ML20236G620
| ML20236G620 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 10/29/1987 |
| From: | Schnell D UNION ELECTRIC CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| ULNRC-1663, NUDOCS 8711030155 | |
| Download: ML20236G620 (22) | |
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E15l 1901 Gratiot Street. St. Louis Donald F. Schnell Vice President
,s.
October 29, 1987 1
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,s U.S. Nuclear Regulatory Commission 3
Document Control Desk j
" Washington, DC 20555' ULNRC-1663 i
Gentlemen:
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i DOCKET NUMEER 50-483 CALLAWAY PLANT CALLAWAY REpCTOR VESSEL HEAD CLOSURE Union Electric ;has completed an evaluation of,tbe status of the reactor vesse.1 head closure studs.as they will exist during cycle 3.
During head disassembly at the beginning of Refuel II, some difficulty in removing studs was encountered.
Five of the 54 vessel head studs were not removed during the outage.
One of those five remains in a partially inserted position such that compilete tensioning of the stud is not considered feasible.
The attached engineering evaluation has been prepared to demonstrate that normal tensioning of 53 studs will assure reactor vessel integrity.
The reactor vessel head has been installed for cycle 3 and stud tensioning of the 53 active studs has been j
satisfactorily completed.
A copy of the reactor vesspi stud
-o tensioning report is available for review by the Callaway Resident Inspector.
Notwithstanding our conclusion that the existing.
configuration of the vessel head closure studs assures primary system integrity and presents no' threat to safe operation of the plant, Union Electric herewith commits to the following actions related to the issue 1.
If during operation, the vessel head inner 0-ring exhibits leakage, the unit will be brought to hot standby within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and to cold shutdown with the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
Leakage indication will be j
based on receipt of an 0-ring leak-off temperature-J alarm in the control room and an increase in 1
reactor coolant system identified leakage greater than 0.5 gpm.
2.
All 54 vessel head studs will be removed during Refuel III.
An action plan will be developed to improve stud bolting operations during future outages.
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Mailing Address: P,0. Box 149, St. Louis, MO 63166
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ULNRC-1663 October 29, 1987 Page 2 Union Electric believes that acceptable reactor vessel closure has been achieved.
The current schedule for attaining criticality is November 4, 1987.
We would appreciate your prompt review of this material to assure that we have addressed any questions you inay have regarding our study and conclusions.
Very truly yours, Donald F.
Schnell i
DFS/WRC/bjp l
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s STATE OF MISSOURI )
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i Donald F.
Schnell, of lawful age, being first! duly sworn upon oath says that he is Vice President-Nuclear and an officer of Union Electric Company; that he has rea:1 the foregoing document and I
knows the content thereof; that he has executed the same for and on l
behalf of said company with full power and authority to do so; and l
that the facts therein stated are true and correct to the best of I
his knowledge, information and belief.
By Donald E'.
Schnell Vice President Nuclear SUBSCRIBED and sworn to before me this dfEO day of M
, 1987
/ 4Ada- 0' h_/ 4 UAHohhPJ. PWJF[/ f NOIARY PUBLIC, STATE OF MISSOURI MY COMMISSION EXPIRES april. 22, 1989 ST. LOUIS COUNTY I
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ULNRC-1663 October 29, 1987 Page 3
)
I cc:
Gerald Charnoff, Esq.
Shaw, Pittman, Potts & Trowbridge 2300 N Street, N.W.
Washington, DC 20037 I
I Dr. J. O. Cermak CFA, Inc.
4 Professional Drive (Suite 110)
Gaithersburg, MD 20879 W. L.
Forney
)
Chief, Reactor Project Branch 1
)
U.S. Nuclear Regulatory Commission Region III f
799 Roosevelt Road Glen Ellyn, IL 60137 j
l Bruce Little I
Callaway Resident Office l
U.S. Nuclear Regulatory Commission RR#1 Steedman, MO 65077 T. W. Alexion (2)
Licensing Project Manager, Callaway Plant l
Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mail Stop 316 7920 Norfolk Avenue Bethesda, MD 20014 Ron Kucera, Deputy Director Department of Natural Resources P. O.
Box 176 Jefferson City, MO 65102 l
I Manager, Electric Department I
Missouri Public Service Commission l
P. O. Box 360 Jefferson City, MO 65102 i
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JUSTIFIC ATION FOR THE OPERATION OF CALLAWAY PLANT DURING CYCLE 3 WITH ONE REACTOR VESSEL STUD DETENSIONED I
TABLE OF CONTENTS I.
Introduc tion II.
Sequence of Events III.
Short Term Corrective Actions Taken IV.
Long Term Corrective Action V.
Interim Action Proposed VI.
Safety Evaluation VII.
Summary Attachment A Dominion Engineering Finite Element Model l
l Attachment B Configuration of Reac tor Vessel Head and Studs I
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October 29, 1987 Page 1 SAFETY EVALUATION FOR OPERATION I.
Introduction During reactor pressure vessel head removal operations at Callaway Plant associated with the second refueling outage, it was determined that five vessel studs could not be removed.
One of the five studs became lodged in a position which does not allow the stud to be fully tensioned.
The objectives of this report are to document the investigations and actions taken by Union Electric to date and to provide the results of evaluations which have been performed which indicate the Callaway plant can safely operate during Cycle 3 with 53 of the 54 vessel head closure studs installed and tensioned.
II.
Sequence of Events A.
CONSTRUCTION EXPERIENCE After hot functional testing and prior to initial fuel load, the Callaway reactor vessel stud hole threads were gauged for major and minor diameter tolerances.
During this evolution the go-gauge became lodged in stud position
- 12.
Because of the extraordinary efforts necessary to dislodge the gauge, position 12 and 7 other stud positions were not gauged.
Instead, Union Electric relied on a functional check of stud installation to assure acceptable hecd closure.
See Attachment B for stud locations involved (#54, 1,
2, 4,
6, 7 and 9).
B.
REFUEL I EXPERIENCE During Refuel I, difficulty was experienced in removing studs in the soutwest quadrant of the vessel cavity; however, these studs were subsequently removed with the use of oil of wintergreen and additional effort.
Stud #7 could not be removed but still retained adequate thread engagement.
A waterproof protective cap was developed and refueling operations proceeded.
An evaluation of thread lubricant was completed resulting in a change of lubricant.
C.
REFUEL II EXPERIENCE The following situations were encountered during vessel head removal:
1.
Forty-seven studs were removed without unusual difficulty.
2.
Two studs (#42, 53) came out with difficulty using a slugging wrench.
October 29, 1987 Page 2 3.
Five studs became lodged and remain in the following configuration:
a.
Removal of stud #2 was attempted subsequent to removing Stud #53.
It turned out approximately four to five inches prior to sticking.
This stud was turned back into its present position which is approximately 4-9/16" out (36.5 turns withdrawn).
i b.
Studs #4, 5,
7 and 9 became stuck almost immediately when attempts were made to turn them out.
These studs have been left in their stuck positions.
Their exact positions are as follows:
Disengagement From the Normal Installation j
Stud #
Position (2 Turns Out) i 4
3.5 turns withdrawn i
5 3.5 turns withdrawn
{
7 2.5 turns withdrawn i
i 9
1.5 turns withdrawn r
III.
Short-Term Corrective Actions Taken Union Electric responded to the stud removal problems with the following short-term corrective actions:
A.
Assessing ability to perform extraordinary removal efforts during this outage, j
B.
Cleaning, inspection, and rework of all reactor vessel studs and accessible stud holes.
C.
Revising procedures concerning reactor vessel studs, stud hole threads and head tensioning as required.
These revisions were made to reduce the potential of l
experiencing any further stuck studs during future refuelings.
1 Union Electric investigated the availability of Westinghouse, Combustion Engineering and Babcock & Wilcox to provide stud 1
extraction services.
None of these suppliers could support l
the removal of the studs during Refuel II.
Also, no threaded I
inserts were available to support potential stud hole thread l
repairs.
This led to-our investigation of the acceptability of head closure with 53 of the 54 stude suitably tensioned.
Union Electric contracted with Combustion Engineering to provide engineering and craft support for cleaning, inspection, and rework (as needed) of studs and accessible
October 29, 1987 Page 3 stud hole threads.
Following core off-load, the refueling pool was drained and shielding was erected.
Stud hole threads were cleaned using an air-driven wire brush.
Each hole was then visually inspected.
If the thread form appeared acceptable, a go-gauge verification of the thread form was attempted.
The final cleanliness and acceptability of the threads were verified by both CE and QC who witnessed all activities performed.
If the thread go-gauge would not pass freely, further inspections were performed in areas where the go-gauge stopped.
If required, an adjustable hand-turned tap was used to chase the threads.
After chasing, the threads were recleaned and gauged again.
This process was repeated until all threads were successfully gauged.
The reactor vessel stud threads were also cleaned, gauged and reworked (if required).
Each stud was gauged with a go-gauge on each end and a no-go gauge on the vessel end.
Studs in i
positions 24, 42 and 53 were replaced with spares because of j
minor thread damage.
The three studs taken from the warehouse I
were UT and MT inspected in accordance with Union Electric's ISI procedures.
The procedures for reinserting reactor vessel studs have been revised to incorporate recommendations from CE and Westinghouse concerning installation, lubrication and tensioning in order to reduce the potential for any additional head closure problems in subsequent refuelings.
The procedural changes made are ac follows:
j a.
Gauge use is required to determine the acceptability of l
all threads; b.
Lubrication:
Fel Pro N-5000 has been specified for use on the stud only; c.
Stud installation:
use of a load cell and chain fall will be required to ensure excess weight is kept off the threads until the stud is fully engaged; and d.
Tensioning:
the order of tensioning has been revised to adjust for not tensioning stud #2.
l During refueling operations, the five stuck studs were encased in protective enclosures.
Upon drain-down for work on the stud holes, indication of borated water ingress into the enclosures was detected.
This situation was documented and evaluated per 10 CFR 50.59.
Combustion Engineering subsequently recommended that wet lay-up of the stud holes and the encased studs using a 200 ppm concentration of hydrazine and demineralized water would prevent boric acid exposure and ruat formation.
This wet lay-up technique was used for the continuation of refueling operations and was very successful.
October 29, 1987 Page 4 i
This technique will be used to protect stud locations during all future refuelings.
IV.
Long-Term Corrective Action Union Electric will undertake appropriate actions to remove all reactor vessel head studs during Refuel III.
Removal will be accomplished using the technique and vendor evaluated to best address the Callaway plant situation.
Further, all studs j
and stud holes will be cleaned, gauged and accepted by QC for i
use.
Union Electric will have threaded inserts on hand for use if the threaded stud holes are not repairable.
All j
replacement studs will be cleaned, gauged & UT inspected in j
order to ensure their acceptability for use.
l Prior to Refuel III, procedures for stud detensioning, removal and cleaning will be revised to incorporate Combustion Engineering recommendations.
Training will be provided for craft personnel performing the reactor vessel head disassembly services.
This. training will cover the procedures to be used for removal, cleaning, inspection and reinstallation of the reactor vessel studs.
V.
Interim Actions l
This section deals with actions to be taken prior to Mode 2 entry following Refuel II activities.
Evaluations were made to assess the suitability of plant l
operation with 53 of the 54 vessel head closure studs properly tensioned.
Calculations show that the vessel head inner 0-ring will remain 9aled and no ASME Section III stress limits will be exceeded by leaving stud #2 detensioned.
The evaluations also confirm that thread engagement for studs 4, 5,
7 and 9 will support full tensioning.
Union Electric will impose a more stringent administrative operating limit on 0-ring leakage, as described in ULNRC-1663.
VI.
Safety Evaluation This section of the report describes actions Union Electric has taken to investigate the suitability of plant operation with one vessel head stud detensioned.
Westinghouse and Dominion Engineering were contracted to i
perform calculations and evaluations to determine the I
acceptability of operating with one or more studs detensioned.
}
The following is a brief summary of calculational techniques used by each investigator:
October 29, 1987 Page 5 A.
Westinghouse employed a simplified model with these assumptions:
The reactor vessel flange is rigid.
Stresses increase at a constant percentage around.the flange based on a summation of moments.
E.
Dominion Engineering used a finite element model as shown in Attachment A.
This model consists of 1100 nodes and 4900 degrees of freedom.
The principal elements of these investigations and the 1
conclusions reached are as follows:
1.
Justification for Use of Studs 4, 5,
7, and 9 Westinghouse determined.the minimum required thread engagement.
Studs 4 & 5 are the limiting studs on the basie of measured disengagement. ' Dominion Engineering was requeated to evaluate the amount of thread damage which could have been sustained by studs #4, 5, 7 and.9 during removal efforts.
This calculation is based on the energy input into the studs during removal efforts.
The energy assumed to be input to the threads was conservatively evaluated for a value 3 times greater than-field information indicates was used.
The Dominion Engineering-calculation shows a maximum predicted thread damage of 2.6 threads.
Assuming this damage and the maximum known stud disengagement, the minimum required thread engagement criteria are met with margin remaining.
The next concern investigated was potential boric acid exposure during Refuel II.
During initial flood-up, the stuck studs were protected by placing protective enclosures around them.
However, upon drain-down to attempt stud removal, signs of borated water ingress were noted.
Westinghouse and CE were contacted.
Neither vendor felt any change in stud integrity had occurred due to the low temperature and short duration of any potential exposure (i.e., two weeks max.)..
Their conclusions.were based on EPRI report NP-3784 and Westinghouse studies on boric acid corrosion which provide the following data:
Boric acid is not corrosive'in dry form.
Boric acid in solution has the following corrosion rates at low temperature:
i i
70 F -
2 mils / year 100 F -
8 mils / year 140 F - 15 mils / year
October 29, 1987 Page 6 High corrosion rates are experienced when a high temperature condition is coupled with a constant flow of concentrated boric acid solution.
I Based on the fact that boric acid exposure occurred at low i
temperature and that exposure to wetted boric acid solutions will not be present during plant operation, no detrimental corrosion has or will take place.
As corrective action, Union Electric filled the protective enclosures and stud holes with a 200 ppm solution of hydrazine and demineralized water to protect them against borated water ingress.
In addition, exposed threads on each of the stuck studs were cleaned and gauged prior to tensioning.
1 At this time, the tensioning process is complete and all studs are tensioned to prescribed values.
The elongation of all studs is between.049 and.053 inches which is within acceptable tolerances.
2.
0-Ring Leakage Evaluation Westinghouse calculated the 0-ring relaxation resulting from one detensioned stud.
The elongation of the remaining studs was alsocalculated.
Using the predicted elongation and an assumed pivot point at the edge of the bolt circle, the 0-ring relaxation is calculated to be 0.00384".
Per Combustion Engineering, the 0-rings have a minimum sping-back of 0.013" upon relaxation, which indicates the 0-rings will remain sealed.
Dominion Engineering Inc. independently evaluated the effects of detensioning of one, two and three adjacent
{
studs.
The results of tha Dominion Engineering finite element analysis confirmed the Westinghouse evaluation for one stud detensioneo.
Extrapolation of the Westinghouse study by Union Electric to evaluate the effects of two and three adjacent studs detensioned was also confirmed by Dominion.
The Westinghouse rigid head analysis and the Dominion Engineering' finite element analysis determined that leakage through the 0-rings would not occur with one or two adjacent studs detensioned.
Leakage past the 0-rings.
is probable with three adjacent studs not tensioned.
A tabulation of the results of these evaluations is provided in Table VI-1.
Leakage past the inner 0-ring.would be detected by an increase in temperature on the leak-off line from the annular space between the two 0-rings.
This increase in temperature would be detected by installed temperature
)
October 29, 1987 Page 7 indicators and alarmed at 160 F in the plant control room.
The leakage would also be detected as an increase in reactor coolant system identified leakage.
3.
Component Stress Analysis Combustion Engineering has determined that the maximum average service stresses for the Callaway reactor vessel head studs during operation occurs during the heat up cycle.
The maximum average service loads were determined by the development of a continuity matrix calculation which included thermal, accident and Operating Basis Earthquake seismic transient conditions.
Westinghouse has determined, based on stud tensioning procedures, that maximum tensile stresses for the studs occur during the tensioning process.
Westinghouse calculations have determined that the average stress increase in the remaining 53 studs caused by not tensioning one stud is 5.2%.
Assuming that the reactor head is rigid, this results in an increase in the stress in the adjacent studsbolts from 49.75 ksi to 52.337 ksi.
The resulting stress of 52.337 ksi remains well below the l
code allowable 2Sm val'le of 69.6 ksi.
]
Westinghouse stress calculations have been independently verified by Dominion Engineering Incorporated.
Dominion-Engineering was requested to perform finite element analysis to determine the. stresses on stud bolt materials and head and flange materials for the following cases:
l l
l Case #1:
Number 2 stud untensioned; Case #2:
Number 2 stud untensioned and #1 and #3 studs failing in service; Case #3:
Number 2 stud untensioned and #4 5,
7 and 9 studs failing in service.
For cases 1 and 2, the Dominion Engineering evaluation verified that the average and maximum service stresses for bolting materials calculated by Westinghouse were conservative.
I Westinghouse did not perform an analysis similar to Case
- 3 performed by Dominion Engineering because of the more involved (finite element) analysis required to evaluate the effects of detensioning non-contiguous studs.
In case number 3, with #2 untensioned and #4, 5,
7 and 9 assumed failed in service, Dominion Engineering determined that 0-ring leakage is probable, but that all material stresses
4 i
October 29, 1987 Page 8 for studs, vessel and head would be within ASME Section I
III code allowables.
A summary of the evaluation results is shown in Table VI-II.
Additional verification of the acceptability of operation with one reactor vessel stud detensioned exists as a result of a successful shop hydro performed by Combustion Engineering on a reactor vessel similar to Callaway's with one stud missing.
This shop hydro was conducted at 125%
of nominal design pressure.
VII.
Summary A review of the results of the calculations presented in Table VI-I and VI-II provides a basis for the following conclusions:
A.
Operating with one stud detensioned is acceptable because the pressure boundary is maintained and stresses are within code allowables.
J l
B.
Operating with two adjacent studs detensioned would be acceptable because the pressure boundary is maintained and stresses are within code allowables.
C.
Operating with three adjacent studs detensioned would provide a possible path for 0-ring leakage because local deformation, as determined by finite element analysis, would exceed the minimum 0-ring relaxation of 0.013".
Stresses are within code allowables.
l D.
Operating with the #2 stud detensioned and assuming the failure during service of #4, 5,
7 and 9 studs would produce 0-ring leakage.
However, stresses imposed on the remaining intact studs would still be within code allowables under operating conditions.
Additionally, 0-ring leakage would be detected and the unit could be shut down in an orderly manner.
Based on the foregoing, Union Electric has determined that
- peration of Callaway Plant with 53 of the 54 reactor vessel studs tensioned is acceptable because adequate 0-ring compression is maintained to prevent leakage and ASME Section III stress limits for affected components are not exceeded.
Furthermore, partially disengaged studs number 4, 5,
7 and 9, are capable of being fully tensioned.
The ability of the studs to withstand all loading conditions has been demonstrated by the analysis performed by Westinghouse.
1 l
October 29, 1987 j
Page 1
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4 TABLE VI-I I
0-RING SPRINGBACK l
- of Studs Westinghouse Dominion Engineering j
Detensioned Methodology Finite Element 1
0.00384" 0.003" 2
0.008" 0.009" l
3 0.013" 0.020" Per Combustion Engineering calculations, the minimum spring-back of the 0-ring is 0.013" through which the 0-ring will remain sealed.
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. 0ctober 29, 1987 Page 1 TABLE VI-II CALC BY CONDITION AVERAGE STUD MAXIMUM STUD EVALUATED SERVICE STRESS SERVICE STRESS (BENDING)
CALCULATED ALLOWABLE
- CALCULATED ALLOWABLE
- W
- 2 Not 52.34 69.6 91.25 104.4 Tensioned i
W
- 1, 2 and 3 58.6 69.6 97.4 104.4 Not Tensioned DE
- 2 Untensioned 60.9 69.6 81.4 104.4
- 4, 5, 7 & 9 failing during s ervice
- The lower material allowables for stud materials at operating temperatures are shown for comparison to actual i
anticipated stress values.
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