ML20081G973
| ML20081G973 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 11/01/1983 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | Adensam E Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8311070258 | |
| Download: ML20081G973 (5) | |
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L TENNESSEE VALLEY AUTHORITY CHATTANOOGA, TENNESSEE 374o1 400 Chestnut Street Tower II November 1,1983 i
Director of Nuclear Reactor Regulation Attention:
Ms. E. Adensam, Chief Licensing Branch No. 4 Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Dear Ms. Adensam:
In the Matter of
)
Docket No.
50-327 Tennessee Valley Authority
)
50-328 Enclosed is our response to your August 18, 1983 letter to H. G. Parris regarding additional information on the hydrogen mitigation system for the Sequoyah Nuclear Plant. The additional information pertains to the CLASIX code and environmental conditions for which equipment survivability is to
- be e aluated. As stated in the enclosure, a response to the question regarding the CLASIX code will be provided on or before February 4, 1984.
If you have any questions concerning this matter, please get in touch with i
J. E. Wills at FTS 858-2683 Very truly yours, TENNESSEE VALLEY AUTHORITY L. M. Mills, Manager Nuclear Licensing
_ Sworn t and subsc ibed before me this /. - - day of t/1983 i
hh) i Notary Public My Connission Expires Enclosure cc:.U.S. Nuclear Regulatory Commission (Enclosure) i Region II i
Attn:
Mr. James P. O'Reilly Administrator j
101 Marietta Street, NW, Suite 2900
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Atlanta, Georgia 30303
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0311070258 831101
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PDR ADOCK 05000327 P
PDR 1983-TVA SOTH ANNIVERSARY An Equal Opportunity Employer
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ENCLOSURE RESPONSE TO AUGUST 18, 1983 LETTER FROM E. G. ADENSAM TO H. G. PARRIS REQUEST FOR INFORMATION ON HYDROGEN MITIGATION SYSTEM SEQUOYAH NUCLEAR PLANT NRC Question 1 With regard te the CLASIX code, the Staff has previously requested clarification of the structural heat sink heat transfer models.
Provide justification for the models incorporated in CLASIX or provide the results of analyses with acceptable models as outlined above. The anslyses should encompass selected sensitiviity studies to assure that the effects of the changes are determined for both containment integrity and equipment survivability considerations.
TVA Response A response will be provided by February 4,1984.
NRC Question 2 Provide,a complete evaluation of fan (both air return and hydrogen skimmer, as applicable) operability and survivability for degraded core accidents. In this regard, discuss the following items:
A.
The identification of conditions which will cause fan overspeed, in terms of differential pressure and duration, and hydrogen combustion events.
I B.
The consequences of fan operation at overspeed conditions. The response should include a discussion of thermal and overcurrent breakers in the power supply to the fans, the set points, and physical locations of these devices, and the fan loading conditions required to trip the breakers.
l C.
Indication to the operator of fan inoperability, corrective actions which may be possible, and the times required for operators to complete these actions.
D.
The capability of fan system components to withstand differential j
pressure transients (e.g., ducts, blades, thrust bearings, housing), in terms of limiting conditions and components.
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TVA Rriponco TVA has formally provided information on air return fan survivability to the NRC in three previous submittals.
(Note that the hydrogen skimmer system at Sequoyah Nuclear Plant is operated by the two air return fans; there are no seoarate hydrogen skimmer fans.) In the submittal of December 17, 1980, the survivability of the fans was addressed for the temperature effects associated with upper compartment burns.
In the submittals of April 6 and November 1,1982, the fans' survivability was addressed for the pressure effects of lower compartment and postulated upper compartment burns.
As stated in the November 1, 1982, submittal, no hydrogen burns were predicted by CLASIX to occur in the upper compartment for the base case or best estimate parameter assumptions. Upper compartment burns only resulted in some of the sensitivity studies where the ignition limit was arbitrarily set lower in the upper compartment than in other regions. The.refore, we do not believe that differential pressures due to postulated upper compartment burns are a realistic loading condition. However, the differential pressure transient from a sensitivity case where the ignition limit in the upper compartment and the dead-ended region was assumed to be six volume percent with 60-percent burn completeness will be examined. For this case, as stated in the November 1, 1982, submittal, the duration of an upper compartment burn was about 12 seconds, the peak differential pressure 2
was about 7.5 lb/in d, and the integrated load was about 36 2
lb/in -seconds.
Each air return fan is a direct-drive, vaneaxial fan with a 50-horsepoker, three-phase motor. When subjected to the postulated loading condition, the motor would resist the fan's attempt to overspeed by acting as an induction generator. If a sufficient overcurrent condition developed in the power supply to the motor, the circuit breaker would trip before the motor could be damaged. The overcurrent circuit breaker is set to trip at a current of 90 amperes which is well above the current of 65 amperes normally drawn by the motor. Thus, we judge that the motor would likely continue to operate during the brief transient without tripping the breaker.
However, if the circuit breaker did trip due to overcurrent, the operator would have several indications of the inoperability of the fan which would enable him to take the appropriate corrective action.
i Annunciation would be provided in the main control room upon the tripping of the breaker at which time the operator could identify the tripped breaker by referring to the breaker position on the status monitoring system. In addition, annunciation would also occur if the flow rate through the fan should drop below 20,000 cfm which would i
l occur shortly after the pressure transient ended. Indication of the flow rate is also available in the main control room. Having recognized that the fan is inoperable and that the tripped circuit l
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breiktr is the causa, the optrator would proceed to reset the breaker to restart the fan motor. The circuit breakers for the air return fan motors are located on the 480 volt shutdown boards 1 A1-A and 1B2-B which are located in the auxiliary building within 100 feet of the main control room on the same floor elevation. Once the circuit breaker has been locally reset, the successful restart of the motor could be verified from the main control room. We estimate the oparator could complete these actions within a few minutes.
As discussed in the submittals of April 6 and November 1, 1982, TVA requested the air return fan supplier, Joy Manufacturing Company, to structurally analyze the fans for overpressure in the direction of fan flow. Their analysis bounded any transient loading that would occur during a postulated upper compartment hydrogen burn by assuming a constant 50 lb/in2d force acting downward on the fan assembly in addition to all the loads the fan experiences during operation at its normal speed., As discussed above, the fan should not overspeed significantly due to the retarding effect of the motor acting as a generator. Neither should the fan overspeed significantly after a possible circuit breaker trip during the transient since the remainder of the pressure transient would have a relatively small impulse effect on the inertia of the fan assembly. Thus, the structural analysis is conservative since significant overspeed should not occur and since 2
the 50 lb/in d load greatly exceeds the worst peak transient differential pressures during a postulated burn. The structural analysis examined the critical areas of the fan assembly including the blades and bearings and the calculated stresses were not above yield.
Neither the ductwork nor the fan housing at Sequoyah would be exposed to the differential pressure since they are embedded in the divider d eck. Therefore, the entire air return fan assembly has been shown to be structurally capable of withstanding any differential pressure transient calculated to occur during a postulated upper comoartment hydrogen burn.
NRC Ouestion 3 Provide an evaluation of the ultimate capability of ice condenser doors to withstand reverse differential pressures.
TVA Response 3 There are three sets of ice condenser doors: the top deck blanket separates the ice condenser upper plenum from the upper compartment, the intermediate deck doors separate the uoper plenum from the ice basket region, and the lower inlet doors separate the ice basket region from the lower compartment. The top deck structure consists of radial beams cantilevered from the crane wall, bolted stringers spanning between the radial beams, grating that rests on the stringers between the radial beams, and an insulation mat that covers the entire deck to form a seal between the crane wall and the containment shell.
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Tha int;rmediata d;ck doors are horizontal, fo2m-insulation-filled 2
steel plate doors supported in a steel frame. The steel frame is supported on the structural steel framework that contains the ice baskets. The lower inlet doors are vertical, foam-insulation-filled stainless steel plate doors. The doors are stiffened internally with horizontal channels and a fiberglass plate on the inside face of the d oor. The doors are butted against a steel frame embedded in the crane wall.
The assumption was made for the top deck blat <et that the reverse differential pressure would force the insulation mat against the grating, so the grating would experience the full effect of the pressure. The top deck was conservatively calculated to fail at 4 2
lb/in g.
The assumption was made for the intermediate deck doors that the structural steel framework that supports the steel doorframe would be considered rigid in comparison to the doors. The intermediate dcck doors were conservatively calculated to fail at 6 2
lb/in g.
The assumption was made for the lower inlet doors that the steel doorframe would be considered rigid on all sides, except for the center post between the double doors. The lower inlet doors were 2
conservatively calculated to fail at 7 lb/in g.
These pressures were, calculated on the basis of minimum specified yield strengths and represent the lower bound pressure capabilities. Since actual yield strengths are generally greater than the specified minimum, and some deformation could occur before failure, the actual failure pressures would be somewhat higher.
A number of other factors should be considered in any evaluation of the ice, condenser doors for their capability to withstand reverse differential pressure. As discussed in the response to NRC question 2, no hydrogen be 7ts were predicted by CLASIX to occur in the upper compartment for the base case or best estimate parameter assumptions.,
Only in some of the sensitivity studies where ignition limits were adjusted nonuniformly did upper compartment burns result. There fore,
we do not believe that reverse differential pressures on the ice condenser doors are a realistic loading condition. Also as discussed in the response to question 2, the calculated transient pressure during a postulated upper compartment burn (12 second duration with a 2
peak differential pressure of 7.5 lb/in d and an average 2
differential pressure of 3 lb/in d) does not significantly exceed the calculated lower bound capability of the doors and only exceeds it for a few seconds before rapidly declining. Since neither the magnitude nor the duration of the transient differential overpressure would be extreme, the doors could realistically be expected to survive although some deformation might occur. In the worst case, if deformation of the top deck blanket did occur it would increase the existing vent area and limit further pressurization.
Due to the great extent of the ice condenser flow area, subsequent upward flow through the ice condenser should be able to redistribute itself around any potential blockages without significant effects. We b?lieve that a consideration of these factors as a whole should resolve concerns about the capability of the ice condenser doors during postulated upper compartment burns.