ML20107L685

Forwards Addl Info Justifying Interim Operation of Byron Units Re Potential Environ Effects of High Energy Steam Line Break Outside Containment.Revs to Pages 4 & 5 of Encl

ML20107L685
Person / Time
Site:	Byron, Braidwood, 05000000
Issue date:	02/08/1985
From:	Tramm T COMMONWEALTH EDISON CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
9729N, NUDOCS 8503010029
Download: ML20107L685 (7)

Text

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Commonwealth Edison

@ One First National Piar. Chicago. Illinois Address Reply to. Post Othee Box 767 Chicago, lihnois 60690 February 8, 1985 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Byron Generating Station Units 1 and 2 Braidwood Generating Station Units 1 and 2 Environmental Qualification of Equipment NRC Docket Nos. 50-454/455 and 50-456/457 Reference (a): January 8, 1985 letter from T. R. Tramm to H. R. Denton.

Dear Mr. Denton:

This letter provides additional information to justify interim operation of the Byron units with respect to the potential environmental effects of a high energy steamline break outside the containment. This supplements the information provided in refereco (a) to satisfy License Condition 5.a of the Byron 1 Operating License, NPF-23.

Enclosed with this letter are revisions to pages four and five of Attachment A to reference (a). Two new graphs are also included. These pages have been revised to include more data on the temperature transient which could be experienced by components in the valve houses which are required for safe shutdown.

Please address further questions regarding this matter to this office.

One signed original and fif teen copies of this letter are provided for NRC review.

Very truly yours, f/2, [AA=

T. R. Tramm Nuclear Licensin0 Administrator 1m cc: Byron Resident Inspector wL G503010029 050200 8 PDR ADOCK 05000454 P PDH 9729N J

In cvalunting tha potcntici offccts of thosa two becaks tho sm311cr brcck was found to bo limitine. Although tha rate of

. temperature rise is less, the tot 3'. time at 3 temeerature higher than tha cri;;nal qual;f t:a::an temperature is greate r and the potential for heating a component to unacceptable levels is higher.

The predicted temperature transient for the 0.2 ft 2 break is shown in Figure 1. Steam Generator tube uncovery occurs at 530 seconds af ter the break. Main Steam isolation occurs at 1114 seconds when the temperature in the valve room is ,

approximately 339 F. The tenperatu:e transient for the 0.5 ft-break is shown in Figure 2.

3. AFFECTED SAFE SHUTDOWN EQUIPMENT The Category I electrical equipment located in the safety valve rooms has been identified and is listed in this section.

No Category I equipment is located in the steam tunnel itself.

The only components listed which are required for safe shut-down following a main steam line f ailure are the Main Steam Isolation Valves (MSIV's) and the Main Steam Pressure Trans-mitters. These are required to isolate the steam generators.

Following the isolation valve closure, the components are not required to function during the remainder of the transient.

The only cables in the valve rooms which must remain operable until MSIV closure are the cables associated with the MSIV's and the pressure transmitters. The function of all Class 1E equipment located in the Safety Valvo Room is described below.

A. Main Steam The Main Steam Isolation Valvos (MSIV's),MSIV Bypass Valves and the Steam Generator Power Operated Relief Valves (SG PORV's) are required to isolate the SG pressure boundary and control cooldown. The MSIV bypass valve is used during start-up at low flow to tempor the lines and is closed during normal operation and, therefore, not required to open during an MSLB avent.

The PORV's are not required to maintain hot stand-by conditions. The Main Steam Safety Valves (which contain no non-metallic parts) will provent ovorpressurization of the secondary system. Secondary depressurization can bo

. accomplished with hydraulic hand pumps if the electrical controls on the PORV's are inoperable. PORV operation is not required during an MSLB. An analysis shows that the hand pump will be accessible within 30 minutes after Main

. Steam Isolation.

The MSIV Safety function (closure within 5 seconds at a maximum specified qualification temperature of 325'F) must be completed in order to isolate the steam generators to prevent blowdown of all steam generators. Qualification of the MSIV actuator during an MSLB is required and is described in Section 4.

L ._

PcgG 4 of 8

. '4 . MSIV QUALIFICATION The Main Steam Isolation Valves (MSIV's) are required to close to prevent blowdown of all steam generators. When a valve is

. closed the differential pressure across the valve will maintain the closed position. Therefore, the valve actuator is required to remain functional only until the valve is closed.

The MSIV's have been qualified using an accident transient.

which peaks at 328'F. To evaluate the effects of higher temper-atures, the individual components of the valve actuator have been reviewed for possible non-metallic material degradation

• or other high temperature effects which could adversely affect the performance.

The non-metallic materials in the MSIV which are required to withstand the transient are listed below. The normal service limits have been compared with the predicted temper-atures of the individual parts obtained by a conservative heat transfer analysis. A significant margin exists between the service limits of the and MSIVthe predicted temperatures demonstrating the adequacy design.

MSIV Materials

. Material Use Normal Allowable Temperature Viton Seals on 3-Way solenoid 425*r valves Viton Seals on 4-Way hudraulic 425'r valves Viton Seals on pilot check 425'r valves Viton & Seals on hydraulic 425'r telfon accumulators EPR (Ethylene-End seal (internal) on 300'r pneumatic reservoir and propylene rubber) seals on hydraulic actuation cylinder .

By comparing the allowable temperature with the transient shown in Figure 1, it can be seen that the only material of concern is the EPR used to seal the pneumatic reservoir and the piston of the hydraulic cylinder which actuates the valve. A conservative heat transfer analysis has been completed to determine the expected temperature which the seals would experience. The limiting seal

temperatures for the hydraulic cylinder and pneumatic reservoir at

! the time of MSIV closure (1114 seconds) are 240*F and 250*F, respectively. Therefore, temperature margins exist of approximately i

60*F for the cylinder and 50*F for the reservoir.

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Pago 5 of 8 .

Figures 3 and 4 show the temperature profile for the case where MSIV closur isolates only three of the four steam generators from the 0.2ftgbreakat1114 seconds. The room temperature rises more rapidly after 1114 seconds because all of the blowdown steam flow comes from only one steam generator. As shown in Figure 4, the time required for the seal on the hydraulic cylinder to reach the allowable temperature (3000F) exceeds the duration of the 1800 second transient. The cylinder seal temperature was estimated to be 288af at 1800 seconds. As shown in Figure 3, the pneumatic reservoir seal temperature would not reach the allowable temperature until 9 cinutes after closure. The calculation indicates they would reach

. 299aF at 1656 seconds. Based on the analysis of continued blowdown after MSIV closure extending to 1800 seconds, substantial margins on the hydraulic cylinder and pneumatic reservoir seal temperature cxist.

Instrumentation cabling for the MSIV's has been environmentally qualified at 3410F for a duration of six hours followed by 3200F for three hours and 300aF for four hours. Power cables are qualified to a higher temperature. Since the valve house temperature reaches a peak of 3390F at the time of MSIV closure and the operability of the MSIV control circuitry is not required following closure, the cabling is qualified to operate an additional six hours at temperatures slightly above the peak 3390F. The terminal blocks are quallflod for 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> ut 3858F. These margins are sufficiently conservative to justify the MSIV and associated cabling performance during the MSLO.

5. STEAM LINE PRESSURE TRANSMITTERS OUALIFICATION The pressure transmitters are required to function only long i enough to provide the main steam isolation signal. Sufficient margin exists in the environmontal qualification testing to assure the operability of the transmitters at the maximum calculated temperaturo et 339'r at the timo of main steam line isolation (1114 seconds) . The transmitters are qualified for a maximum of 420'F (duration throo minutos) followed by 340*P for 15 minutos and 250*F for 16 days (Ref. EQDP Rev.4, March, 1983). The qualification transient is significantly more sovoro than the predicted transient shown in Figuro 1.

Additionally, the transmitters are mounted on the valvo room concrote wall where temperatures will bo lower duo to steam condensation.

Instrumentation cablino from the pressure transmitters has been environmentally qualified at 341oF for a duration of six hoursSince followed by 320*F for three hours and 300*F for four hours.

the valve house temperature reaches a peak of 3398F at the time of MSIV closure and the operability of the pressure transmitters is not recuired following closure, the cabling is quallflod to opurate an adcitional six hours at temperatures slightly above the peak 3398F.

These margins are sufficiently conservative to justify the transmitters and associated cabling performance during the HSLD.

Page 6 of 8 k

I

6. CONSERVATISM AND MARGIN IN ANALYSIS i l

The re-evaluation of the MSIV and Pressure Transmitter environ-mental qualification is consistently conservative in assump-tions and procedures. The postulated initiating event, a main steam line rupture with a break area of between 0.2 and 0.5 Ft2, is a very low probability event. Prior to occurronce of superheat conditions in the pipe the plant operators will ',

receive alarms for low lovel in a steam generator and largo amounts of steam will be rolcased to the turbine building and the environment. It would be excocted that the operators would quickly isolate the main steam linos.

Assuming the event continued, temperatures in the valve room were calculated assuming no delay in flow from the break to the safe shutdown equipment. No credit was taken for the amount of heat transferred from the steam to the structure and equipment, resulting in a higher prodicted environmental temperature.

When ovaluating the potential heat transfer to the temperature sensitive components, there is somo uncertainty involved with the heat transfer rate to a surfaco below tho steam saturation temperature (approximately 212'r for this case) because of the variability of condonsing hoat transfor. To model this is a very concorvativo assumption was made that the surface of the component in questign was initially at 212'F.

7. CONCLUSION In the limiting steam superhont caso, components could be exposed to temperaturen slightly above the qualification temperature for about 10 minuten. The most critical components contain seals made of EPR which in rated for only about 300*F.

Conservativo calculations show that the temperaturo at the seal location the safoty would function. rise only about 30'F prior to the completion of This results in a temperaturo margin of about 60*F and a timo margin estimated to be at least equivalent to the duration of the event.

Given the extremoly low probability of the defined initiating event and the adoquacy of the materials for the predicted temperaturos, the components in the main steam tunnel have boon shown to be qualiflod for the conditions caused by superhonted steam due to low lovel in a stonm gonorator, t

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