ML20090G784
| ML20090G784 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 07/20/1984 |
| From: | BALTIMORE GAS & ELECTRIC CO. |
| To: | Jaffe D Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8407250340 | |
| Download: ML20090G784 (8) | |
Text
.O B ALTIMORE G AS AND ELECTRIC COMPANY P.O. BO X 14 7 5 B ALTIM OR E. M A RYL AN D 212o3 NUCLEAR POWER DEPARTMENT CALVE 9T CLIFFS NUCLEAR POWER PLANT LUSOY, MARYLAND 20657 U. S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Division of Licensing Washington, DC 20555 ATTENTION: Mr. D. H. Jaffe, Project Manager Operating Reactors Branch #3
SUBJECT:
Calvert Cliffs Nuclear Power Plant Unit No. 2; Docket No. 50-318 Additional Information Regarding Steam Generator Water Hammer Event Gentlemen:
In response to your recent request for additional information, Enclosures (1), (2), and (3) are being forwarded for your information and use.
Should you require additional information regarding the above event, please contact us.
Sincerely yours, h
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General Supervisor -
Training and Technical Services RED /LES/gla l
Enclosures G. F. Trowbridge, Esquire k
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D. A. Brune, Esquire T. Foley, NRC fl Olh t,,
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Mr. D. H. Jaffe July 20,1984 Page 2 bec:
- 3. A. Tiernan L. B. Russell S. E. Jones D. W. Latham L. E. Saiyards
ENCLOSURE (1)
REQUESTED INFORMATION REGARDING WATER HAMMER EVENT ON CALVERT CLIFFS UNIT 2 - APRIL 21,1984 (1)
OCCURRENCE DESCRIPTION Calvert Cliffs Unit 2 was in MODE 3 (1400 psia,405 F) cooling down in preparation for its fifth refueling outage. An Auxiliary Feedwater (AFW) System test was completed just prior to the event, with #21 Steam Generator water level at -65".
In preparation for increasing Steam Generator level from -65" to -40", tne Operator opened the Main Feedwater Isolation Valves.
The water hammer occurred immediately.
The affected system was main feedwater, with the damaged components being the #21 Main Feedwater Isolation Valve (2-MOV-4516) motor operator and casing, a broken casting where air enters #21 Main Feedwater Regulating Valve (2-CV-Illi), and a cracked yoke on the worm gear drive of the nianual isolation valve to #21 Main Feedwater Regulating Bypass Valve (2-FW-132).
(2)
DESCRIPTION OF DAMAGE AND POST-EVENT EXAMINATIONS The damage resulting from the water hammer event is described in Item (1) above.
The means of detecting the event was a loui, audible noise. The damage was initially assessed by visual examination of accessible piping.
The post-event examination efforts were extensive.
The actual examinations conducted are summarized in Enclosure (2). The overall results of the examinations are summarized as follows:
There were no indications of damage to the feedwater piping or supports which were examined.
The pressure test described in Fnclosure (2) revealed no leakage from the feedwater piping subjected to the pressure test.
(3)
DETERMINATION OF UNDERLYING CAUSE(S):
The root cause of the event was failure to observe the General Precaution of the Unit 2 Main Feedwater System Operating Instruction-12A, which states in part that the AFW System should be used to feed the Steam Generator any time Steam Generator water level is below -26". The event was induced by plant operations, as described above. No new systems, procedures, design or control features were involved in the event.
ENCLOSURE (1)
REQUESTED INFORMATION REGARDING WATER HAMMER EVENT ON CALVERT CLIFFS UNIT 2 - APRIL 21,1984 (4)
PLANT DESIGN FEATURES AND OPERATIONAL PRECAUTIONS (To minimize water hammer):
In late 1978 for Unit 2 and mid-1979 for Unit 1, we modified the Steam Generators by providing non-reducing 3-Tubes to the top of the Feed Rings and covering the bottom exit nozzles. This reduced the possibility of a feedwater hammer event by extending the period of time required for the feed ring to drain once it was uncovered.
These modifications were recommended by our NSSS Supplier (Combustion Engineering), and were previously installed at S t. Lucie-1 and Millstone-2, which have Steam Generators similar to those at Calvert Cliffs.
Prior to installing the 3-Tubes, our Unit 2 Operating License, DPR-69, contained a License Condition specifying the following feedwater addition rate limitation:
" Steam Generator Water Level Rise Rate Except for the purpose of performing Steam Generator fe:dwater flow stability tests, the licensee shall whenever the secondary side water level in a Steam Generator is below the level of the feedwater sparger, limit the secondary side water level rise rate in the Steam Generator to less than 1.2 inches per minute, and shall reduce the rise rate to within this limit within two (2) ninutes if this limit is exceeded. This condition shall be removed only by amendment of this license after the licensee has demonstrated to the satisfaction of the Commission that secondary side flow instability (water hammer) either will not occur, or does not result in unacceptable consequences."
' This license condition was removed by License Amendment 25, dated March 10,1980. Additional procedural guidance was incorporated in June 1980.
This guidance specified that if main feedwater was lost and Steam Generator levels dropped below -26" (conservative value for top of feed ring), the AFW system should be used to restore Steam Generator levels. The AFW system should te used until Steam Generator level was greater than -26".
A review of procedural guidance provided by Combustion Engineering revealed a brief review was performed in 1979 of Steam Generator drainate paths which determined that following a reactor trip, if 5% of full main feedwater flow is maintained, the feed ring shculd be expected to remain filled with water.
Following t!)is event, an engineering review of the water hammer event and existing stress calculations was conducted by the Architectural Engineer, Bechtel Power Corporation.
Based upon recommendations from this review, we developed the examination schedule described in item (2) above.
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ENCLOSURE (1)
REQUESTED INFORMATION REGARDING WATER HAMMER EVENT ON CALVERT CLIFFS UNIT 2 - APRIL 21,1984 Although ASB BTP 10-2 was not specifically referenced in the Main Feedwater System testing performed following installation of the 3-Tubes in the Calvert Cliffs Steam Generator feed rings, we have reviewed the BTP and can address each of the positions, as follows:
POSITION 1:
Prevent or delay water draining from the feed ring following a drop in Steam Generator water level by means such as 3-Tubes.
RESPONSE
This modification was performed on Calvert Cliffs Units 2 and 1 in late 1978 and mid 1979, respectively, as described above.
POSITION 2:
Minimize the volume of feedwater piping external to the Steam Generator which could pocket steam using the shortest possible (less than 7 feet), horizontal run of inlet piping to the Steam Generator feed ring.
RESPONSE
The Main Feedwater feed ring inlet piping to the Steam Generator at Calvert Cliffs contains horizontal piping approximately 9' 7" maximum length between the inlet reducer and the first elbow external to the Steam Generator. The AFW piping external to the Steam Generator contains approximately 6" of piping between the first cibow external to the Steam Generator and the inlet to the Steam Gene-ator.
This information was provided to the NRC by BG&E in a letter dated March 15,1979. This design was reviewed by the NRC and determined acceptable by a safety evaluation issued on March 10, 1980, from Mr. R, W. Reid, NRC, to Mr. A. E. Lundvall, Jr., BG&E.
POSITION 3:
Perform tests acceptable to the NRC to verify that unacceptable feedwater hammer will not occur using the plant operating procedures for normal and emergency restoration of Steam Generator water level following loss of normal feedwater and possible draining of the feed ring. Provide the procedures for these
, tests for approval before conducting the tests.
Response
We provided a summary of the test procedures used for the feedwater testing performed following the 3-Tube modifications in a letter to the NRC dated March 15,1979. These tests were reviewed by the NRC and described in the_ safety evaluation dated March 10,1980.
r ENCLOSURE (1)
REQUESTED INFORMATION REGARDING WATER HAMMER EVENT ON CALVERT CLIFFS UNIT 2 - APRIL 21,1984 (5)
CORRECTIVE ACTION TAKEN Following the event, we conducted an extensive investigation to identify the root cause of the event and to recomraend corrective and preventive measures to minimize the possibility of recurrence.
The damaged feed water system components were repaired.
The engineering evaluations performed, post event inspections and tests are described in items (2) and (4) above. Procedural precautions already in place for water hammer concerns were.trengthened. Licensed personnel were informed of the event, including root cause and consequences, via a memorandum from the General Supervisor-Operations. This memorandum also reinforced the need for procedural compliance in this area.
Steam Generator water hammer events are covered in initial license training during feed water control panel lectures.
The training includes events which have occurred at Calvert Cliffs. Procedures for Steam Generator refill are also covered, stressing the significance of notes and precautions concerning this condition. These items are also discussed during initial license training on heat transfer and fluid flow.
Several concerns relating to water hammer events will be reemphasized in the licensed operator requalification training program. These concerns are presently scheduled to be covered during the October and November 1984 requalification training sessions.
r ENCLOSURE (2)
FEEDWATER SYSTEM INSPECTION
SUMMARY
DATA FIGURE EXAM ITEM POINT DESCRIPTION NUMBER TYPE NUMBER i
6 Pipe to Nozzle Weld at 1
UT 1
- 21 Steam Generator 10 Both Welds on first piping i
UT 2
elbow external to #21 St. am Generator inside Containment 20 Structural Steel of vertical 1
Visual A
snubber at data point 20 60 Structural Steel of the 2
Visual A
support at data point 60 45 Structural Steel of the 2
Visual B
support at data point 45 80 Structural Steel of the 2
Visual C
support at data point 80 45 90 Elbow at data point 45 1
UT 3
35 90 Elbow at data point 35 1
UT 4
3,4 Valve to Pipe Welds on 3
UT 3,4 2-FW-illlCV 2
Elbow Welds on 1st elbow 3
UT 5
upstream of 2-FW-131 NA Bolts in 2-FP-4156 3
Visual 1,2 75 Valve Yoke on 2-FW-4516-MOV 2
PT 1
NA Valve Yoke on 2-FW-illlCV 3
PT 6
NA Piping between 2-FW-131 NA Pressure Test NA and 2-FW-4516 MOV (900 psi for 20 min.)
In addition, visual examinations were conducted on the main feedwater ring inside #21 Steam Gener ator and the Structural Steel of all piping supports from #21 Steam Generator to #26 A&B High Pressure Feedwater Heaters.
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