ML20046B204

Responds to NRC Request for Addl Info Re Relief Request from ASME Code,Section Ii,Article 9 to Allow Stop Valve to Remain Installed Downstream of Thermal Overpressure Relief Device for Regenerative HXs of Both Units

ML20046B204
Person / Time
Site:	Calvert Cliffs
Issue date:	07/26/1993
From:	Denton R BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
TAC-M83999, TAC-M84000, NUDOCS 9308030295
Download: ML20046B204 (8)

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l BALTIMORE

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GAS AND

. ELECTRIC l I

1650 CALVERT CLIFFS PARKWAY . LUSBY, MARYLAND 20657-4702 l

! i t 1 ROBERT E. DENTON 1 VicE PRESIDENT l NUCLE AR ENERGY (460)260-4465 i

i I 4 1 i U. S. Nuclear Regulatory Commission l Washington,DC 20555 1

A'ITENTION: Document Control Desk i

SUBJECT:

Calvert Cliffs Nuclear Power Plant  ;

i Unit Nos.1 & 2: Docket Nos. 50-317 & 50-318 l Response to NRC Request for Additional Information Concerning Relief

Request from the American Society of Mechanical Engineers (ASME) Code,Section III, Article 9, for Calvert Cliffs Nuclear Power Plant, Unit 1 (TAC a No. M83999) and Unit 2 (TAC No. M84000) 4

REFERENCES:

(a) Letter from Mr. D. G. Mcdonald, Jr. (NRC) to Mr. R. E. Denton i (BG&E), dated June 8,1993, Request for Additional Information &

i Concerning Relief Request from the American Society of Mechanical

] Engineers (ASME) Code,Section III, Article 9, for Calvert Cliffs i j Nuclear Power Plant, Unit 1 (TAC No.M83999) and Unit 2 (TAC ~  ;

J No. M84000)

! Letter from Mr. G. C. Creel (BG&E) to Mr. D. G. Mcdonald, Jr.

(b)  !

! (NRC), dated June 30,1992, Request for Relief from 1968 ASME I

Boiler & Pressure Vessel Code Section III, Article 9 .

This letter provides Baltimore Gas and Electric Company's (BG&E) response to your request for l- additional information (RAI) (Reference a). The RAI pertains to our request for relief from 1968

ASME Boiler & Pressure Vessel Code Section III, Article 9 (Reference b). This relief request

. would allow a stop valve to remain installed downstream of a thermal overpressure relief device for the Regenerative Heat Exchangers (RHX) of both units. The specific information you requested and our responses are contained in Attachment (1).

i Should you have any furthe'. questions regarding this matter, we will be pleased to discuss them with l You-I Very truly yours, s  !

$ b 9,(,()6,

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RED /DJM/ dim I'y i l, Attachment (1) Response to NRC Request for AdditionalInformation Regarding the Relief 1 9g* est from ASME Code,Section III, Article 9 9308030295 930726 y PDR ADOCK 05000317 T d$

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Document Control Desk July 26,1993 Page 2 I

i cc: D. A. Brune, Esquire l J. E. Silberg, Esquire j R. A. Capra, NRC  ;

D. G. Mcdonald, Jr., NRC

T. T. Martin, NRC P. R. Wilson, NRC i R. I. McLean, DNR J. H. Walter, PSC

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A'ITACIIMENT m l l

Response to NRC Request for AdditionalInformation Reguiding the l Relief Request from ASME Code,Section III, Article 9 l

NRC Reanest If the Regenerative Heat Erchanger (RHX) fails, this will result in the loss of auxiliarypressunzer spray (APS). The November 10,1992, submittal states that the APS is not required to mitigate a Loss-of-CoolantAccident. However, the Generic Letter (GL) 90-06 response, dated December 20,1990, states l that the APS system is safety-related and is used to depressurize the primary system during normal cooldown andfor a steam generator tube nipture (SGTR) event. Therefore, describe the required safety functions of the APS system in mitigating any transient or accident event, such asfor depressurizing the l pdmary system dudng an SGTR event. l l

HG&E Resumse i

Calvert Cliffs Emergency Operating Procedures (EOPs) allows the operator to use APS, if it is .

available, for all events to reduce Reactor Coolant S3stem (RCS) pressure to control subcooling. l The impact of a failure of the RHX on the various uses of APS is discussed below. j i

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> Normal RCS Boration An RHX failure would eliminate our normal flowpath for boration of the RCS. If such a j failure occurred, Operating Instructions provide directions and appropriate valve lineups to  ;

allow boration of the RCS from the Chemical Volume and Control System through the  !

Safety Injection cross-connect, MOV-269. Manual isolation valve CVC-183, located in the l

Auxiliary Building, would be closed in order to isolate the damaged portion of the charging l l header and to ensure that the borated water reaches the RCS via MOV-269. Therefore,  !

even with an RHX failure, boration of the RCS can still be accomplished. j

> Core Flush A pressure boundary failure of the RHX would remove the capability to use APS. Auxiliary Pressurizer Spray provides a core flush function by using High Pressure Safety Injection pumps to provide containment sunip water to the APS header / charging header. An RHX failure would make the charging system header inoperable. However, post-Imss-of-Coolant i Accident core flush can be accomplished via Hot Leg Injection (Low Pressure Injection i pumps take suction from the containment sump and deliver water to the RCS hot leg via the shutdown cooling return header).

> Steam Generator Tube Ruoture Auxiliary pressurizer spray is not credited in the SGTR accident analysis, as discussed in Section 14.15 of the Updated Final Safety Analysis Report. Therefore, an RHX failure would not impact the results of the SGTR accident analysis. The GL 9-06 response, dated December 20,1990, stated that the APS system is used to depressurize the primary system for the SGTR event. This statement should have been clarified to say that the EOPs allow the operators to use the APS to cooldown following an SGTR event if it is available, but we do not take credit for APS in the accident analysis.

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1 ATTACIIMENT (1) l Response to NRC Request for Additional Information Regarding the Relief Request from ASME Code,Section III, Article 9 I NRC Request j

! If the R1LXfails and the tube side (i.e., the letdown side) alsofails as a result, are the letdown isolation valves capable ofclosing against the resulting RCSpow andpressure loading? Are these valves included 1 in the scope of the GL 89-10 program?

l HG&E Response The letdown isolation valves are capable of closing against the resulting RCS flow and pressure loading if the RHX fails and the tube side also fails. The valves are designed to handle a shut-off differential pressure of 2485 psid. The letdown isolation valves are two-inch air-operated globe l l valves and are, therefore, not included within the scope of GL 89-10. )

i NRC Request If the RHXfails, are the chargingpumps secured such that there is nopaw and differentialpressure load on manual valve CVC-183? Ifnot, would any condition prevent an operatorfrom closing itfor the local environmentalconditions orfor thepow andpressure loading on the valve disk?

HG&E Resmmse If the RHX fails, charging pressures will be lower than the RCS pressure. In this condition, the Abnormal Operating Procedures instruct the operators to secure the charging pumps. This would climinate any flow or pressure loading on the valve disk of CVC-183 which would prevent its being closed. There are no radiological or other local environmental reasons which would prevent operation of CVC-183.

NRC Request The licensee stated that CVC-435 is to be modified to controlAPSpow. Does this modipcation involve adding an actuating mechanism to the valve, and could it prevent the valve from accomplishing its thennalpressurerelievingfunction? Describethismodification.

BG&E Resamse Since the time the original Relief Request was submitted (Reference b), the proposed modification to CVC-435 to control the APS 110w has changed. It does not involve adding an actuating mechanism to CVC-435. Rather, it involves installing an in-line orifice in the two-inch bypass line between CVC-435 and CVC-188. The orifice will be sized to allow the flow required to prevent overpressurization of the RHX and to provide sufficient differential pressure to ensure rated flow during auxiliary spray operations. CVC-435 will remain as is and CVC-188 will be placed and locked in the full open position.

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ATTACIIMENT m 1

Response to NRC Request for Additional Information Regarding the Relief Request fmm ASME Code,Section III, Article 9 1 NRC Request The licensee stated that their should be no major damage to reactor coolant pumps, steam generaton, orletdown isolation control valves as a result of RHXfailure. However, the licensee indicated that this i equipment is in the closepraximity to the RHX. If the RHX ruptures andfragments ofsufficient energy or brok.en pipe ends impact this equipment, major damage could occur. Describe the configuration f and/or shielding which wouldprevent damage to any safety components.

i BG&E Response f

Baltimore Gas and Electric Company determined the failure mode due to overpressurization of the RHX. If the RHX were to fail due to overpressurization, the failure mode would be a self-contained ductile failure with no fragmentation of the heat exchanger.

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The shell side of the heat exchanger is similar in design and operation to a thin-walled pressure 2 vessel. Compressed liquids within a thin. walled pressure vessel cause circumferential and

, longitudinal stresses. Normally, the maximum tensile stress (in absence of a stress concentration) is a 3 circumferential hoop stress. Overpressurization ultimately causes longitudinal splitting of the shell j near the midpoint of the long axis of the vessel.

The RHX tubes, tube sheets, shell, shell cover, channel and baffles are fabricated from 304 stainless steel. Type 304 stainless steel is a very tough and ductile alloy. When subjected to overload stresses,

the failure mode of 304 stainless steel is a self-contained ductile failure unless the temperature is j extremely low. When the heat exchanger (shell side) reaches a pressure causing a stress level beyond

the material's yield point, the shell side of the heat exchanger walls begin to thin and bulge. If the

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pressure continues to rise, at some point the thinning wall will no longer be able to sustain further i stress and the wall will rupture. This will immediately relieve the pressure. The rupture may occur at '

a stress concentration such as a welded inlet / outlet, vent or drain. However, this will still be a self- l

contained rupture with no fragments of sufficient energy to cause major equipment damage due to j impact.

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ATTACllMENT m Response to NRC Request for AdditionalInformation Regarding the Relief Request from ASME Code,Section III, Article 9 I

NRC Request  !

lI If the Regenerative Heat Erchanger (RHX) fails, this will result in the loss of auxiliarypressunzer spray l (APS). The November 10,1992, submittal states that the APS is not required to mitigate a Loss-of- 1 Coolant Accident. However, the Generic Letter (GL) 90-06 response, dated December 20,1990, states that the APS system is safety-related and is used to depressurize the primary system during normal cooldown andfor a steam generator tube mpture (SGTR) event. Therefore, describe the required safety functions of the APS system in mitigating any transient or accident event, such asfor depressurizing the primary system during an SGTR event.

I HG&E Response l 1

Calvert Cliffs Emergency Operating Procedures (EOPs) allows the operator to use APS, if it is available, for all events to reduce Reactor Coolant System (RCS) pressure to control subcooling.

The impact of a failure of the RHX on the various uses of APS is discussed below.

l

> Normal RCS Boration An RHX failure would eliminate our normal flowpath for boration of the RCS. If such a failure occurred, Operating Instructions provide directions and appropriate valve lineups to

, allow boration of the RCS from the Chemical Volume and Control System through the  ;

Safety Injection cross-connect, MOV-269. Manual isolation valve CVC-183, located in the '

Auxiliary Building, would be closed in order to isolate the damaged portion of the charging 1

header and to ensure that the borated water reaches the RCS via MOV-269. Therefore, even with an RHX failure, boration of the RCS can still be accomplished.

• Core Flush A pressure boundary failure of the RHX would remove the capability to use APS. Auxiliary Pressurizer Spray provides a core flush function by using High Pressure Safety Injection pumps to provide containment sump water to the APS header / charging header. An RHX failure would make the charging system header inoperable. However, post-Loss-of-Coolant Accident core flush can be accomplished via Hot Ixg Injection (Low Pressure Injection pumps take suction from the containment sump and deliver water to the RCS hot leg via the shutdown cooling return header).

• Steam Generator Tube Runture Auxiliary pressurizer spray is not credited in the SGTR accident analysis, as discussed in 4

Section 14.15 of the Updated Final Safety Analysis Report. Therefore, an RHX failure would not impact the results of the SGTR accident analysis. The GL 9-06 response, dated December 20,1990, stated that the APS system is used to depressurize the primary system for the SGTR event. This statement should have been clarified to say that the EOPs allow the operators to use the APS to cooldown following an SGTR event ifit is available, but we do not take credit for APS in the cecident analysis.

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} A'ITACilMENT (1) i  :

J , Response to NRC Request for Additional Information Regarding the I Relief Request from ASME Code,Section III, Article 9 l

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t 1 NRC Request j i

, if the RHXfails and the tube side (i.e., the letdown side) alsofails as a result, are the letdown isolation l l valves capable ofclosing against the resulting RCSflow andpressure loading? Are these valves included l 1 in the scope ofthe GL 89-10 program? l l

HG&E Response l I

He letdown isolation valves are capable of closing against the resulting RCS flow and pressure l l loading if the RHX fails and the tube side also fails. The valves are designed to handle a shut-off i differential pressure of 2485 psid. The letdown isolation valves are two-inch air-operated globe valves and are, therefore, not included within the scope of GL 89-10.

! j i NRC Reauest i 1

l If the RHXfails, are the chargingpumps secured such that there is noflow and differentialpressure load l

, on manual valve CVC-183? Ifnot, would any conditionprevent an operatorfrom closing itfor the local environmental conditions orfor theflow andpressure loading on the valve disk?

l I l HG&E Response If the RHX fails, charging pressures will be lower than the RCS pressure. In this condition, the i

. Abnormal Operating Procedures instruct the operators to secure the charging pumps. This would eliminate any flow or pressure loading on the valve disk of CVC-183 which would prevent its being

,' closed. There are no radiological or other local environmental reasons which would prevent operation of CVC-183. l

, NRC Request

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The licensee stated that CVC-435 is to be modified to control APSflow. Does this modification involve adding an actuating mechanism to the valve, and could it prevent the valve from accomplishing its thenna!pressurerelievingfunction? Describethismodification.

BG&E Response  !

Since the time the original Relief Request was submitted (Reference b), the proposed modification i to CVC-435 to control the APS flow has changed. It does not involve adding an _ actuating .

mechanism to CVC-435. Rather, it im'olves installing an in-line orifice in the two-inch bypass line l between CVC-435 and CVC-188. He orifice will be sized to allow the flow required to prevent overpressurization of the RHX and to provide sufficient differential pressure to ensure rated flow during auxiliary spray operations. CVC-435 will remain as is and CVC-188 will be placed and locked in the full open position.

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l A'ITACIIMENT (1)

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j Response to NRC Request for Additional Information Regarding the j Relief Request from ASME Code,Section III, Article 9

! NRC Reauest k The licensee stated that there should be no major damage to reactor coolant pumps, steam generators, i orletdown isolation contml valves as a result ofRHXfailure. However, the licensee indicated that this equipment is in the close prarimity to the RHX. If the RHX ruptures andfragments of sufficient energy l or broken pipe ends impact this equipment, major damage could occur. Describe the configuration f and/or shielding which wouldprevent damage to any safety components.

t HG&E Response j Baltimore Gas and Electric Company determined the failure mode due to overpressurization of the 4 RHX. If the RHX were to fait due to overpressurization, the failure mode would be a self-contained j ductile failure with no fragmentation of the heat exchanger.

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{ The shell side of the heat exchanger is similar in design and operation to a thin-walled pressure vessel. Compressed liquids within a thin. walled pressure vessel cause circumferential and j longitudinal stresses. Normally, the maximum tensile stress (in absence of a stress concentration) is a 3 circumferential hoop stress. Overpressurization ultimately causes longitudinal splitting of the shell

near the midpoint of the long axis of the vessel.

The RHX tubes, tube sheets, shell, shell cover, channel and baffles are fabricated from 304 stainless j i

steel. Type 304 stainless steel is a very tough and ductile alloy. When subjected to overload stresses, the failure mode of 304 stainless steel is a self-contained ductile failure unless the temperature is i extremely low. . When the heat exchanger (shell side) reaches a pressure causing a stress level beyond the material's yield point, the shell side of the heat exchanger walls begin to thin and bulge. If the pressure continues to rise, at some point the. thinning wall will no longer be able to sustain further ,

stress and the wall will rupture. This will immediately relieve the pressure. The rupture may occur at a stress concentration such as a welded inlet / outlet, vent or drain. However, this will still be a self- -

contained rupture with no fragments of sufficient energy to cause major equipment damage due to  ;

impact.

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