ML20138C537
| ML20138C537 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 03/24/1997 |
| From: | TENNESSEE VALLEY AUTHORITY |
| To: | |
| Shared Package | |
| ML20138C530 | List: |
| References | |
| SQ970649PER, SQ970649PER-R, SQ970649PER-R00, NUDOCS 9704300134 | |
| Download: ML20138C537 (57) | |
Text
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- SQ970649PER REV.O C.9 CONTIN. Pg. of ;
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PRESSURIZER LEVEL EVENT
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SO970649PER 1
MARCH 24,1997 l
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1 9704300134 970421ADOCK 05000327<l-' j PDR-g PDR A- ;
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SQ970649PER REV.0 C.9 CONTIN. Pg. of TABLE OF CONTENTS
~
I. EXECUTIVE
SUMMARY
. ...... . ... ...... .. . . . . . . . .. 3 II. DESCRIPTION OF THE EVENT... .. . . .... . . . . . . . . . . . . . . . . .. .. . 4 A. INITIAL CONDITIONS.. . ... . . . . . . .. .. .. ....4 B. SEQUENCE OF EVENTS .. . . . . .. . . . . .. .5 C. IMMEDIATE CORRECTIVE ACTIONS . . . .. .6 III. ANALYSIS OF THE EVENT.. .. .. . . . . . . . . . . . .. .6 A. EVALUATION OF PLANT SYSTEMS / COMPONENTS ... . . . . 6 B. EVALUATION OF PERSONNEL PERFORMANCE. . 10 C. SAFETY CONSEQUENCES AND IMPLICATIONS. .. . 12 IV. DISCUSSION OF EXTEND OF CONDITIONS . .. . . . . . . . .13 V. DISCUSSION OF PREVIOUS SIMILAR EVENTS.. . . . . . . . .13 1
VI[ ROOT CAUSE STATEMENTS... .. . . . . . . . . .15 i VII. CORRECTIVE ACTIONS. .... . . . . . . . . . . . . .. .. . . . .... 16 l VIII. OTHER OBSERVATIONS AND ACTIONS. . . . . .19 IX. DESCRIPTION OF INVESTIGATION... . . . . . . .21 j
X. ADDITIONAL SUPPORTING INFORMATION/ DOCUMENTATION. 21 XI. ACRONYMS. . . . . . . . . . . . . . .. .. . . 22 )
l ATTACHMENTS f
1 - PZR COLD CAL./ HOT CAL. TRACKING l
2 -
THEORIES FOR LOSS OF REFERENCE LEG i
3 -
PZR VOLUME DRAINED - SHIFT MANAGER 4 -
PZR VOLUME DRAINED - PEDS / BETA / STRIP CHART PZR VOLUME DRAINED - RCS PRE'SSURE 5 -
DESCRIPTION OF INVESTIGATION 6 -
PZR COLD CAL. LEVEL BACKFILL MAINTENANCE HISTORY 7 - TIME LINE FOR PEDS / BETA / STRIP CHART 2
SQ970649PER REV.0 C.9 CONTIN. Pg. of PRESSURIZER LEVEL EVENT
~ SO970649PE~R I. EXECUTIVE
SUMMARY
On March 23,1997 at approximately 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br />, the Unit One Operations crew initiated the draining of the PZR from solid water operation using 0-G0-7," Unit Shutdown From Hot Standby To Cold Shutdown" PZR fluid temperature was approximately 140aF as required per 0-GO-7. As the cold cal. (1-LI-068-321)
PZR level channel and the three hot cal. PZR level channels (protection set level transmitters calibrated for 653 'F PZR fluid temperature) came on scale from greater than 100% , the operating crew recognized there was a difference in indication between the cold cal. channel and the three hot cal. channels. The operating crewjudged this to be as expected and did not verify the accuracy of the level indications using graphs and comparison of scales provided in 0-GO-13,
, " Reactor Coolant System Drain and Fill Operations".
The operating crew continued to decrease the PZR level to approximately 56%
cold cal. and 25% hot cal. not realizing that the cold cal. channel (1-LI-068-321) was indicating approximately 33% higher than actual level as result of a loss ofits reference leg. The decrease in PZR level was terrninated at approximately 56% l cold cal. and 25% hot cal. to maintain PZR heater availability for D/G testing I which was in progress as directed by SI-26.
The operating crew reinitiated draining of the PZR using 0-GO-7 at approximately 0658 on March 24,1997 once the SI-26 testing requiring PZR l heaters was complete. The PZR level was at 56% cold cal. and 25% hot cal, when draining was restarted. The objective was to drain to 25% cold cal. level.
l The day shift operating crew assumed the shift at approximately 0735 on March 24,1997 with the PZR level at approximately 44% cold cal. and 0% hot cal.. The day shiR crew also considered this difference to be as expected and did not verify the accuracy of the level indication. Shortly after assuming shift the operating crew recognized that the cold cal, level channel was no longer decreasing from approximately 38% with PZR draining sill in progress. They also noted that RVLIS was indicating approximately 92%. (This investigation determined that RVLIS was isolated as a planned outage activity by this time, per Plant Engineering Data System (PEDS) data.)
The operating crew referenced 0-G0-13 graphs to verify PZR level and then increased charging flow such that charging flow low alarm returned to normal (257 GPM) at 0816. The PZ.R hot cal. level came on scale at 0% by approximately 0858 and was stabilized at 26% by approximately 0916.
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SQ970649PER REV.0 C.9 CONTIN. Pg. of Subsequent evaluations of PZR inventory drained show that level dropped to a minimum of elevation 708.2 feet and that approximately 3300 gallons of water was required to refill to 24% hot cal. PZR level. Calculations determined that it would have taken at least an additional 13 hrs. and 20 min. to reach midloop elevation at a conservative drain rate of 50 GPM.
No safety functions were lost as a result of the loss of reference leg for 1-LT-068- s 321, PZR cold cal. level. This level channel is used for indication only. All safety functions are derived from the hot cal. PZR level channels which have sealed capillary reference legs and are not susceptible to displacement ofliquid in the reference leg due to air or gas expansion.
The root causes of draining the PZR below 25% actual level are: (1) erroneous cold cal. indication caused by loss of reference leg sometime between taking the RCS solid at approximately 325 PSIG and starting the draindown from solid water at approximate 30 psig and (2) lack of a questioning attitude with respect to verifying PZR level by more than one means during the critical. evolution of draining the PZR. ;
Corrective actions have been taken and/or will be taken to address the following areas: ;
e Loss of reference leg
. Personnel performance
. Training
. Procedures II. DESCRIPTION OF TIIE EVENT A. INITIAL CONDITIONS Unit One was in Mode 5 with RCS average coolant temperature less than 140 2
degrees F and PZR pressure at approximately 30 psig with a N gas cover seing supplied from the PRT. Preparations were in progress to decrease the PZR level from a solid water condition to between 80% and 25% cold ;
calibration indication using 0-GO-7 Umt Shutdown From Hot Standby To j Cold Shutdown. 0-GO-7 covers PZR draindown to 25% cold cal... l Transition to 0-GO-13 is required for further drauung. I l
Unit Two in Mode 1 at 100% power. l 1
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l SQ970649PER REV.O C.9 CONTIN. Pg. of B. SEQUENCE OF EVENTS NOTE: All times noted as = are from Operator interviews. PEDS and BETA annunciator times are identified and placed in a time line in Attachments 4 and 7 respectively and are considered to be the accurate times.
3/22/97 1330 -
PZR cold cal. level channel 1-LT-068-321 checked for calibration by MIG 3/23/97 0955 - PZR solid water per 0-GO-7
=2300 - Started decreasing PZR level per 0-G0-7 3/24/97 0025 -
PZR cold cal. on scale at 99% level )
0038 -
PZR hot cal. on scale at 99% level
=0150 -
Stopped decreasing PZR level while indicating ,
approximately 56% cold cal. and 25% hot cal, in order to !
maintain PZR heater availability for D/G SI-26 testing. l l
=0630 -
Cold cal. indicating = 56%
- Hot cal. indicating = 25%
RVLIS indicating = 102% l
=0710 -
Night shift Unit Supervisor directed night shift Unit Operators to start decreasing PZR level to 25% cold cal. after confirming that SI-26 no longer required PZR heater availability
=0735 -
Day shift Unit Supervisor and Unit Operators assumed shift. PZR cold cai. was indicating = 44% and hot cal, was indicating = 0%.
=0745 - PZR cold cal. indicated = 38% and no longer decreasing with PZR draining in progress. Hot cal, was indicating
=0%.
Operator also noticed that RVLIS was indicating = 92%.
This investigation determined that RVLIS was isolated as a planned outage activity by this time, per PEDS data.
Operator terminated PZR draining
SQ970649PER REV.0 C.9 CONTIN. Pg. of
]
=0750 -
O~perator started increasing PZR level. .
, l
=0815 -
PZR hot cal. level indication came on scale from 0%. I
=0840 -
PZR hot cal. level indication at = 25%. I PZR level stabilized at = 25% hot cal. indication s
C. IMMEDIATE CORRECTIVE ACTIONS
- Day shift Operators terminated draining of PZR and started PZR level
- _ - recovery. PZR level was retumed to = 25% hot cal. instrument channel and .
stabilized. This corresponds to an actuallevel of= 22% based on Appendix E l 4
of 0-GO-13.
i Operations Superintendent briefed operating crews on PZR draining incident. !
. 1 1
III.* ANALYSIS OF THE EVENT j
A. EVALUATION OF PLANT SYSTEMS / COMPONENTS ,
1 l 1. 1-LT-068-321 (cold cal.)
l PZR cold cal. level channel was checked for calibration on Saturday I March 22,1997 using PM #1-L-068-0321 *01. The Maintenance )
, Instrument Group (MIG) found the milliamp output slightly high and )
adjusted the zero down. The as found level reading was 40% and the as i left reading was 38%.
While filling the PZR per 0-GO-7, the hot cal. PZR level channels and 1-LT-068-321 (cold cal. channel) tracked properly (See attachment 1).
The PZR was taken solid with water at 0955 on March 23,1997 per log entry. The hot cal. and cold cal. channels indicated greater than 100%
PZR level. During this period Loop 2 RCP was in service with RCS c pressure at = 325 psig and PZR fluid temperature at approximate 140 'F.
PEDS data indicated that 1-LT-068-321 reference leg was full during PZR level increase.
4-Draining of the PZR from solid water operation was initiated at approximately 2300 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.7515e-4 months <br /> on March 23,1997 per 0-00-7. The Operators noted that 1-LI-068-321 came on scale from 100% prior to the hot cal. !
. channels at 0025. The hot cal. channels came on scale from 100% at =
0038. Immediately prior to starting the draining from solid water, the last
, RCP was removed from service and PZR pressure was reduced from = 325 psig to = 30 psig over a 15 minute period. j 6 l
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SQ970649PER REV.0 C.9 CONTIN. Pg. of Evaluation of PEDS data for the hot cal. and the cold cal. PZR level channels indicates that the cold cal. channel,1-LT-068-321, was reading high shortly after it came on scale from 100%. Therefore the reference leg for 1-LT-068-321 had been lost sometime between taking the PZR solid.
with water, stopping all RCPs, decreasing PZR pressure from =325 psig to
= 30 psig over 15 minutes and starting the PZR draining from solid water operation.
The reference leg for 1-LT-068-321 was backfilled per 1-PI-IFV-068-443.0 on March 24,1997 at approximately 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br />. No additional backfills have been required. This eliminates a leak on the reference leg as a cause for the loss of the reference leg. Additionally, walkdown of 1-LT-068-321 sense lines were performed and no extemal leaks were identified.
Several failure modes to account for the loss of the reference leg were postulated (See attachment 2). Two of these failure modes have the !
highest probability for causing the loss of the reference leg. The first is air l or gas entrapment in high or low points of the reference leg which migrate under changes of pressure and/or temperature and thus displace water from i the reference leg. The second is gas coming out of solution at reduced j pressure and temperature and displacing water from the reference leg. !
The reference leg sense line for 1-LT-068-321 was walked down on March 27, and March 31,1997 to check for any grossly abnormal slopes or other abnormalities. During these walkdown, a downward bow of approximately 2" was noted in a 5 foot span between two supports. This ,
bow does not meet the criteria of approximately 1/8" slope per foot of run for instrument sense lines of this size. WRC04964 was written to replace damaged tubing and the WR was noted as a Mode 5 entry restart restraint.
This bow was most likely caused by Outage related work activities (e.g.
scaffolding etc.). Another downward bow of approximately 2 inches was found at a bent flex line connection from the condensate pot to the 3/8 inch sense line tubing. WRC213203 was written to repair connection and was noted as a Mode 5 entry restraint. WRC213204 was also written on missing unistrut clamps on hot cal. channel I (not associated with cold cal.) which was allowing the flex hoses to bow downward. A sense line walkdown has been performed to determine if the 1-LT-068-321 sense line is acceptable. Other sense lines will be walked down per Corrective Action 6.B.
Other problems found during the walkdown include missing insulation on the lines from the PZR to the condensate pots (all three channels) and piping from the PZR to the root valves was found bent downward. These conditions will be evaluated for acceptability and corrective actions will be taken as necessary.
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.. .- - -. ._. -.. = . - - .. . - -. . - .
SQ970649PER REV.0 l C 9 CONTTN. Pg.' of l
J The bowed area has the potential to act as an air dam and therefore could i have released air entrapped previously and thus cause the loss of the i reference leg for 1-LT-068-321. It has been determined that this bowed area has existed since 1988 by reviewing 1988 sense line walkdown -
documentation. j l
s Following is the results of maintenance history review for Unit 1 & Unit 2 !
PZR cold cal. level channels. !
5 B ACKFILL DUE TO LOSS OF REFERENCE LEG 1-LT-068-321 (PZR COLD CAL.)
NOTE: (See attachment 6 for Unit status)
DATE WR# l 4/28/80. MR43369 3/4/86 MR116393 4 7/18/92 C126839 2/14/93 C174392 4/9/93* C174513 l 7/16/93** C174513 9/6/93 C171328 9/11/95 C194689 10/15/95 C211928 10/21/95*** C202412 l
II S-93-023.
- 2"d Backfill using this WR.
- Backfilled to support vacuum fill process for UIC7, was not a loss of reference leg. j i
2-LT-068-321 (PZR COLD CAL.)
NOTE: (See attachment 6 for Unit status) 9/8/81 A157098 A028591 9/23/83 4/25/92 C134784 4/24/96 C280340 !
In summary, there is no record oflosing the reference leg on 1-LT-068- ;
321 from 10/15/95 until the loss of reference leg on 3/24/97.
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.: - - . . =. . . . .- - - . . ,
SQ970649PER REV.0 C.9 CONTIN. Pg. of 1
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- 2. Volume Drained From PZR ]
Following refilling of the PZR to approximately 25% as indicated on the hot cal. PZR level channels an estimate of how low the level decreased was performed by the on-shift crew. Their estimate (See attachment 3) indicates the level was between 705' el. and 707' el. For reference, the top s
of the Rx. Vessel head is 709' el. Additionally, this estimate indicates that a total of approximately 4500 gallons of water was required to return PZR level to 25% on the hot cal. channels. This estimate is considered to be a rough estimate and is conservative.
1 During this investigation two methods were utilized independently by Engineering to estimate how low the level was decreased.
PEDS DATA AND STRIP CHART FOR HOT CAL. PZR LEVEL (See attachment 4)
This method used an average drain rate of 34 GPM starting at approximately 0658 on 3/24/97 until 0744 at which time a 48 GPM drain rate was used. Operator interviews determined that the rate of draining was not changed after PZR level went below 0%. PEDS data indicated that the drain rate increased at 0744. The refill rate was determined to be 88 GPM and was not changed during refilling. This method estimated that
, approximately 3300 gallons of water was required to return PZR level to 24% on the hot cal. channels and that the minimum water level did not go l below elevation 708.8 feet. See diagram X.3 for illustration of 708.8 feet level. A time line for this method is provided in attachment 7.
CHANGE IN PZR PRESSURE (See attachment 4)
This method used the PZR Exosensui presser values before and after the drain down to calculate how low the level was decreased. This method estimated that the level went to elevation 708.2 feet.
In summary, by evaluating the levels and volumes given by these two methods it is safe to conclude that the PZR level was no lower than approximately elevation 708.2' feet and that the amount of water necessary to retum to 24% hot cal. PZR level was approximately 3300 gallons. At no time during this event did the level decrease below the bottom of the PZR.
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SQ970649PER REV.0 C.9 CONTIN. Pg. of- t
- B. EVALUATION OF PERSONNEL PERFORMANCE The following six inappropriate actions were identified during the investigation ,
of this event:
(1) Failure of both night and day shift crews to verify that the observed relationship between the hot and cold cal. level indications was correct.
Both crews accepted the observed difference as normal (with one i exception as discussed below in Section B.(4)), without further verification. In fact, the loss of water from the cold cal. reference leg had caused a reversal of the normal relationship between the two instruments (above 21%, cold cal. normally reads lower than hot cal.; ;
it was higher during this event). This relationship could have l been checked by reference to either Appendix B or E of 0-GO-13.
Additionally, calculations of the volume of water drained could also have alerted the crew to a problem. Two alarms, one at 17%
hot cal. level (Heater off; letdown secured at 0711) and a second at 16% ,
hot cal. level (PZR low level at 0712) also did not cause the night shift ,
crew to question the validity of the cold cal level indication. Note that the day shift crew, when given two additional indications of a level ,
problem (RVLIS on scale at = 92% and cold cal. level no longer -
dropping) referred to 0-GO-13.
Three external factors may have contributed to the crews' failure to apply i a questioning attitude and QV&V techniques in this situation. While l changing any or all of these external factors probably would have caused i either crew to question the observed hot and cold cal. level indications, it is Operations Management's expectation that the existence of two or more different indications for the same parameter is sufficient to prompt application of QV&V to confirm consistency between the different indications. The three factors are: j a) The procedure being used (0-G0-7) did not require comparisons of hot and cold cal. indications as does 0-00-13, nor did it caution the Operator to check the indications by referring to 0-GO-13. ;
b)- Training for Operations with decreased PZR levels focused on reduced inventory and midloop conditions (covered by 0-GO-13). l The last training was conducted in March,1997. Training specific l to draining from solid water operation during Mode 5 operation and I specifically on how to monitor PZR level using hot cal. and cold j
- cal. instruments has not been emphasized.
c) Poor pre-job briefing, as discussed below. j 10
.. . ~ - . . . . . . . - -. - -
SQ970649PER REV.0 C.9 CONTIN. Pg. of (2) The pre-job briefing was inadequate in that it did not provide specific guidance on what hot cal.- cold cal. relationship to expect, nor did it review or refer to the relevant information in 0-G0-13.
Additionally, no specific termination criteria was specified, nor did it cover the need to maintain positive inventory control by tracking the volume of water drained based on drain rate and time. Also, both crews involved in this event were returning to work after three off-days, a s
situation which increases the potential for mistakes. In such situations, good pre-job briefings are essential.
(3) The Outage Shift Manager (OSM) position was established to provide additional Senior Operations management support and oversight in the Control Room during outages. However, the OSM was not attentive to the draindown evolution. He did not participate in the pre-job briefing or reinforce management expectations for confirming consistency between the two different level indications.
(4) While the draindown was on hold pending completion of SI-26, a SM noted that the difference between hot and cold cal. readings appeared unusually large, but did not take immediate action to investigate. The SM had been informed during turnover that draindown would continue to be delayed, and stated he planned to investigate after the 0730 meeting.
When he returned from this meeting, the event had already occurred.
(5) The Unit Supervisor did not infonn either the SM or OSM in advance that i the draindown was being restarted. He considered the restart to be a l continuation of a previously approved activity. l (6) Log keeping by the night shift crew was inadequate. For example, start and stop times for the draindown were not logged. Crew members stated that they were focused on doing SI-26 correctly, and because of the high activity level, log entries received lower priority. The failure to keep adequate logs did not contribute to this event. However, the lack of complete logs made it difficult to recreate the time line for this event. ;
Other factors evaluated during the investigation included schedule pressure and the effect of the OSM position on the chain of command. The investigation concluded that there was no inordinate schedule pressure. While the Operators were aware of and motivated to meet the outage schedule, the PZR draindown was not critical. path, and had 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> float on March 24,1997. With regard to the effect of the OSM position, the investigation concluded that additional clarification of the OSM and SM roles is needed. This is discussed further in Section VIII below.
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SQ970649PER REV.0 C.9 CONTIN. Pg. of One positive factor was the decision to drain the PZR at a more conservative rate from 56% cold cal. as compared to solid water in PZR. The approximate rates for these two periods are (1) approximately 65 gpm average from solid water and (2) approximately 32 gpm to 48 gpm from 56% cold cal. level.
The recovery of the PZR level was evaluated using information from Operator interviews, PEDS, BETA and PZR level strip chart. The Operator interviews provided best estimates of times for actions performed.
Based on all the information available, it appears that the Operators discovered the PZR cold cal. level was no longer decreasing from = 38% closer to 0800 rather than 0745 as indicated in the interviews. This is primarily based on subsequent interviews which determined that the Periodic Instmetions (PIs),
which are performed after assuming shift, were completed prior to discovery of the cold cal. no longer decreasing from = 38%. This PI normally takes 10 to 15 minutes to perform and longer during refueling outages due to level of activity in the MCR. This implies that the time of discovery was close to 0800.
Two sources ofinformation were used to determine when PZR level recovery was initiated. The BETA annunciator system shows that charging flow low i alann returned to normal at 0816. This means charging flow was 2 57 GPM ,
which is the reset value for the low flow alarm. Also, the electronic card readers l for access to the MCR from the Turbine Building were checked. This shows )
that the day shift Shift Manager returned to the MCR from the morning meeting at 0810. He indicated in his interviews that when he entered the MCR he was informed of the PZR draindown event and that PZR level recovery was in progress.
C. SAFETY CONSEQUENCES AND IMPLICATIONS Draining of the PZR was terminated with level no lower than 708.2' el.. This in no way challenged operation of RHR and therefore was not a challenge to core decay heat removal.
Subsequent calculations have determined that it would have taken at least an additional 13 hrs. and 20 min. to reach midloop elevation at a drain rate of 50 GPM (actual drain rate was 32 to 48 GPM). Midloop elevation is the minimum level at which RHR pump NPSH is ensured (695'7"). RHR pump cavitation is expected to occur at 695'2".
No safety functions were lost as a result of the loss of reference leg for the PZR cold cal. level channel since it is used for indication only.
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SQ970649PER REV.0 C.9 CONTIN. Pg. of
(
IV. DISCUSSION OF EXTENT OF CONDITION l Ea_uipment This investigation determined that the extent of condition is limited to open reference legs which have the potential for air entrainment and/or gas coming out of solution with pressure and temperature changes. The reference legs susceptible to these conditions will be identified and corrected as appropriate by corrective actions 6(B) and 6(D).
Personnel Performance Two different crews, failed to question the correctness of the hot and cold cal.
IndicoMns. This is indication of a lack of application of a questioning attitude relative to instrumentation operating outside ranges for which they are calibrated..
It is managements expectation that all information used by operations personal to make operational decisions be closely examined.
V.
- DISCUSSION OF PREVIOUS SIMILAR EVENTS The Nuclear Experience Review (NER) Tracking and Reporting of Open Items (TROI) and Licensee Event Report (LER) data bases were reviewed for previous similar events using the following words: draindown, RCS inventory, reference leg, sense line, depressurization and level transmitter. TROI was searched with the additional words level and drain. The searches also identified several industry documents involving reference leg fill losses (e.g. GE-SIL 470, Bulletin 93-03, IN l 92-54) that were useful in determining the root cause for the current event. The J search identified one similar in-house event that was documented in 1993 under the conective action program by IIS93023," Inadvertent Loss of PZR Inventory ;
During Drain Down."
This investigation team reviewed II93023 to determine if the Corrective Actions from that effort should have prevented this event. Plant operational conditions 1 leading up to the 1993 event bear little resemblance to the 1997 event other than the actual inappropriate operator action of draining the RCS without adequately monitoring and questioning why changes to indicated PZR level changes did not equate to known drain rates. One difference in the Operator performance for the 1997 event is that they recognized that the cold cal. instmment had stopped I decreasing within 20 minutes vice the 60 minutes required in 1993. l 1
I The plant operational conditions leading up to the 1993 loss of reference leg to the -
cold cal. instrument have a strong bearing on the conclusions and corrective actions identified in IIS93023. The reference leg for 1-LT-068-321 was backfilled in July,1992 and February,1993 while Unit I was at full power. Unit I was shutdown in March,1993 following an extraction line leak on Unit 2 and a l
13 i
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SQ970649PER REV.0 C.9 CONTIN. Pg. of decision was made to go into the U1C6 refueling outage. On March 2,1993 Unit I was shutdown, on March 9,1993 the decision to go into the U1C6 refueling outage was made, on March 15,1993 the last RCP was removed from service and on April 2,1993 all RCPs were uncoupled. Therefore the unit had been offline for approximately 1 month, depressurized for approximate 2 weeks and PZR level below 30% for several days before the cold cal. problem was discovered.
The root cause for the 1993 event was stated to be drain valve leakage. The investigation report did not indicate if other failure rnodes were evaluated.
Corrective actions focused on drain valve replacement and putting guidance in Operations draindown procedures to ensure these events were not repeated.
Adequate guidance was proceduralized in 0-GO-13 " Reactor Coolant System Drain and Fill Operations". The drain valves were replaced in August of 1993, but one month later the reference leg was backfilled. Therefore the root cause of the continuing reference leg losses was not adequately addressed by IIS93023.
Because the Corrective Actions for IIS93023 were too narrowly focused (e.g. use of PZR cold cal. level indication in 0-GO-7 was not considered) the corrective actions taken could not have prevented this event.
Problems with adequacy of SQN root cause analysis and ineffective corrective actions have been previously recognized and are being addressed by SQ962633PER and SQ962389PER. No additional actions are necessary as a result of this conclusion regarding IIS93023.
The maintenance history for 1,2-LT-68-321 was reviewed for examples of previous similar failures. This history is identified in Section III. A, above, and discussed in detail in attachment 6. This review identified two other cases where loss of reference leg occ~u rred that appears to have been caused by the same phenomena as in the current event. On September 7,1995, the method of RCS depressurization was changed from a slow steam bubble collapse to a more rapid depressurization while water solid. On September 11,1995 during the first use of this new depressurization method, loss of accurate cold cal. indication was identified by Unit i Operators using 0-00-13, App. E. WRC194689 was written and the reference leg was filled to restore indication. There were no adverse consequences, and no PER was written. The second case is when Unit 2 experienced a loss of reference leg on PZR level,2-LT-068-321, on April 24, 1996. The Unit was shutdown on April 20,1996 for U2C7 refueling outage and was in Mode 5 at 2345 on April 20,1996. While following 0-GO-13 and comparing PZR cold cal, level to Mansell it was determined that 2-LI-068-321 was reading greater than 1 foot out of tolerance. WRC280340 was written to.
backfill 2-LT-068-321. There was no adverse consequences, and no PER was written. The 1995 and 1996 loss of reference leg fill appear to be similar to the 1997 loss of reference leg fill. If PERs had been written on the 1995 and 1996 .
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l SQ970649PER REV.0 i C.9 CONTIN. Pg. of .
occurrences, they could have identified additional corrective actions to prevent the 1997 occurrence. .
r Information Notice (IN) No. 92-54 " Level Instrumentation Inaccuracies Caused ;
By Rapid Depressurization," was reviewed relative to this event. The document was prepared for rapid depressurization events as related to design basis accidents and EOP use for mitigation. Correspondence relative to IN No. 92-54 among TVA, Westinghouse, and NRC retrieved from the Nuclear Experience Review data base was focused on safety related instrument loops and how they are used in EOPs. IN No. 92-54 did not address normal operating modes relative to use of PZR cold cal, channels, and no action was taken relative to those channels.
s VI. ROOT CAUSE STATEMENTS ;
Eauipment The Root Cause for loss of reference leg for 1-LT-068-321 most likely is non-condensibles expanding during RCS depressurization and displacing = 182" of liquid in the reference leg once the PZR level was decreased to the top PZR tap.
Personnel Performance ,
The root cause of draining the PZR below 25% actual PZR level is the lack of a i questioning attitude by the operating crews on Unit One with respect to PZR level. While performing a critical evolution (PZR draining) the cold and hot cal. .
PZR level indications were not verified. This could have been done using graphs and scales provided in 0-GO-13 Reactor Coolant System Drain And Fill l Operations. This root cause and the contributing factors described below were J determined using HPES analysis, event and casual factors charting and barrier analysis.
Six contributing factors were identified during the investigation. These were related to training, procedures, pre-job briefing, management oversight misjudgment, and communications.
i Training Traditionally, training for RCS decreased inventories has focused on 0-00-13 which deals to a large degree with reduced inventory and midloop operations.
Training specific to draining from solid water operation during Mode 5 operat2on j and s;udlically on how to monitor PZR level using hot cal. and cold cal. !
instrt4.ents has nd been emphasized. "Just In Time" training for Unit 1 l shutdown and RCS draining evolutions was provided for the night shift crew l involved during March,1997. ,
15 i
- SQ970649PER REV.0 )
C.9 CONTIN:Pg. of Procedures 0-G0-7, Unit Shutdown From Hot Standby To Cold Shutdown did not provide specific guidance to monitor PZR level utilizing the hot cal. instrument channels for comparison with the cold cal. instrument channel during the drauung process so as to provide redundant indications. As discussed in Previous Similar Events above, II S93023 incorporated significant improvements in level information into the reduced inventory and midloop procedures 0-GO-13. However, an opportunity was missed by not incorporating or referencing similar information in other procedures that rely on the cold cal. instruments, specifically,0-G0-7.
Afanagement Oversieht ,
The OSM position was established to provide additional Senior Operations management support and oversight in the Control Room during outages.
However, the OSM was not attentive to the draindown evolution. He did not participate in the pre-job briefing or reinforce management expectations for confirming consistency between the two different level indications.
Pre-job Briefine The pre-job briefing did not provide specific guidance on the expected hot cal.
cold cal. relationship, nor did it review or refer to the relevant information in O-GO-13. Additionally, the briefing did not cover the need to maintain positive inventory control by tracking the volume of water drained based on drain rate and times. Specific termination criteria were not established during pre-job brief.
31isiudement While the draindown was on hold pending completion of SI-26, a SM nofed that the difference between hot and cold cal. readings appeared unusually large, but did not take immediate action to investigate. The SM had been infomied during turnover that draindown would continue to be delayed, and stated he planned to investigate after the 0730 meeting. When he retumed from this meeting, the event had occurred.
Communication The Unit Supervisor did not inform the OSM or SM in advance that the draindown was to restart.
VII. CORRECTIVE ACTIONS TO ADDRESS ROOT CAUSE AND DIRECT CONTRIBUTING FACTORS
- 1. The Operations Superintendent will take appropriate personnel action and )
continue to hold Operatigns personnel accountable for perfbrmance. l Comolete l Operations Superintendent Due Date 16
SQ970649PER REV.0 C.9 CONTIN. Pg. of
- 2. The Operations Training Manager will develop appropriate training material to enhance Operator knowledge of use of PZR hot cal. and cold cal.
instrumentation for PZR draining evolutions. As a minimum shall include how hot cal. and cold cal. track over entire range.
Operations Training Manager Due Date
\
- 3. The Operations Training Manager will provide appropriate training for Operators on use of PZR hot cal. and cold cal. instrumentation during PZR draining evolutions.
Operations Training Manager Due Date
- 4. The Operations Support Superintendent will revise O-GO-7 to enhance guidance for use of PZR hot cal and cold cal. instrumentation during PZR draining evolutions.
Comolete Operations Support Superintendent Due Date 5.A. The Operations Support Superintendent will revise 0-GO-7 to require i
positive inventory control while draining from solid water in PZR (e. .
gallons per percent, etc.) and require backfill of PZR cold cal. level l reference leg after solid PZR conditions per 0-GO-7.
t Operations Support Superintendent Due Date ,
B. Determine if other Operations procedures reference use of PZR cold cal. i level and establish appropriate backfill requirements. Other procedtues l may include sos, GOs, EOPs and AOPs.
i Due Date j Operations Support Superintendent i
i
. 6. A. The Engineering and Materials Manager will determine if failure modes )
identified in this report could have caused a loss of reference leg for J 1-LT-068-321 (e.g. Hydrogen out of solution, etc.).
4/25/97 Eng. & Materials Manager Due Date 17
.j
.. - _ _ - - - - _1
+ >
SQ970649PER REV.O t C.9 CONTIN. Pg. of ,
P B. The Engineering and Materials Manager will determine if other similar reference legs have same problem as 1-LT-068-321 for SQN Unit One prior to completion of UIC8 by performing a walkdown. Problems identified
' will be appropriately resolved. ,
Eng. & Materials Manager Due Date ;
- C. The Engineering & Materials Manager will develop an inspection plan for :
instrument sense lines that as a minimum will verify condition of appropriate instrument sense lines prior to restart from refueling outages. l i
Eng. & Materials Manager Due Date .
1 D. The Engineering & Materials Manager will perform instrument sense line walkdowns on Unit 2 similar to those currently being performed on Unit 1.
Eng. & Materials Manager Due Date E. The Engineering & Materials Manager will evaluate use of 3/8" tubing vs
%" pipe for instrument sense lines.
Eng. & Materials Manager Due Date -
- 7. Review the details of this event with all Operations personnel to emphasize management expectations regarding personal accountability, pre-job briefings, j QV&V, log keeping, communication and misjudgment.
Operations Manager Due Date I
. 8. Develop additional work related and non work related examples of QV&V for use in training.
May 1.1997 Industry Affairs Manager Due Date
- 9. The Lead Civil Engineer will evaluate the effect of piping bent downward and no piping insulation at the condensate pots for PZR level instrumentation.
Lead Civil Engineer Due Date 18
.~. _ ., -
SQ970649PER REV.0 C.9 CONTIN. Pg. of
- 10. To support the walkdown of other similar reference legs under Action 6.B, Operations will identify critical, pressure, differential pressure and level instrumentation used during off-line (depressurized) conditions. In this context, critical. means instrumentation used to ensure core parameters or other vital parameter are monitored accurately.
Complete s Operations Manager Due Date VIII OTHER OBSERVATIONS AND ACTIONS
.1. PM # l-L-068-032) *01 does not provide sufficient criteria for PMT of 1-LI-68-321 following calibration. For example step 5 on page 20 of 21 of rev. 2 requires Operations to agree that the level recorded in step 4 reflects current plant conditions.
ACTIONS Revise PM # l-L-068-0321 *01 and other similar procedures to provide a more specific criteria for PMT purposes.
Responsible Manager Due Date
- 2. During this investigation the team was made aware of a PER written July 16, 1996 (SQ961965PER) relative to compliance with N2E-884 Rev. I as it ;
relates to inspection plans for instrument sense lines. This PER states that Inspection Plans were canceled December,1992.
ACTIONS Resolve the issues identified in SQ961965 PER in a timely manor such that an appropriate inspection program is in place. Note corrective action VII-6.C may be worked parallel with this action to resolve the common issue.
Responsible Manager Due Date J
19 i
j
[
t
! o i SQ970649PEli REV. 0
' C.9 CONTIN. Pg. of i
- 3. Some Operators stated that the relationship between the Outage Shift Manager and the Shift Manager was not always clear. For example, should the crew keep both SMs informed or only the Shift Manager?
ACTION Clarify the roles of tre Outage Shift Manager and Shift Manager.
Comolete Operations Superintendent Due Date
- 4. During this investigation it was discovered that draining the PRT to less the 10% to allow N 2communication with the PZR leaves little margin to covering the sparger holes.
ACTION Revise 0-G0-7 and 0-GO-13 to drain the PRT to less than 5% for good PRT and PRZ communications.
Ops Support Superintendent Due Date
- 5. During this investigation it was discovered that 1-PI-IFV-068-4 43.0 does not have specific criteria for backfilling instrument reference lines (e.g. gallons per minute, velocity, etc.).
ACTION Engineering to establish criteria and provide in appropriate engineering output document to MIG to revise backfill procedure.
Responsible Manager Due Date 20
SQ970649PER REV.0 ,
C.9 CONTIN. Pg. of
- 6. During this investigation it was determined that PZR hot cal. channels 1-LT-68-320 and 1-LT-68-335 have required multiple backfills during 1995 and 1996. There is no evidence at this time that this problem is directly tied to the loss of backfill on the cold cal. channel 1-LT-68-321.
ACTION The Engineering & Materials Manager wil1 evaluate the hot cal. backfill issue and determine if discrepancies which have been identified during on-going walkdowns are the cause of this problem.
Eng. & Materials Manager Due Date 3
- 7. Revising 0-00-7 to require the Mansell level gauge to be in service
. before starting PZR draindown from PZR solid water operation and require Outage Management to include installation of Mansell level gauge .
in refueling Outage schedule per 0-00-7. >
Ops Support Superintendent Due Date .
IX. DESCRIPTION OF INVESTIGATION ,
I i
(See attachment 5)
X. ADDITIONAL SUPPORTING INFORMATION/ DOCUMENTATION
-1. 1-PI-IFV -068-443.0 Rev. O, Attachment 1
- 2. Pictures of sense lines for 1-LT-68-335 and 1-LT-68-321
- 3. 0-GO-13 Rev. 6, Appendix A- PZR Elevations, Appendix B - PZR Level Instrumentation Range Comparison, and Appendix E - Curve for PZR hot cal, and cold cal. comparison. l
- 4. EVENT AND CAUSAL FACTOR CHART.
i 21 i
I i
SQ970649PER REV.O C.9 CONTIN. Pg. of XI. ACRONYMS i PZR -
PRESSURIZER D/G -
DIESEL GENERATOR RVLIS -
REACTOR VESSEL LEVEL INDICATING SYSTEM PEDS -
PLANT ENGINEERING DATA SYSTEM GPM -
GALLONS PER MINUTE 1 PRT -
PRESSURIZER RELIEF TANK s
QV&V -
QUALIFY, VERIFY AND VALIDATE SM -
SHIFT MANAGER OSM -
OUTAGE SHIFT MANAGER RCS -
NUCLEAR EXPERIENCE REVIEW TROI -
TRACKING AND REPORTING OF OPEN ITEMS LER -
LICENSE EVENT REPORT IN -
INFORMATION NOTICE WR -
WORK REQUEST PER -
PROBLEM EVALUTATION REPORT SO -
SYSTEM OPERATING GO -
GENERAL OPERATING EOP -
EMERGENCY OPERATING PROCEDURE AOP -
ABNORMAL OPERATING PROCEDURE PMT -
POST MAINTENANCE TEST PM -
PREVENTIVE MAINTENANCE PI -
PERIODIC INSTRUCTION 22
A 'SQN BACXFILLING SENSING i.INES ON SYSTEM 68 l-PI-IFV-068-443.0 l 0
PRESSURIZER PRESSURE AND LEVEL Rev. 0 1 TRANSMITTERS PROTECTION SET II Page 1 of 1 1
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~ SON 0-GO-13 REACTOR COOLANT SYSTEM Rev: 6 -
0,1,2 DRAIN AND FILL OPERATIONS Page 118 of 235 (( () APPENDIX A
- Page 1 of 1 RCS FEATURES -
DIAGRAM FOR MIDLOOP OPERATIONS SEQUOYAH ELEVATIONS [C.14] gfg Partial Draindown @ Reduced Inventory _............................. --..- ....... . . - 7s4 . Top of scd tw Manas uwstor MN [.=3 O _ ........ ....... . . 754* 8 t/4% 100% COLD CAL LEVEL i pg - .... 738' S% TOP ELEV OF TOP STEAM GENERATOR TUBES
- 73T 2 . t00% HOT CAL LEVEL-(73T 4%U2)
T O a . . O b - b E 720' 6*. HOT and COLD CAL CROSSOVEft POINT (720' 7*.U2) 1 _.................. .. .. I ... -
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SON 0-GO-13 REACTOR COOLANT SYSTEM Rev: 6 , 0,1,2 DRAIN AND FILL OPERATIONS Page 119 of 235 ' APPENDIX B Page 1 of 1 PRIMARY / BACKUP LEVEL INSTRUMENTATION ELEVATIONS COMPARISON CHART PLANT ELEVATION NOTES i. Both Units levels are the same within indicator readability tolerance.
- 2. At least 2 independant indications are required frorn 702' to 717* 11" (15% cold cal) with fuel in the core. See Appendix C for requirements belo_w 702'[C.26]
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~
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I ! SON 0-GO-13 0,1,2 REACTOR COOLANT SYSTEM Rev: 6 Page 127 of 235 [,3 DRAIN AND FILL OPERATIONS
~
APPENDIX E Page 1 of 1 PRESSURIZER LEVEL COFlRECTION CURVE FOR HOT CAllBRATION CHANNEL READ AT COLD CONDITION [C.26] NOTE Formula is to be used to calculate actual lev,el and table is to only be used as a quick reference. FORMULA H2= (0.4921) (H 3) + 10.22% TABLE l 100 9 90 I I I 80 ; ;
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ATTACHMENT 1
~
Time line for pressurizer level loss of reference leg ;
- 1. March,22 at approximately 1350 IM's began cal of 1-LT 68-321 at approximately 1530 cal was completed and valved back in. PM was left open to perform IVs on valve line up. IV's were completed on mid shift March,23 paper work was closed out with Operations verifying that cold cal level reflected correct plant conditions.
- 2. March,22 at approximately 2047 calibration reflected approximately same as cal before beginning of PM.
- 3. March,23 at approximately 0438 hot ca! went off scale high and P250 points became invalid. Cold ca! was reading 69% level. 's ' s 4 March,23 at approximately 0910 cold cal TSC point became invalid .
- 5. March,24 at approximately 0035 hot cals and cold cals came back on scale. Cold ca! was only 5%
lower than highest hot cal level."
- 6. March,24 at approximately 0148 hot cals were approximately 24% and cold cal was approximately 57%." ~
- 7. March,24 at approximately 0714 hot cals were approximately 15% and cold cal was 51% at this
- point P250 and the TSC were removed from service due to the ICS mod.
- 8. March,24 at approximately 1600 cold cal backfilling was completed. ,
" Indications after levels came back on scale, for hot cal and cold cal, had swapped (cold cal was reading higher than hot cal). This is an indication that the cold cal had lost its reference leg causing this ,
instrument to read higher than it should. Due to the hot cals having a sealed reference leg only a few inches of water would have been lost. This caused the hot cals to reflect a level that was closer to it's actual cal. f Experience has shomt that on level loops placed under a vacuum the reference leg can be lost. This is often seen when the MSIV's are opened causing the Stm Gen level to go high . The level then must be back filled. If an inadequate vent path was provided during drain down of the pressurizer a vacuum could have been applied to the reference legs causing the problem experienced en the pressurizer cold cal.
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ATTACHMENT 2 The following are areas that have identified as potentially causing the, loss of reference leg:
- l. flashing of the reference leg under vacuum
- 2. hydrogen in solution causing high point entrapment
- 3. siphoning
- 4. instrument valve open
- 5. leaking pipes or lines
- 6. air bubble in line
- 1. Flashing has not been completely mled out however, it is very unlikely. It was estimated'that the sense line temperature was at approximately 85 degrees and a very high vacuum would have had to been established to have caused flashing. In addition the water temperature in the pressurizer was at approximately 120 degrees which meant that the pressurizer water would have flashed first preventing vacuum from going high enough for the sense lines to flash.
- 2. Hydrogen in solution causing high point entrapment is still being researched in one area and has the potential for causing the loss of reference leg. Tnere is one area being looked at this time. Can enough hydrogen be produced to displace the water in the sense lines?
High points have been identified in the sense lines af*er walk downs and could be a causing factor in the loss of reference in combinanon with the i,ydrogen in solution.
- 3. The siphoning effect was ruled out due to the fact that a leak must be present to allow the water to be
. .. pulled back up the sense line. There is no indication that any leaks exist due to the fact that the reference line is maintaining it's fill and the level indication is reading correct.
4 An open or missed aligned valve has been ruled out due to the fact that the valve would have had to have been opened and then closed after drain down started but before the instrument came back on scale. In addition, there is no history of any person operating valves at the instrument during this time.
- 5. Leaking sense lines or valves has been ruled out due to the fact that the reference leg dropped down to a certain point then stopped and after the reference line was back filled the line maintained it's proper fill.
- 6. Air bubbles in the line is possible however lines were back filled at the end of the previous outage and there were no indications ofincorrect levels after the back fill. in addition, the cold cal read correct!y up till flood up indicating that there wasTo air trapped in the lines. Like number two its believed that a high point must exist to cause this problem .
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.y y ATTACHMENT 4 , DETERMINATION'OF MINIMUM REACTOR RCS LEVEL DURING U1C9 E'JENT ON 3/24/97 Page 1 of IT ~
Calculated by: -(+ 6 b 4/t g ; Checked by: A /p % 9f 4 i SQ970649PER DETERMINATION OF bRAINDOWN/ FILL RATE AND MINIMUM WATER LEVEL This evaluation determines the draindown rate and minimum water elevation reached during the pressurizer level anomaly which occurred on March 24,1997. Data used includes PEDS (untilloss of , P250/ PEDS at 07:51), BETA annunciation history, operator observation / recording of the Exosensor ' Train B RCS pressure, and the pressurizer level recorder strip chart during the event. RVLIS upper plenum readings were not used since the high side sensor bellows was isolated during the event, invalidating the data. The upper plenum high side sensor was isolated at approximately 08:00 as estimated by AUOs (NOMS logged at 08:14 after AUOs exited reactor cavity). Isolating the 1 high side bellows could have caused the RVUS upper plenum reading to immediately track l pressurizer /RCS level and give a false indication of dropping level in the upper head. (Operations noted a minimum RVLIS upper level reading of 92% during the event). Operation's time estimates were not used to calculate the minimurn RCS level since PEDS and BETA ! times, which validated each other, did not agree closely with some of the times reported by l Operations. This is due to the fact that the times reported by Operations were from memory after the ! event was over. For information only, a minimum level was ca!culated using Operation's estimate of 250.0 gallons required to refill to 0% pressurizer hot cal (HC). , The attached graph provides a summary of known and assumed information used in determining draindown/ fill rate. This graph is based on PEDS and BETA information which is also attached. The draindown rate from PEDS data was determined to be 32 to 48 GPM and the fill rate from the pressurizer level recorder strip chart was 88 GPM. Draindown rate is assumed to be constant at 48 GPM during the time period when PEDS data was lost until the BETA low charging flow alarm returned to normal at 08:16 (Iow flow setpoint 55 GPM). It is assumed that when the low charging flow alarm cleared that the operator stabilized the charging / letdown rates. The assumption of essentially constant letdown rate is consistent with information obtained from Operations. Using the availat,le information, a draindown volume below 0% hot calis calculated below: Drain volume below 0% HC = 34 gal.
- 8 min. + 48 gal.
- 32 min. = 1808 gal The total draindown volume (and hence fill volume) from 24% hot cal to the minimum elevation ]
reached was calculated to be 3288 gallons. The fill rate from the hot callevel strip chart was 88 j gpm. Minimum pressurizer level was determined by two different methods as cescribed below: First, the RCS Exosensor' Train 8 pressure observed at 24% hot cal and the minimum pressure observed during the event were used to determine the minimum level reached. The minimum level determined by this method is 708.2 feet. Second, the minimum level was determined from the volume drained (below 0% hot cal) and the pressurizer geometry. The minimum elevation determined by this method is 708.8 feet (level above pressurizer outlet nozzle). Additionally, an information only calculation was performed using the Operation's estimated inventory addition of 2500 gallons. The results of this evaluation shows that the minimum level would have been just below the top of the loop piping, which is inconsistent with available data and is therefore ruled out. ,
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DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 Pegn 2 ef I?~ , Calculated by: I-Q N khgl4-7 , Checked by:%L 9f,go An evaluation was performed to determine the additional time the draindown could have continued until RHR suction was lost. Using conservative estimates, the time until loss of RHR would have occurrad is 13 hours,20 minutes. ; in summary, it is concluded that the draindown rate was 32 to 48 GPM, the fill rate was 88 GPM, I and the minimum water level did not go below elevation 708.2. The pressurizer water level dropped to just above the outlet nozzle but did not enter the surge line. No water drained from the unvented l reactor vessel head. Even the information-only calculation based on Operation's estimates did not !
~
jeopardize RHR cooling. P
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DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING USC8 EVENT ON 3/24/97 Page 3 of lj t
- Calculated by:. f -Q - L g /g [4 ; f Checked by: % % 4fn PURPOSE: Determine minimum level reached using pressureihead correlation basep og9pserved Exosensor pressures. This method uses a wide range pressure instrument whichha's d rafge absolute - r i error. Since a pressure difference is used to compute elevation, the error will be less but the exact i error value is unknown. However, the minimum elevation determined by this method is included as l 1 additionalinformation and tends to support the minimum elevation determined by the drain volume method.
Given: s
- 1. Pressurizer hot callevel = 24% with RCS B Exosensor pressure = 15.3 psig.
i 2. Minimum RCS B Exosensor pressure = 9.7 to 10.0 psig (Average = 9.85 psig)
- 3. RCS temperature approximately 116 deg. F from PEDS
- 4. Hot cal 0% = 715.75 and hot cal 100% = 737.17 from 0-GO 13 i
- Find Elevation at hot callevel = 24 %
. 715.75 + .24(737.17 715.75) = 720.9 ! Determine equivalent ft. of head AP = 15.3 - 9.85 = 5.45 psi Ft. of head at 116 deg. F = (5.45 psi) * (.016188 ft8 /lb)*(144inz/ft2 ) ; 4
= 12.7 ft.
- Determine minimum elevation reached
, I Minimum elevation = 720.9 ft. 12.7 ft = 708.2 ft. j t s ) i
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DETERMINATION dF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 Prgs 4 of [] Calculated by: f O. t/% 4/g j Checked by: R h yf,gy j 8 : PURPOSE: Determine the pressurizer water level, using available PEDS and BETA data. This method is considered to be the most accurate of the methods used. Determine amount of water drained from pressurizer inventory:
- Given plant operational data (from PEDS and BETA): ,
i
. Hot calindication reached 0% @ 0736
. Net inventory loss of 34 gpm from 0736 to 0744
. Net inventory loss of 48 gpm from 0744 to 0816 ;
e Draindown terminated @ 0816 . i Net pressurizer inventory loss: 34 gpm ;(0744 - 0736) = 34 gpm
- 8 minutes = 272 gallons :
48 gpm * (0816 - 0744) = 48 gpm *.32 minutes = 1536 gallons . Total pressurizer inventory loss = 272 + 1536 = 1808 gallons Calculate totalPressurizer volume: Given:
. Pressurizer ID = 84 inches, from pressurizer stress report e
. Hot Cal 0% Elev. 7,15'9" = 715.75', from 0-GO 13
. Cold cal tap in the pressurizer lower shellis Elev. 711'-4-3/4" = 711.4' (from O.
GO-13) :
. Geometry of Pressurizer lower head is approximately hemispherical (radius 3.5 feet) with the he'mispherical radius center point 2.25 inches (0.19 ft) above the lower tap (from pressurizer stress report). .
Calculate volume of Pressurizer heaters: Maximum length of heater (from nozzle to hot cal 0%) = 715.75 708.1 = 7.65 feet Diameter:. 0.875 inches != 0.073 ft from pressurizer stress report Number of heaters: 78 from pressurizer stress report , Volume = (3.14*(.073)2/4] *7.65 *78 = 2.5 cu ft = 18.7 gallons Calculate volume of Pressurizer bottom head: . The diameter is 84 inches = 7 leet . Radius R is 3.5 feet, hemispherical radius center point is 0.19 feet above lower tap (L = 0.19) - V = (213)nR8 nR'L = (2/3) x(3.5)* - n(3.5)2(0.19) = 82.5 cu ft = head volume Therefore, head volume = 82.5 cu ft
" Calculate volume of Pressurizer below Hot Cal 0%
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DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 peg 3 5 cf if , Calculated by:_ 4-d . A 4./A/g ; Checked by:R Q #fy 2 Volume = nr (715.75" - 711.4") + head volume - heater volume ' Volume = 167.4 cu ft + 82.5 cu ft - 2.5 = 247.4 cu ft. , 247.4 cu ft
- 7.48 gallons /cu ft = 1851 gallons Therefore, the volume of the pressurizer below the hot ca! 0% is 1851 gallons Determine elevation of pressurizer outlet nozzle .
! The lower pressurizer tap is 2.25 inches (or 0.19 feet) below the lower head radius of 42 :
inches (from the pressurizer stress report). Using this information and the elevation of the
- lower tap (711.4 feet), the nozzle elevation (lowest internal point of pressurizer) can be determined.
s 711.4 + 0.19 - 3.5 ft = 708.1 ft I t i ,. Determine'levelof waterin the pressurizer:
. .- Volume remaining in pressurizer = 1851 - 1808 = 43 gallons
- The formula for volume of a segment of a sphere is V =(1/3)nh 2(3R h) :
- This results in a value of h of approximately 0.7 feet. Pressurizer water level = 708.1 + 0.7 = 708.8 ft. l Determine drain volume from 24 % hot cal to minimum levelreached: Volume from 24% hot cal to 0% hot cal = .24*(737.17 - 715,75)* x(3.5)2
- 7.48
= 1480 gallons !
! Total drain volume from 24% hot cal = 1480 + 1808 = 3288 gallons ; CONCLUSION: Since the drain volume below hot cal 0% was calculated to be 1808 gallons, and the pressurizer volume below hot cal 0% is 1851 gallons, the water level remained in the pressurizer. The calculated minimum water level is 708.8 ft. using this method. The total drain volume from 24% i hot cal to Elev. 708.8 ft, is 3288 gallons.
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DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 Pags 6 of /7
~
Calculated by: I. 4. N +/R [4 7 - Checked by: Ah_ t,f,a, I
' PURPOSE: Using Operations estimate of RCS refill rates, determine if the minimum level would have entered the RCS loop piping.
Given: 1 The initial condition of the event includes unvented, completely filled RCS loops. 2 100 GPM fill rate, obtained from interviews with Operations personnel 3 25 minutes duration until Hot Callevelindication rose above zero, obtained from interviews with Operations personnel
'4 Pressurizer ID = 84 inches, from pressurizer stress report 5 Hot Cal 0% Elev. 715'9" = 715.75', from 0-GO-13 6 . Cold cal tap in the pressurizer. lower shellis Elev. 711'-4-3/4" = 711.4' (from 0-GO-13) 7 Geometry of Pressurizer lower head is approximately hemispherical (radius 3.5 feet) with the hemispherical radius center point 2.25 inches (0.19 ft) above the lower tap (from pressurizer stress report).
Calculate volume of Pressurizer heaters: Maximum length of heater (from nozzle to hot cal 0%) = 715.75 708.1 = 7.65 feet Diameter: 0.875 inches = 0.073 ft from pressurizer stress report Number of heaters: 78 from pressurizer stress report Volume = [3.14 *(.073)'/4]*7.65 78 = 2.5 cu ft = 18.7 gallons Calculate volume of Pressurizer bottom head: The diameter is 84 inches = 7 feet Radius R is 3.5 feet, hemispherical radius is 0.19 feet above lower tap (L = 0.19) V = (2/3)nR' nR8L = (2/3) n(3.5)' - n(3.5)'(0.19) = 82.5 cu ft = head volume Therefore, head volume = 82.5 cu ft Calculate volume of Pressurizer below Hot Cal 0% Volume = nr2 (715.75" 71).4") + head volume - heater volume Volume = 167.4 cu ft + 82.5 cu ft - 2.5 = 247.4 cu ft 247.4 cu ft
- 7.48 gallons /cu ft = 1851 gallons Therefore, the volume of the pressurizer below hot cal 0% is 1851 gallons Calculate volume containedin the pressurizer surge pipe Length of pipe in surge pipe: ~ 58 ft (1-47W304-1)
Pipe is schedule 160, pipe holds 5.11 gallons per linear foot. (Navco Catalog) 58 ft
- 5.11 gallons /ft - 296 gallons
- . --_. ~ . . . . . -. . - - - _ _ -
..- 4 DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 P:gs 7 of jf .
Calculated by:__ f.d. O g/gh Checked by:_ % g,g 7 r Total volume la pressurizer plus surge line: 1851 gal + 296 gal = 2147 gallons Determine waterlevelin Pressurizer surge line where flashing willoccurin S/G tubes. ; i It is possible that water could drain from the steam generator tubes into the pressurizer surge
- line. This section will determine if this effect could occur with the system conditions existing at the time of the event.
As the RCS level drops, a point will be reached where water begins to come out of the steam ' generator tubes due to the height of the standing water column in the steam generator tubes. The standing water column height is determined by subtracting the vapor pressure of water at the given RCS temperature from the atmospheric pressure and then adding the PRT pressure. ; The top of the S/G tubes is El 738'5" = 738.4'- (from 0-GO 13). The RCS temperature at the time of the event was 116F (conservative from PEDS data). j i Vapor pressure @ 116F = 1.5133 (steain tables) l l PRT pressure at the time of the event was approximately 6 psig (conservative from PEDS l data) Density of water @116F = 61.77 lbicu ft, converting units => 0.429 psi /ft Standard atmospheric pressure (14.7 psia) 1.5133 psia + 6 psi = 19.2 psia
~
19.2 psi / 0.429 psi /ft = 44.8 ft standing water column Therefore, when the water levelin the pressurizer is 73 8.4' - 44.8' = 693.6 ft, flashing will ) occur in the steam generator tubes. As this is below the level of the hot legs, flashing will j not occur before S/G tube venting occurs. The top of the reactor head is El 709', therefore l the same conclusion is true for the reactor head.
Conclusion:
No water will drain from the S/G tubes r,r the reactor head prior to nitrogen being allowed to enter the S/G tupes. Estimate the amount of waterpumpedinto the RCS The duration is from the estimated time Operations began the refill to the estimated time l when hot cal indication rose above zero. The estimated amount of water used to refill the' system was = 100 gpm
- 25 minutes =
2500 gal. Determine if the minimum waterlevelcould have entered the RCSIcops. Since the total volume below the Pressurizer hot coi 0% to the RCS toop piping is 2147 gallons, the minimum water level would be just below the top of the loops if 2500 gallons had been required to refill to 0% hot cal. As water drained from the RCS, nitrogen would have entered the loop 2 hot leg and traversed along the top of this leg to the reactor head v
DETERMINATION O'F M!NIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 Pzga 8 of 17~ Calculated by:_ f.d . O 4 / thy } Checked by: W Q 47,,jn ' and to the steam generator tube bundles, The level would have remained ndar the top of the loops until essentially all water drained from the steam generators and the reactor head. A '
- margin of approximately 39,000 additional gallons would be available for draining before loss of RHR suction. j i
CONCLUSION , If Operation's estimate of 2500 gallons required to refill to 0% hot calis used, the minimum levelis just below the top of the loop piping (approximate elevation 696) and nitrogen would have entered the loop piping and RCS components. The amount of nitrogen that could have entered the RCS loops would not have endangered RHR suction, with a large margin. Y e o I D
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DETERMINATION OF MINIMUM REACTOR RCS LEVEL DURING U1C8 EVENT ON 3/24/97 ) Prgs 9 of /f Calculated by: f.4. b QLk/g Checked by:__ Afh v/,9fo DETERMINATION .0F DRAINDOWN TIME UNTIL LOSS OF RHR SUCTION IS LIKELY '
- 5 i
PURPOSE: Determine the time until RHR suction is lost Assumptions: Constant draindown rate of 50 gpm - Conservative as the highest draindown rate observed in this event is less than 50 gpm. , 4 {as l$1 d Per 0 GO 13, Appendix Y, ~ 53,000guare required to fill the RCS from the minimum midloop i operating level to 100% cold callevel under vacuum fill conditions. This value is conservative for this evaluation as approximately 200 standard cubic feet of air remain in each S/G (reference RIMS B38 950908 806).' The Pressurizer volume is approximately 12,500 gallons measured from 0% cold
. cal, to 100% cold cal or 11,250 gallons measured from 0% hot cal to 100% cold cr.l. Therefore the total RCS volume from minimum midicap level to 0% hot cal is 53,000 - 11,250 or 41,750 gallons. -
Since the drain volume was calculated at 1,808 gallons, an additional 41,750 - 1,808 or approximately .40,000 gallons would have to drain before RHR suction would be in jeopardy. Assuming a draindown rate of 50 gpm, it would take an additional drain time of 40,000/50 or 800 minutes (13 hours 20 minutes) for this to occur. Note-IF the Operators estimate of 2500 gallons refill volume is used, the additional drain time would be approximately 13 hours (1000 gallons /50 apm = 20 minutes less drain time and 39,000 gal margin). l CONCLUSION: The additional drain time to lose RHR suction (after minimum level was reached) would be greater than 13 hours and 20 minutes. 1 i i l
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a 2 ATTACHMENT 5 , _ INVESTIGATION PLAN SQ970649PER PRESSURlZER (PZR) LEVEL DRAINDOWN BELOW 25 PERCENT - Scope This investigation will determine the cause(s) for the unintended draindown of the Unit
- One PER level below 25 percent that ' occurred on March 24,1997.
~
Personnel
- The investigation team consists of the following personnel
J. R. Walker - OPS (Team Leader) . J. D. Robertson - NA&L i Ed Elam - Technicalhupport Gregg Clark - mig Larry Begley - NE ' John Thomas - NE Schedule Draft report by April 2,1997, i Actions
- 1. Ensure evidence is preserved, as necessary. - Walker ;
\
- 2. Evaluate the need for immediate action to prevent recurrence. - Walker 1
- 3. Assemble and brief team, make assignments. - Walker j l
- 4. Obtain written statements from involved personnel. - Walker )
l
- 5. Collect information relative to thp, event (control room togs, maintenance history, ;
control room charts, prints, etc.). - Clark / Walker O. Interview involved personnel. - Robertson/ Walker l
- 7. Determine root cause analysis technique and perform analysis. '
i a) Equipment problems-Clark -- b) Human performance - Robertson
- 8. Prepare draft section of the final report in accordance with the attached outline.
- 9. Review investigation results in meeting (s) with site managers and involved personnel.
JDR:EAM - 1:WD: Invest. plan.97083PER 4 4' l l
. . .-. .- . _ . . . . _ - = . . . . .. .
ATTACHMENT R8 PORT OUTUNE 4
- 1. Executive Summary Walker .
; ll. Description of the Event Walker Ill. Analysis of the Event
\
A. Evaluation of Plant Syptems/ Components Clark B. Evaluation of Personnel Performance Robertson C. Safety Consequences and Implications Walker IV. Discussion of Extent of C6ndition Walker /Robertson i V. Discussion of Previous Similar Events Bajraszewski VI. Root Cause Statement (s) Clark /Robertson I Vll. Corrective Actions to Address Root Cause(s) Clark l and Direct Contributing Factors Vill. Other Observations and Actions Waixer i IX. Description of Investigation Robertson i X. Additional Supporting Information/ Documentation 4 I f a mesa 1 WDC;-- "7'--myesepgn
. . . .- --- =. . - . , ,.
- SQ970649PER REV.O l C.9 CONTIN. Pg. of ]
l i I ATTACHMENT 6 UNIT-1 l 2 Date Mode Pwr Lvl RCS Level Rapid Depressurization 1 4/28/80 Unknown Unknown Unknown Unknown 3/4.'36 5 or 6 0 Unknown No 1988 Bowed sense line on 1-LT-68-321 reference leg identified and , evaluated as acceptable 7/18/92 1 100 % PZR 60% No 2/14/93 1 100 % PZR 60% No 4/9/93 5 0 PRZ = 10% No 7/16/93 6 0 Unknown No -- RCS drained and i filled since 4/5/93 9/6/93 5 0 Unknown No , 1 9/7/95 0-GO-7 effective dat u; introduces water solid operation and rapid depressurizaion of RCS. Prior to this date RCS depressurization 1 was performed by a steam bubble collapse method resulting in a slower depressurization. ] 9/11/95 5 0 Unknown Yes - Identified by use of O-GO-13, App. E ) 10/15/95 5 0 =700' el. No - RCS drained and filled since 9/11/95 10/21/95 5 Ref. leg. filled for support of vacuum refill of RCS i l 3/24/97 5 0 Near water solid Yes l l The most probable cause for the current loss of reference leg fill (nonsoluable gas release 1 due to rapid depressurization, coupled with bowed sense lines) may also explain the ] 9/11/95 event. However, none of the other reference leg problems appear to have this i Cause. I 1 I l
. _ _ . _ . _ . . - _ . . . _ . _ ._ _. _ ~ . . .. ._ _ _.. _ _._ . _ . __ . . . _ _ . . , _ . . . .
** e
- SQ970649PER REV.O C.9 CONTIN. Pg. of v
ATTACHMENT 6 ., UNIT-2 E Date Mode Pwr Lvl RCS Level Rapid Depressurization
^
i 9/8/81 5 or 6 0 PZR Empty No i ~
. s 1 9/23/83 5 or 6 0 PZR Empty No 4/25/92 5 0 RCP Sweeps ' No and Vents i'
4/24/96 5 0 Unknown Yes- IDENTIFIED BY , USE OF 0-GO-13 WHEN COMPARED TO MANSELL ; { 4 i ! I 1 l 1 i l i 1 4 i 4
- i O" " l SQ970649PER REV.0 C.9 CONTIN. Pg. of ATTACHMENT 7 l PZR LEVEL CHANGE TIME LINE BASED ON PEDS, STRIP CHART AND BETA ANNUNCIATOR 3/24/97
=0658 -
Charging flow reduced from = 95 GPM to = 60 GPM with PZR hot cal. level indicating = 24% based on PEDS and hot cal, strip chart Drain rate was = 35 GPM based on PEDS
=0711 - PZR to level alarm received (17% setpoint) based on
. BETA annunciator
=0736 -
PZR hot cal. level at zero based on PEDS
=0744 - Charging flow reduced from = 60 GPM to = 44 GPM based on PEDS
- Charging flow low alann received (setpoint < 55 GPM) ]
based on BETA annunciator Drain rate was = 48 GPM based on PEDS
=0751 - P250 computer and PEDS data lost due to computer outage
=0816 -
Charging flow low alarm cleared (57 GPM reset) based on BETA annunciator
=0838 -
Estimated time when fill rate increased to 88 GPM (assumes no ramp up time)
=0858 - PZR hot cal. level reached 0% based on strip chart
=0908 - PZR level lo alarm cleared (17% setpoint) based on BETA annunciator
=0916 - PZR level returned to 26% on strip chart I
1 I
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