ML20206C410

Proposes to Remove Rod Sequence Control Sys & to Lower Rod Worth Minimizer Low Power Setpoint from 20 to 10% at Util

ML20206C410
Person / Time
Site:	Limerick
Issue date:	11/09/1988
From:	Kemper J PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	Murley T Office of Nuclear Reactor Regulation
References
NUDOCS 8811160265
Download: ML20206C410 (3)

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PHILADELPHIA ELECTRIC COMPANY 2301 MARKET STREET P.O. BOX 8699 PHILADELPHI A PA.19101 (215)841 45o0 JOHN S. KEMPER camon vic enssionar . nuci. ^" November 9, 1988 Docket No. 50-353 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Conmission Attn: Docunent control Desk Mal' Station P1-137 Washington, DC 20555

SUBJECT:

Limerick Generating Station, Unit 2 Deletion of Rod Sequence Control System ,

and Lowering of Red Worth Minimizer Low Pcwer Set Point to 10 Percent

REFERENCE:

NRC Letter from A. C. Thadant to J. S. Charnley of General Electric dated Decerrber 27, 1987 FILE: GOVT 1-1 (NRC)

Dear Dr. Murley:

Philadelphia Electric Ccopany proposes ; ranove the Rod Sequence Control System (RSCS) and to lower the Rod Worth Minimizer (RWM) Low Power Setpoint (LPSP) frcm 20 percent to 10 percent at Lirnerick Gener-ating Station, Unit 2. The proposed changes reouested herein are based upon the NRC Safety Evaluation relating to the General Electric Topical Report NEDE-24011-P-A, Revision 8, Amendment 17, "General Electric Standard Appilcation for Reactor Fuel".

The RSCS is a hard wired rod pattern control system that provides insert or withdrawal blocking logic te the Reactor Manual Control i

System to prevent excessive rod worth during reactor low pcwer conditions where a rod drop accident (RDA) would be trost severe.

The RSCS Is designed for mitigation of the RDA and is active only ,

during Icw pcwor operation (less than 20 percent pcwer). A similar rod pattern centrol function is provided by the RhM. As discussed in the referenced doeurent, the NRC had concertis about the reliability of the RWM when It was first placed in service and therefore required the Installation of a redundant system (RSCS). The RhM has hcwever proven to be re11able enough to operate in a stand-alone rrode. Refined tredeling using incroved rnethodology (and to a lesser extent Irroroved 0011160265 Of1109 g(

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rod pattems) Indicate that a RDA Involving no errors In the rod pattern would result in peak enthalpy far below NRC limits. The RWM at LGS-2 is a NLA%C system that utilizes the Banked Position Withdrawal Sequence (BPWS) patterns reccmnended by the referenced letter. The design of this system does not allow rod sequences in a non-BPWS pattern. RSCS rerroval is justified because it is redundant to the RhM and f s therefore not needed to mitigate the control rod drop accident.

Additionally, a more restrictive RWM technical specification will enforce use of the RWM. The elimination of RSCS will reduce startup and shutdem times and will reduce operational complexities.

In addition to the removal of the RSCS, Philadelphia Electric Conpany proposes to lower the RWM LPSP frcrn 20 percent to 10 percent power. More accurate modeling for the RDA (BNL-NUREG 28109, "Thermal Hy*raulic Effects on Center Rod Drop Accidents in a BWR", October 1980) show that peak fue; enthalples caused by an RDA are truch icwer than previously detennined by earlier mathematical models. The modeling derronstrates that lowering the RWM LPSP to 10 percent will not exceed the Ilmit of 280 cal /gn in the event of an RDA above the 10 porcent setpoint.

The reference letter states that the NRC does not intend to repeat Its review of the report subject matter except to assure that the material presented is appitcable to the plant involved. The letter hovever does request that any technical specification changes along with a discussion of the review of verification procedures used by a dedicated (second) operator be included in the submittal. Additional-ly, the letter reccmnends that rod patterns used be at least eaulvalent to the Banked Position W!thdrawal Sequence patterns.

The Technical Specificatient for LGS-2 are currently under review and revisions will be proposed to minimize operations without the active use of RhM. The proposed Technical Specification revisions will be In confornunce with the suggested revisions found acceptable in the j NRC approval of NEDE .%011-P-A. Programs and procedures for e, second operator are provided, for Instances when the RhM is not In use, .e independently verify the correctness of the first operator's actions ,

during rod movements. The procedure specifically requires the  ;

following:

1. Plant management approval prier to the operator bypassing the RWM. 1

2. A second operator, without any other duties, is rtculred to verify the first operator's actions wh11e the f'. st operator Initiates rod rrovennnts.

f 3. Separate startup and shutdown sequences to avoid sequence order confusion.

4 Both operators are provided with shutdown Instructions that will not allcw a control rod pattern different frcm RW4, assun!ng the RWM was not bypassed and was controlling. These ,

instructions trientify, for the operator, the Rui shutdown '

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.. o step to be Initiated for further rod Insertion below the RW LPSP.  !

Philadelphia Electric Conpany has conmitted to inform the NRC of any major design differences between LGS-1 and LGS-2 during the con-struction and licensing of LGS-2. Although this proposed change on l LGS-2 is consistent with a proposed Technical Specification snendment .

request for LGS-1, NRC approval of the LGS-2 RSCS deletion and inwering l of the RW LPSP ls requested by January 1989 so that physical trodifica-tions can be concleted prior to the scheduled preoperational testing of this system. The Technical Specification revision proposals and FSAR changes will be ccrreleted after approval of this request.

i Sincerely, 7 t

J. S. Kenper Senior Vice President Nuclear WEB /dEP/sw/10138802 cc: Addresseo R. d. Clark, USNRC Project Manager W. T. Russell, Administrator, Region I, USNRC R. A. Granm, USNRC Senior Resident inspector, LGS-2 i

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