of particular interest with j
respect to the recorder output:
(1) only water deaths greater than one inch are recorded; and (2) only the first echo received at a depth greater than one inch is recorded.
I When no water is present in the header, the echo from the pipe i.d. is the only return pulse.
Since this initial pulse and its multiples indicate less than one inch, nothing is gated to the recorcer.
The second pulse indicating i
water level never comes.
The recorder sees this as a long uelayed second pulse.
Thus, the normal empty header condition reads full scale on the recorder.
The recorder full scale reads er 'nches.
Since the foll pipe condition woulc reac only six inches, there is ri. confusio i in the reading.
When a pulse is 4
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gated, indicating the presance of water in the header, the recorder pen is driven down toward the lower part of the scale.
(See lower portion of Figure 5).
At Browns Ferry Unit 3, two separate UT devices and recorcers are provided to monitor both the East and West SDV headers indepencently.
On Units 1 and 2, however, a single UT device is used to drive and monitor two transducers at the same time, one on the East SDV header and one on the West 50V header.
This provides another characteristic of the system that must be recogni:ed by those operating the system.
The gating device passes the first pulse which returns corresponding to a depth greater than one inch, and the recorcer i
responds to this pulse. With water in both headers, the indication seen on the recorder corresponds to the first returning echo greater than one inch.
l Inus, if both East and West headers have a water depth above one inch, the smaller depth indication is recorded because it is the first pulse to return.
Individual measurements for either side can be made by disconnecting.the cable from the transducer on the header opposite the side of interest.
The following is a brief discussion of the recorder output from a scram test at Browns Ferry 2 (Refer to Figure 5).
Figure 5 shows only the calibrated water depth trace.
The amplitude trace has been omitted for simplicity.
Increasing time is from the bottom upward with one division equal to approxi-mately 5 minutes. Water depth is measured in inches starting from zero on the right hand side of the trace to 10 inches on the left.
Note that the pen location prior to the scram at 0202 hours0.00234 days <br />0.0561 hours <br />3.339947e-4 weeks <br />7.6861e-5 months <br /> is full scale left (10 inches).
This is because no water is present and the second echo never returns, which tne instrument interprets as maximum distance from the bottom mountad trans-The momentary readings where the pen is driven downward (to the right) cucer.
prior to the scram are due to the instrument reacting to the "walkie-talkies" used for communication.
These momentary readings also activate the visual and aucio alarms which clear eacn time the walkie-talkie transmission stops.
Since Unit 2 was scrammed at 0202 hours0.00234 days <br />0.0561 hours <br />3.339947e-4 weeks <br />7.6861e-5 months <br /> the water level indicates 6 inches.
The trace has been blacked in below the 6 inch level for emphasis.
At about 5
0230 hours0.00266 days <br />0.0639 hours <br />3.80291e-4 weeks <br />8.7515e-5 months <br /> the indicated water level falls frem 6 inches to 1 inch.
This is cue t: the West header going emoty. At the time when the level incicatcr reaches 1 inch, the pen is driven back up to the 6 inch level.
This is because on Unit 2, a single UT instrument is used to monitor both East and West headers.
Since the gating device passes the first returning signal above one inch, the recorder tracks the header which empties first (West side) and when the echo from the West side indicates less than one inch, the gating device begins to pass the echo from the East side header.
Since the East side header has not yet emptied, the pen is driven back up to about 6 inches.
Between 0234 and 0256 hours0.00296 days <br />0.0711 hours <br />4.232804e-4 weeks <br />9.7408e-5 months <br />, the East side header continues to crain.
When the level reacnes one inen on the East sice, no returning echo is gated and the pen returns to tne 10 inch position.
As indicated on the trace, a series of momentary indica-tions of water are present at 0440 hours0.00509 days <br />0.122 hours <br />7.275132e-4 weeks <br />1.6742e-4 months <br />.
These are due to some CRD survillance tests which were run at that time.
2.1 Calicration and Oceration Ali tranducers used in the system were tested prior to use to assure adequate performance.
The gain of the signal generating and receiving equipment is adjusted to provide an adequate signal output from the least responsive trans-ducer.
The minimum acceptable signal for reflection from the water interface is adjustec for 80% full scale output on the CRT display.
The gating cevice is set to pass any signal with an amolitude more than approximately 20% of full scale so as to provice an adequate margin of sensitivity.
The time scale on the CRT is adjusted to read the depth of the water in the heacer in terms of horizontal divisions on the CRT screen.
As shown in Figure 4, a total of ten horizontal divisions are used on the CRT display.
The sweep time and the horizontal centering of the CRT are acjusted so that the sixth civision on the screen corresponds to a water cepth of 6 inches while the first division on the screen corresponds to a water depth of one inch.
- Thus, tnc CRT displays water depth directly.
If no water is present, only the echo from the i.d. of the pipe is cisplayed on the CRT screen at a position belcw the one inch mark.
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Initial system calibration and later checking of the calibration is cone by use of standard pipes filled with known amounts of water. Once per snif t, a
level two QC inspector takes a reading from a standard containing 2 inches of water and from a standard containing 6 inches of water.
This is ccne cy cisconnecting the cable from the transcucers on the heacers and connecting it to hand-helc transducer which is held against the cottom of the starcard sample pipes.
If the reading on the CRT and the recorder does not agree with the known depth of water in the standard pipes, the gain and amplitude of the UT instrument are adjusted to recalibrate tne system.
At tnis time, all transducers are functinally checked by examining the CRT display for indica-tions of transducer deterioration.
The two UT instruments on Unit 3 are physically located at the ends of the rows of HCUs, one on the East side and one on the West side.
On Units 1 and 2, the UT instruments are located on a me::anine level above the HCU level apcroximately midway between the two sides.
Browns Ferry has an auxiliary operator on eacn shift who coserves the UT system recorder strip chart for each unit every 30 minutes.
The operator is not qualified or required to monitor the CRT output.
His sole responsibility is to monitor tne strip cnart recorder and the alarm.
The calibration and operability of each UT device and each transcucer is cnecked once per shift by a level two QC inspector trained in the use of UT equipment. Communication witn the control room concerning SDV water accumulation is by a hand held wa l kie-tal ki e.
As mentioned earlier, a separate UT transducer, CRT and recorder is proviced for each sice on Unit 3, while Units 1 and 2 each have a single UT, CRT and recorcer to monitor both the East and West 50V header transducers.
Tnus for Units 1 anc 2, since tne recorder tracks the first returning pulse for a water level greater than one incn, it is necessary to disconnect One leac at a time to cetermine secarate 50V water levels.
If water is cetec ed, a level two QC inspector must be called to verify the readings.
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- 2. 2 Coeratinc Procedures Procedures have been written for the control room operator to respond to the presence of water in the SDV as detected by the UT system.
If the water level reading in the SDV is less than 1-1/2 inches, procecures call for an operator to:
(1) visually verify that the SDV vent and drain valves are open; (2) check for leaks in the scram discharge valves by observing CR0 tem::erature probe outputs and by touching the HCU discharge risers; and (3) request QA verification of the UT reading.
If the water level reading is between 1-1/2 and 2 inches, procedures call for the control room operator to:
(1) immediately recuest QA to dispatch a level two QC inspector to verify the reading; and (2) unless the QC inspector deter-mines that the water level is less than 1-1/2 inches, begin an orderly shutdown within one hour.
If water level exceeds 2 inches, procedures call for the control room operator to immediately begin an orderly shutdown without verifying the UT reading.
Plant personnel estimate that a level two QC inspector can reach the area of the UT device within approximately three minutes of being notified. At least one level two QC inspector is available for this duty at Browns Ferry during each shift.
2.3 Other Leaksce Detection Cacabilities i
Leakage of water through a scram outlet valve to the SDV is recognized as one of the ways for water to reach the SDV.
This leakage may be detectable by means other than the UT system or SIV instrumentation.
Flow out of a scram cutlet valve would change the flow of the CRD cooling water nrough the CRD seals so as to increase the water temperature at the location of tne CRD temperature probe.
At this time, no good data is available to correlate CRD temperature with scram outlet valve leakage.
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the rate of temperature change for a given leakage is unknown.
However, for the leakage rates postulated.in this section, it is reasonable to assume that the temoerature probe alarm set point would be reachec within a relatively short time; on the order of a few minutes.
Althougn tne CRD temoerature probe alarms in the control room, not all 185 CRD temperature probes are reac-simultaneously.
A sequential scan is used and it is estimated by GE that the cycle time to read all temperature probes is approximately six minutes.
Another means of inferring scram outlet valve leakage is the observation of control rod drift.
Leakage past the scram valve in excess of the CRD seal leakage would cause the associated control rod to begin to crift into the I
core.
A rod drift alarm is available in the control room.
Another indication of scram outlet valve leakage is movement of the scram outlet valve stem sufficient to actuate the scram valve position indication switches.
This requires a stem movement of approximately 1/32", out of a total valve stroke of approximately 3/16".
Actuation of the stem mounted
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switen will light the associated scram outlec valve position indication light in the control room.
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j One means by which the scram outlet valves can open sufficiently to leak is j
cegraded air pressure in the HCU air header.
A low pressure alarm is provided
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to alert the operator at approximately 70 psia.
The actual pressure reading on the HCU header is available locally in the area of the HCus.
4 2.4 Procedures for Loss of Control Air The procedures at Browns Ferry for loss of control air were modified in response I
to IE Bulletin 80-17, Supplement 3, to protect against scram valve leakage on gradual loss of control air.
The details of the concern for loss of control air are discussed in the AE00 memorandum of August 18, 1980.(2) 4 At Browns Ferry, control room incication of the air pressure in the HCU air header is limited to a single alarm with a setpoint at 7C psia.
A local air
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pressure guage is available at the HCUs.
Normally air pressure is maintainec between 70 and 75 psia.
Since only a slight degradation of air pressure initiates the alarm, the licensee considers it undesirable to initiate a scram based on this alarm alone.
Upon receipt of the 70 psia alarm, procedures call for the control room operator to dispatch an auxiliary unit operator to read the local air pressure gauge.
Plant operations personnel at Browns Ferry have told the NRC resident inspet+or that an operator will be dispatched to read the local pressure guage no later than 2 minutes after receipt of the 70 psia alarm.
If air pressure in the HCU air heacer is found to be 60 psia or less, the auxiliary operator informs the control room operator. The control rocm coerator then initiates a manual scram.
Communicaticn between the auxiliary and control room operators is maintained via walkie-talkie.
In addition to the above procedure far gradual loss of air to the HCU ai header, other procedures have been implemented in response to IE Bulletin 80-17, Supplement 3.
These call for manual scram initiation in the event of:
(1) multiple rod drift-in alarms; or (2) a marked change in the number of control rods with high temperature probe alarcs.
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- 2. 5 Procedures for Standby Licuid Control Initiation i
Bulletin 80-17, Supplement 1, requested that operating procedures be revised to provide clear guidance to the control room operator regarding initiation of the stancby liquid control system (SBLC) following a failure of control rods to fully insert.
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At Browns cerry, mandatory SBLC system actuation is required by operating l
procedu e if either of the following conditions exist:
(1) five or more 1
acjacent rods are not inserted below 06 position and eitner reactor water level cannot be maintained or suppression pool water temperature limit of 110 F is reached; or (2) thirty or more rods are not inserted below 06 position and either reactor water level cannot be maintained or suppression pool water temperature of 110 F is reached.
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3.
ANALYSIS AND EVALUATION For purposes of analysis and evaluation of the Browns Ferry failure to scram event, an effort was made by AE00 to identify water sources tnat could fill the SDV.
The AE00 report of August 1980, identified the follow ~ ; sources of water:
- 1) water left frcm a previous scram; 2) purge line inflow; 3) water or steam backing up from the clean radwaste drain system; and 4) inficws through the scram discharge valves.
It is recognized that this list of water sources may not be complete. However, at this time, neither operating experience nor a
design review has revealed any other sources.
As discussed in Section 2 of this report, the current capability at Browns 1
Ferry to detect and responc to accumulation of water in the SDV is based on tne following elements:
previously installed SIV instrumentation, recently installed UT instrumentation; other previously installed instrumentation such as CR0 temperature probes, CR0 drift alarms, control air flow pressure alarm, i
etc; and operating procedures for response to the aforementioned instrumenta-tion.
The response of the interim system at Browns Ferry is dependent on both the instrument capability and the operator response.
Botn aspects are addressed in this analysis and evaluation.
The original design, as understood prior to analysis of the Browns Ferry event, was thougnt to have provided continuous, redundant, safety grade and automatic protection which was functional for all water sources.
It was thought to also fail safe on loss of HCU control air pressure.
However, the i
analysis of the Browns Ferry event snowed that this system did not work for all situations of water accumulation.
Accordingly, the original system was supplemented with a functional UT system.
This interim system is neither continuous, redundant, safety grace, nor automatic 1.
many cases.
Further-more, its capability may be inadequate for a loss of control air pressure.
That is, the interim system does not provide the same level of protection as was cerceivec of the original design.
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At this point, a short discussion of the capability and reliability of the UT system will be presented.
As statec in Section 2, tne UT system includes a i
CRT display of the return echo.
This is snown in Figure 4 As statec earlier, an echo is received frcm tne i.d. of the pipe and is disclayed on the CRT as i
tne left most peak.
The presence of this so-called " reference pulse" is interpreted by the inspector performing the calibration as verification of the operability of the transducer wnicn is being monitored.
The calibration technique also requires that the UT system be connected to a separate trans-ducer to detect a known depth of water in a standard pipe.
These surveillance and calibration procedures are performed once each shift by a level two QC insoector who has extensive training in UT techniques.
We believe that cecause of the cresence of the " reference pulse" and the high level of training of the level two QC personnel who performed the surveillance and calibration of this instrument, any degradation of the operation of the UT system due to heat, vibration, radiation or other failure mode would be discovered during the scheduled surveillance.
It is recognized that during the period of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> between surveillance of the UT system, it would be possible for equipment failure to go undetected.
However, because of the unlikelihood of a rapid water inflow with an accompanying need to scram occurring during the same perioc of the equipment failure (except for the degraded air case), this surveillance interval is judged to be adequate.
The following analysis and evaluation addresses the capability of the interim system to detect and respond to water from various postulated sources.
3.1. Water From the Previous Scram Water lef t from the previous scram after scram reset will be detected by the ultrasonic system.
Because of the time required between a scram and startup cperations, a number of ultrasonic readings and equipment calibrations would normally take place during the shutdown.
For this situation, rapid detection is not required and no immediate action is neeced if water is detected.
Startup would simply te delayed until it was assured that the situaticn was corrected and no water remained in the SDV.
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3.2 Purce Line Inflow Purge line inflows would be detected in time depending on the rate of inflow and the time of purging.
Purging of the SDV is an operation which is per-
)
formed when the reactor is shutdown in order to reduce accumulations of radio-activity in the SDV and its associated piping. The ultrasonic system would be used to check for the presence of water prior to startup.
Enough time would be available during the plant shutdown for the operator action required to detect and remove all water frca the SDV prior to startup.
An administrative or operator error, which allowed purging during normal operation, could provide a flow rate of water into the SDV which might not be detected soon enough by either the interim system or the original system.
However, the likelihood of a properly informed oper-'-' performing this unprecedented action is remote.
3.3 Water or Steam From the Drain System Water backing up from the clean radwaste (CRW) drain system would most probably be detected by the SIV instrumentation of the original cesign.
An exception to this would be a SIV drain line blockage which could prevent flow from the CRW drain system into the SIV but would allow flow uo through the SDV vent lines to fill the SDV.
However, at Browns Ferry, positive vent paths to atmosphere have been provided on the SDV vents.
Any water backing up in the vent line would be released via this path rather than to fill the SDV unless the backflow rate was high due to a large water release to the drain system.
Operating experience at Browns Ferry to date has shown that water backing up from the CRW drain system has actuated level switches in the SIV.
Low pressure steam backing up from the CRW drain system due to flashing hot water in the drain pipe would not be detected by the SIV instrumentation.
If the drain line between the SDV and SIV became plugged by the slow drain of condensed vacor mixed with rust, the steam backing un through the SDV vent lines wnuld slowly fill the SDV with concensed vapor.
The positive vent paths 13
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te atmosphere would vent a portion of the low pressure steam but woulc not j
prevent the SDV from filling with condensate.
Operating experience at Browns Ferry Unit I has shown that flashing hot water can appear in the drain pipe from v.,ve leakoff connections or other sources.
Therefore, the ultrasonic 4
t system must be used to monitor this situation and the surveillance interval must be short encugh to assure timely discove y during a large hot water j
release.
Thus, with r.espect to the three identified scurces of water listed above, we believe that the UT system provides adequate interim assurance that water can be detected anc actions taken before the plant reaches a condition where tne SDV is filleo and a scram is recuired.
It appears that this woulc be true with 4
a surveillance interval longer than the 30 minutes currently used at Browns Ferry provided that surveillance was performed prior to any start-up. Hcwever, if protection for currently unidentified water sources and ficw rates is to be provided, continuous monitoring of the UT system in the control room would be preferable to the current 30 minute surveillance interval. This would allow for a more rapid control rocm operator response.
As an intermediate method (between centinuous acnitoring and-lengthy surveillance intervals) for providing response tc unidentified water sources and rates, an alarm outout of the UT device could be provided in tne contrcl room. This would alicw more timely coerator response withcut the complexity of locating the complete UT system output in the control
'Upon receipt of the sustaiaed alarm, an auxiliary operator could be dis-rocm.
patched to the UT readcut located by the HCUs. We believe this approach wculc also provide acequate protection for unidentified water scurces and flow rates.
3.1 Single Scram Cutlet '!alve Leakage To evaluate the adequacy cf tne interim system for various leak rates from the scram outlet valves, it is necessary to identify the causes of leakage.
First, it should be noted that scram valve leakage curing normai operation is quite low.
At Browns Ferry Unit 3, tests following tne June 28,19S0 event incicated an aggregate leakage of from 0 to 3 gallons oer hcur.
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i Discussions with GE on the leakage characteristics of the scram outlet valves incicate that any leakage is likely to cause degradation of tne valve seats and could lead fairly rapidly to greater leakage. Rapid ceterioration of the seating surface of one valve would result in obvious problems with the associated control rod drive but would not affect others.
One aspect of the scram outlet valve leakage problem that must be addressed is the difference in character between a leak arising from a single valve failure and that which could arise from a common mode failure leakage of many valves.
With respect to a single valve failure, the maximum inficw of water into the SDV is limited by leakage past the CR0 seals.
GE has estimated that with the CRD seals comoletely destroyed, a leak rate of 10 to 12 gpm into the SDV is tne maximum that could occur.
This is tne rate if the flow is completely un-restricted by the scram outlet valve.
If the scram valve is only partially open or leaking, the flow rate would be less.
If the CR0 seals are intact, leakage would be expected to be in the range of 1 to 5 gpm.
Thus, a single j
failure of a scram valve results in only a limited flew into the SDV which would drain out with no accumulation for the current SDV drainage characteristics. (1)
Indications of scram valve leakage would be available to the operator. The CRD temoerature probe alarm would be actuated.
If the scram outlet valve leakage is greater than the corresponding CRD seal leakage for a pressure differential across the piston of approximately 550 psig, the rod would move into the core.
bhen assessing the probability of an event that could cause problems for the SDV, it must.be recogni::ed that tne probability of a simultaneous failure of j
more than one scram outlet valve at a given time is very low.
Multiple valve failures would have to occur simultaneously cefore tne drainage capacilities of the current system would be challanged.
Because of (1) the low procability of tnis event, (2) the likelihocd of early detection by rod drift alarm or CRD temoerature crcbe alarm, and (3) because the event does not cause an accompanying 1
piant disturbance, this postulated event is not considered to be a serious concern for the interim period.
The interim precautions snould be acequate to orotect against failures of tnis type.
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3.5 Multiple Scram Outlet Valve Leakace from a Common Cause Multiple scram outlet valve leakage due to a common cause can raise serious concerns about the ability to scram the plant successfully.
To date, the only plausible common cause which leads to substantial leakage of a large numoer 6f scram outlet valves is degraded air pressure in the control air header for the HCus.
Loss of air pressure in the control air header has occurred due to a variety of reasons such as fail;re of an air compressor, improper valve alignment, clogging of filters and dryers, and severance of an air line.
As air pressure in the header decays, the scram outlet valves, which are held closed by air pressure, begin to open.
Although the exact pressure at which a given valve begins to open depends on manufacturing tolerances, the pressure i
for a group of valves is in the range of about 40 to 45 psia.
Information from GE indicates that a leakage flow of from 1 to 3 gpm out of a scram outlet valve for a given drive could occur without produciag rod motion.
The actual value for a given drive would depend on the condition of the seals in that particular drive.
GE has stated that for a typical reactor, if the scram discharge valve flow rate to produce rod motion for each individual CRC was averaged with the scram discharge valve flow rate to produce rod motion of all other CRDs, the average would be in the range of 2 to 3 gpm.
1 With this information it can be postulated that a degraded control air pressure condition could exist for which leakage from a large number of scram outlet valves could exist without producing a scram, In fact, depending on the numcer of scram valves which partially open and the leakage rate of these valves, it would be possible to generate a significant flow of water into the SDV without producing significant rod motion.
It is recognized that the possibility of the actual occurrence of hign flow rates without rod insertion deoends on three factors:
(1) the control air pressure degradation pattern, (2) the range of air pressure over which the scram cutlet valves open, and (3) the seal leakage rate of the CRD associated with each particular scram outlet valve.
However, with the data given above, a flow rate in the range of 1 to 16
2 gpm per drive witnout significant rod insertion could be possible for certain degraded air pressure sceneries.
3.6 Deoraded Control Air Assuming an average leak rate that could be generated without significant rod motion (given a specific degraded air pressure) of 2 gpm per CRD, a total of 2 x 185 or 370 gpm flow into the SDV would occur.
Although this large flow rate appears feasible within the characteristics of the system, lower rates of leakage to the SDV could also be generated by tne same mechanism, and indeed are more probable.
ihese are discussed below in a framework of average steady-state flow rates.
It is recognized that an actual air system failure would likely lead to continuously changing leakage rates, but the air pressure cegradation might level off and thereby stabilize the leakage rate at any point.
For purposes of evaluation, inflow rates into the SDV can be separated into those for the East SDV header and those for the West SDV header.
Test data show that for Browns Ferry Unit 3 the average drain rate of the East SDV header is normally about 12 gpm with its vent and drain valves open.
- Thus, any steady state in-leakage of less than 12 gpm would not result in water accumulation in the East header unless the East sice drain line were blocked.
Similarly, test data show that the average drain rate of the West SDV header, wita its vent and drain valves open is normally about 2A gpm.
Thus, any steacy-state in-leakage of less than 24 gpm would not result in water accumu-lation in the West header unless the West side drain line were blocked.
Test cata also show that the average drain rate of the SIV, with the vent and drain lines and valves functioning normally, is about 35 gpm. To a first a:oroximation, from the above test data and assumptions, the following general statements can be made:
i i)
For a steacy-state in-leakage below approximately 12 gpm per side, no water accumulation would occur, no water measurement with UT is required, and no operator action is necessary.
17
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2)
For a steacy-state in-leakage between approximately 12 and 24 gom per sice, water would accumulate on the East side.
As an example, for a steady state flow rate of 24 gpm into each header, the West side would remain empty and the East side would fill within apor0ximately 25 minutes.
The current 30-minute surveillance interval at Brcwns Ferry using the UT system might not detect this accumulation before filling of the East side.
Also, because the 5IV drain rate is greater than the inflow rate from both the SDV sides to the SIV, the 50 gallons scram level switch would probably not activate.
However, the 3 gallon and perhaps the 25 gallon level switches might be activated.
This level of inleakage could result in a scenario similar to the Browns Ferry event where the West side rods scrammed successfully but the East side rods did not.
3)
For a steady-state in-leakage above approximately 24 gpm per side, water would accumulate in both the East and West side SDVs. As an example, for a steady state flow rate of 36 gpm into each header, the East header would fill within approximately 12-1/2 minutes and the West header within acproximately 25 minutes.
The current 30-minute surveillance interval at Browns Ferry using the UT system might not detect this accumulation cefore filling both the East and West sides.
For this case, the SIV 50 gallon level switches would probably activate somewhere between 12-1/2 and 25 minutes and initiate an automatic scram.
However, the scram capability would be limited on both the East side and the West side due to the previous water accumulation.
4)
For in-leakage at very high rates (approaching 150 gpm per side) water would accumulate in both the East and West side SDVs.
Each side would fill within 3 minutes and probably before sufficient water could flow to tne SIV to activate the automatic scram switches at the 50 gallon level.
Tne current 30 minute surveillance interval at Browns Ferry using UT j
would not detect this accumulation before a probable loss of scram caca-bility.
Procer operator action would probably be required within less l
than 2 minutes following the initiation of this scram valve leakage rate te avoid reaching a point where it would beccme impossible to scram.
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In summery, the above analysis adresses conditions of degraded cressure
~
in the HCU control air heacer which can lead to aggregate leakage rates to the SDV in the range of 24 to 300 gpm.
Flow rates at the hign end range probably produce at least some rod motion and perhaps some rocs might fully insert.
However, at this time there is no assurance eitner by analysis or testing tnat a range of leakage rates does not exist whicn could fill the SDV quicxly with insufficient indication to tne operator or time for manual scram before tha ability to scram is lost.
Tne above discussion of the scram system behavio-is for different but constant flow rates.
This would prooably not be the case for an actual degraded control air event.
The scram valve flow rate would likely pass through the different regimes as discussed above and the characteristics of a particular flow rate would apply at that time.
However, analysis or test results for a variable flow rate, whicn show acceptable system behavior, do not exist at this time.
Thus, inadequate basis is available
)
to justify disregarding these concerns.
1 A degraded air supply can also affect the perfcrmance of the SDV vent and the SIV drain valves.
Tests done at Browns Ferry show that the SDV vent and the SIV drain valves begin to close at a control air system pressure of about 17 psig.
Thus, the drain and vent valves will remain open during tne type of degraded air condition that mignt lead to loss of scram capability.
It should be noted that the time available for operator action to respond to a degraded air condition can be separated conceptually into two phases:
]
(1) time available before air pressure degrades from the normal alarm set-point of 70 psig to the pressure at wilich scram discharge valves leakage begins (about 45 psig) and (2) time available following the beginning of scram discharge valve leakage to the time where the SDV fills to the point where a scram is no longer possible.
The analysis shows that the time available for operator action following the beginning of scrac discharge valve leakage can be as little as 2 minutes.
Because of the i
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short time available for. operator action following initiation of scram discharge valve leakage, operator action snould be taken prior to reaching a degraded or lost scram capability.
Because HCU concrol air degradation preceeds opening of the scram discnarge valves, addec time would be available if operator action were basec on air pressure incications.
For a rapid air pressure degradation which stabalized at a point where large scram discharge valve leakage occurred, the benefits of operator action based on air pressure indication would be ciminished.
From the standpoint of improved assurance of a successful scram during a dagraded HCU control air event, however it is preferrable to scram on the indications of degraded air pressure than on the UT system.
This would be true even if UT readout were continuous in the control room.
The UT system (on indica-tion of water in the scram discharge volume) to initiate a manual scram for the degraded HCU control air event does not provide sufficient assurance that adequate time will be availaole for the required operator diagnosis and action.
The same can be said for reliance on CRD temperature probes, rod drift alarms, and scram outlet valve indicator lights.
IE Supplement 3 to Bulletin 80-17 requires an immediate manual scram on low HCU control in pressure at a minimum pressure of 10 psi above the opening pressure of the scram outlet valves.
This provides additional time for operator diagnosis and action prior to possible filling of the SDV following receipt of the low pressure alarm.
However, because of the lack of any control room indication of HCU control air pressure (except the low pressure alarm at 70 psig) current procedures at Browns Ferry require an operator to be sent to the HCUs to read a local HCU air pres-sure gauge.
This local operator then reports back the local reading to the control room by walkie talkie.
Given the raoidity of the water inflow possible with the degraded air pressure condition, we judge this arrangement to be inadequate. We believe that the rapid operator response required by a degraded air system condition ncessitates that adequate alarms and instrumentation be available in the control room.
- However, because the present alarm is not safety grade and is a single channel, we 20
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believe that reliance on the alarm alone to initiate a manual scram is not adecuate.
Short of installing safety grade instrumentation for this i
function, we believe that adequate instrumentation could be provided by redundant pressure indication in the control room along with a distinctive alarm on degraded air pressure.
Furthermore. since the instrumentation is not qualified to function curing certain postulated events (e.g.
earthquakes), procecures which require immediate manual scrams for such 4
events should be considered.
t It is our judgment that if the upgraced instrumentation and procedural i
changes discussed acove are provided, then the system will be adequate to respond to cegraded HCU control air for the interim period.
We believe that this analysis supports the position that a scram on degraded HCU control air is sufficient to respond to the complete range of 'gregate leakage rates arising from degraded HCV control air pressure l
as enu.nerated earlier in this section.
1 l
inis judgement is based on C) the additional time available before any discharge valve leakage begins and (2) the relatively low probability of a rapid air pressure degradation which stabilizes in the range of serious scram cutlet valve leakage.
3.7 Oceratina Excerience With Degraded Control Air Sucoly An effort was made to look at reactor operating experience relative to scrams caused by the consequences of a degraded control air supply. By looking through the Annual Report on Nuclear Power Plant Operating Experie.nce(3-6) for the l
years ic74-78, a total of 21 events were found for SWRs where the description of the event mentioned a loss of control air as the initiat'ing event leading to the scram.
The dates of these events are listed in Table 1.
Because of the brevity of the descriptions anc the lack of records available to make a more careful study of eacn event, it is probable that not all of the 4
21
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ever.ts describe a loss of control air which would or coulc affect the HCus.
On the other hand, some of the events seemed to be very close descriptively to the type of rod behavior that would be expected given a loss of control air to tne HCUs.
For example, one event description mentioned massive rod drift.
Another event generated an autcmatic scram due to high level in the SIV.
This event occurred at Browns Ferry Unit 1 on November 24, 1976. Because of the known crain characteristics at the Browns Ferry units, it is likely that curing this event the SDV was at least partially filled.
Because the SDV is cesigned to provice approximately 3.3 gallons per drive free volume, and a typical scram requires less than one g?.11on per drive, enough volume was available for a successful scram.
However, there is no doubt that the volume margin was recuced. An air degradation of a sligntly different character could have lead to a water filled SDV and inability to scram.
An effort was made by AEOD to find data that would indicate filling of the SDV curing some of these events.
From the event at Browns Ferry 3 on June 28,19S0,one bit of evidence that leads to tne conclusion that water was in the SDV prior to the first attempted scram was tnat the SIV high level scram switches were
]
activated more quickly than expected during a manual scram (18 seconds vs. 45 I
seconds).
This is because for a full SDV, water entering the SDV during the j
scram will more quickly pressurize the SDV and force water through the drain line to the SIV than if the SDV were not pressurized.
For events wnere an eventrecorderoutputwasavailable,nosuchchangewashoted.
However, most i
events had no data from an event recorder available and no ather way of recalling this data.
In general, this search for operating experience data was unsuccessful. On the other hand, the argument that because a plant has successfully scrammed 21 times during degraded control air events does not provide a large statistical basis on which to judge the adequacy of the scram system (eachine and man) for resconding to sucn events.
From the observation of 20 successful scrams in 20 scram attemots, one can conclude that the 95% upper (one siced) conficence li;rit for the probability of a scram failure is accroximately 3/20 =.15.
22
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Alternatively, the 95% icwer (one siced) conficence limit for the probability of successful scram is approximately 1 - 3/20 =.85.
Both confidence limits are computed on.the assumption of a common probability of a scram attempt failing and the assumption of statistical independence of scram attemets.
These assumptions have been made for matnematical convenience; they are not necessarily plausible.
In fact, there is no doubt that the list of successful scrams includes some events wnere the HCU control air header system was not affected.
These would not be incitded in a list developed through a closer investigation of the event whicn would disclose that fact.
A lower number of successful scrams, due to legitimate control air degracation, even if all are successes, only detracts from the merits of the argument whicn claims that since no scram failures have occurred to date, the system is adequata.
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FIN 31NGS l
Easec on the system cescription and evaluation discussed in Sections 2. and 3.
of this report, a numcer of fincings have ceen ceterminec.
Again, it should be emphasized tnat these are based on Browns Ferry only.
1 The present system (ultrsonic level instrumentation, existing SIV instru-e i
2 mentation, and special operating procedures, etc.) shoulc be capable of providing acequate protection during the interim against filling of the SDV due to all identified water sources except for tPose related to scram discharge valve leakage due to degraced HCU control air pressure.
Degraded HCU control air pressure cocid result in scram outlet valve leakage to the SDV which would require operator action to manually scram the plant within a few minutes before scram capability would be completely lost.
This event would likely be accomoanied by a plant disturbance recuiring a scram cue to other control air related disruptions in tne plant.
Such an event would be accompanied by numerous control room alarms and indications wnich could distract the operator from a prompt manual scram actuation.
Operating experience indicates that a significant number of reactor a
scrams attributed to loss of HCU control air pressure have occurred.
These provide evidence that rapid filling of the SDV is a credible event.
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1 5.
RECO!<MENDATIONS Tne principal recommendations of the stucy are as follows:
i e
An immediate manual scram shoulc be recuired based on control room indi-cation of degraded HCU control air pressure.
Review of licensee proposals should incluce consideration of the available pressure indications and procedures to assure t:1at other alarms and indications do not divert operator attention from this priority actior, e
Recuncant HCU air heacer pressure instrumentation should be providec in tne control room.
A distinctive alarm for degraded air pressure shculd be provided to aid the operator in cuickly focusing his attention on the t
need for protective action.
Because of the possibility that a currently unidentified water source e
j could result in water accumulation in the 50V, it would be prudent to monitor the ultrasonic system alarm output in the control room and recuire an immediate verification of a sustained alarm by operator dispatch to the equipment. Operability and calibration checks of the system should be continued on a schedule cf once per shift.
i e
25 i
,m-
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6.
CONCLUSIONS AECD has reviewed the interim surveillance system at Browns Ferry used to detect the presence of water in the 50V.
Tne AE00 assessment considers the Orccecures and equipment changes initiated in response to IE Bulletin 80-17 with Supplements, 1, 2, anc 3 te be adequate for continued interim cperation of the Browns Ferry Nuclear Plant, if the recommencations of this report for rescanse to degraded control air pressure are implemented.
i As of the cate of this recort, the instrumentation and procedures in piace to respond to the loss of control air scenario at Browns Ferry are judgec to be inacequate.
For this event the operator must respond prcmotly to a single in-distinctive alarm for loss of control air pressure during a period when numerous alarms may be occurring.
Additionally, the operator must take actions outside the control recm in a very limited time frame because he lacks a pressure readout in the centrol room.
IE is currently taking steps to upgrade the procedure for response to the degraded control air pressure event.
In the past, operator action to perform a vital safety function within less than 10 minJtes has not been considerec acceptable by the NRC.
However, pro-viding the operator with both a distinctive icw pressure alarm and reliable air pressure instrumentation in the contrcl room, would help assure adequate coerator response within the required time period.
Such an arrangement should be acceptable for the interim.
A dedicated operator with adequate alarms and instrumentation in the control room could provide even gr2ater assu nce of a timely manual scram.
If the provisions made to accomplish a manual scram are found to be untimely or inaceqiate, provisions should be made for an automatic scram on low HCU control air pressure.
For the long-term, the scram system shculd be upgraded according to the recom-mendations of the AEOD report of July 30, 1980.
However, the consequences of degraded air pressure in the HCU air headers were not fully recognized at the time of that report and were not directly addressed.
Although tne reccmmended scram system modificatiens may be sufficient tc enable the scram system to 25
respond to rapid inflows of water from tne scram outlet valves due to degraded HCU air header pressure, design review of the long-term modifications shouic include specific consiceration of the effects of degraded air pressure.
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REFERENCES 1.
AE00 MEMO (Michelson) to NRR (Denton) datec August 1, 1980 with enclosures.
2.
AE00 MEMO (Michelson) to NRR (Denton) dated August 18, 1980.
3.
USNRC NUREG-0227 dated April 1977.
a.
USNRC NUREG-0366 dated December 1977.
5.
USNRC NUREG-0483 dated February 1979.
6.
USNRC NUREG-0613 dated Decemcer 1979.
25
Table i Scrans Attributed to Loss of Air (1974-19731 Browns Ferry 1:
S/1/70, 10/19/75, 11/24/76, 8/15/73, S/iS/7S Brc',.ns Ferry 2:
S/18/78 Erunsaick 2:
4/5/77 i
Dresden 2:
9/7/77, 7/2S/78 Dresden 3:
S/15/74 Duane Arnold:
1/9/7S Haten 1:
3/4/75 Millstone 1:
S/5/77. 5/29/78 Nine Mile Point 1: 12/21/74 Pilgrim:
1/19/76 Quad Cities 1:
1/3/77, 4/30/78 Quad Cities 2:
7/1/74, S/31/74, 10/25/77 29
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