ML20216H604

Advises That Based on B&W Analytical Studies for Primary Sys Blowdown Using RELAP5,use of Blowdown Valves Not Practicable for Feed & Bleed,Per 851204 Commitment.Listed Proposed Mods Will Be Implemented to Enhance Makeup Sys

ML20216H604
Person / Time
Site:	Davis Besse
Issue date:	06/25/1987
From:	Shelton D TOLEDO EDISON CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
1382, TAC-66415, NUDOCS 8707010465
Download: ML20216H604 (3)

Text

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Docket No. 50-346 License No. NPF-3 TOLEDO Serial No. 1382  !!EI!E!EI!d June 25, 1987 s DONAU3 C. SHELTON Vce Pressdent-Nuclear (419]249-2399 United States Nuclear Regulatory Commission Document Control Desk Washington, D. C. 20555 Gentlemen:

The purpose of this letter is to:

1. Briefly report to the Staff, the results of the detailed analyses of enhanced blowdown capability undertaken by Babcock and Wilcox and Toledo Edison.

2. Identify the alternate method of enhanced feed and bleed now being designed for the Davis-Besse Nuclear Power Station.

Since 1979, Toledo Edison procedures have been in effect at the Davis-Bess 2 Nuclear Power Station to provide feed and bleed cooling to the reactor core in the event that there was a suctained loss of all cooling throus;h the steam generators. Detailed analysis and testing in 1986 were used to verify the effectiveness and improve these procedures.. The success of these efforts was documented in NUREG-1177, "The Safety Eval-uation Report Supporting the Davis-Besse Restart" (Log No. 1997, dated June 10, 1986). However, substantive improvements have been made to the steam generator cooling capacity of the plant to render the complete loss of feedwater event even less likely. These improvements include main feedwater and auxiliary feedwater system improvements, the addition of a new electric motor driven feedwater pump and~ modifications to the Steam /

Feedwater Rupture Control System. These efforts were documented in NUREG-1177.

Toledo Edison, in its December 4, 1985 submittal (Serial No. 1207), made a long-term commitment to further enhance the existing feed and bleed capability by installation of primary system blowdown capability. This capability was committed to be redundant, have safety-grade components and be designed to decrease the reactor coolant system pressure to that which is within the High Pressure Injection (HPI) System capability. Conceptually, the RCS blowdown valves were to be located on top of each steam generator and would have been the only primary system discharge valves required for feed and bleed operation.

The above commitment was based on preliminary engineering evaluations addressing'the feasibility of enhanced blowdown capability. The detailed analyses are now completed and demonstrate that use of blow-down valves is not practicable for feed and bleed.

1 0 THE TOLEDO EOISON COMPANY EDISON PLAZA 300 MADISON AVENUE' TOLEDO, OHIO 43652 h0 8707010465 8706253, /

PDR ADOCK 05000346 ~[O P PDR

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' Docket'No. 50-346 License No. NPF-3 Serial No. 1382 Page 2 Analytical studies were performed by Babcock and Wilcox for the primary system blowdown, using RELAP5. The computations, in each case considered, indicated a resulting unacceptably low reactor vessel water level. A summary of the analyses is presented below: ,

1. The analyses consisted of parametric studies considering various RCS blowdown paths, valve sizes and initiation times for feed and bleed.

To be consistent with sin 7l e failure criteria, credit was taken for operation of only one injection train. The effect of low pressure injection (LPI) in combination with HPI or makeup was considered. J

2. In all cases, the plant response proceeds in two distinctive steps:

a. An initial reduction of collapsed water level to approximately one foot above the top of the core and depressurization to 1600 )

psi prior to initiation of HPI flow into the vessel. f

b. Continued RCS depressurization to approximately 600 psi with a net loss of water inventory despite HPI or makeup system opera-tion. . Total water loss from the RCS in each case exceeded 320,000 LBM. The minimum collapsed reactor vessel water level was highly sensitive to small changes in valve size and dis-charge coefficient, and varied from one to six feet below the top of the core. 'n the case of the current feed and bleed capability (two makeup pumps and the PORV), the core remains covered.

Based on the above results, Toledo Edison does not intend to pursue the installation of enhanced blowdown capability. Detailed results of the analyses can be made available to the Staff if desired. 'J Toledo Edison presented the initial results of the blowdown valve analysis, discussed above, to the Commission and Staff on November 21, 1986. At  ;

that time, alternatives for enhancing.the feed capability, in lieu of blowdown velves,'wcre briefly presented. Toledo Edison is currently pursuing this enhanced feed approach to improve feed and bleed capability.

This would involve a modification of the existing makeup system. The existing feed and bleed capability requires operation of both makeup pumps and the'PORV to be successful. The proposed upgraded makeup system would provide increased flow and add independent flow paths for each pump. As a result of these improvements, feed and bleed would be functionally success-  ;

ful following the failure of either makeup pump or the.PORV, in addition to the multiple failures of steam generator cooling systems required to neces-sitate-feed and bleed operation. Therefore, the following minimum equipment combinations would support successful feed.and bleed operations. i l

1. Two makeup pumps and the RCS Code Safety Valves, '

2. One makeup pump in piggyback with the LPI Pump and the PORV.

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< Docket No. 50-346 iLicense No. NPF-3 I

Serial No. 1382 s Page 3 ,

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Also, successful feed and bleed operat;fon may be accomplished with'ene f makeup pump and the PORV if a reactor / turbine trip is initiated by actuation of the Anticipatory Reactor WIp System. As befcre, return of stena .

generatcr cooling can be expected to successfully cool the core from the j secondary side, l Analytical results show dho vinimum collapsed reactor vassel water level i I

for Cane 1 (2 makeup pumps, end code sefeties) would exceed the Z 2 foot

/ level which maintains the core fully covered. Case 2 (L makeup pump /LPI b pigyback and the PORV) would result in a ecllapsed reactor vessel water ,

h w l of 11.6 feet and would assure depressurization to HPI operating

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l condit ions. Although the minimun collapsed water. level,foc Case 2 indicates I 1

a invel 0.4 feet below the cop of tbc core, the cooling medium will he a two-phase mixture (water / froth) which would cover the cere and ma'.ntain adequate cooling. ]

The pnhancement of the makeup r stem, for feed and bleed cooling, is to bi implemented in twq phases; l l

1. Plow enkneements, conetenng of new piping and valvec, are planned Nr installation during the next refueling outage. This would provide e functional sys' tem. All nev> equipment will bd purchased and installed l aa nuclear safety-related, Setsmic Class I. I

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2. Upgraden, where possible, of' the existing equipy nt to nuclear safety-related. would be imp 1mented during the fif th or sixth refueling outage.

Detailed engineering to support O is modification is now underway.

In thC planrod upgrade:, ceraporents need only be seismically and environmen- j tally qualifteJ for operation following the loss of feedwater event. Since j t'n makwp purge can be pmered from the emergency diesel generators,

{ they would be operabid after loss of offsite power. The system would be l

I operable after any cingia'setive mechanical failure and any single active AC power system failure. In general, the system would be also operable following any single D0 rwer system failure except a single failure affecting both 125 VLC essential distribution panels of DC Load Group 2 that would require compensatory operator action outside the control room.

Jioledo Edison believes that implementation of these proposed modifications "will result in significantly enhanced and reliable feed and bleed cooling capability. 'Eince the makeup pumps can be powered from the emergency diesel generatopo, they would be operable after loss of offsite power.

Very tr 17 yours, A D DCS:WD:plf cc: DB-1 NRC Resident Inspector A. B. Davis, Regional Administrator (2 copies)