ML17037C362

Letter Responding to the May 18, 1972 Letter Requesting Information Relating to the Design and Administrative Procedures Employed at Unit 1 to Preclude Occurrence of a Control Rod Drop Accident

ML17037C362
Person / Time
Site:	Nine Mile Point
Issue date:	06/16/1972
From:	Brosnan T Niagara Mohawk Power Corp
To:	Skovholt D US Atomic Energy Commission (AEC)
References
Download: ML17037C362 (10)

Text

'1 AEC DISVRIBUTION FOR PART 50 DOCKET MATERXAL TEMPORARY FORM CONTROL No: FROM: Niagara Mohawk Power Corp.Syracuse, N.Y.13202 T.J.Brosnan DATE OF DOC: 6-,16"72 DATE REC'D 6-16-72 LTR-MEt40 RPT OTHER TO t Mr.Donald;S o holt CLASS: OP INFO ORIG CC No CYS REC'D 40 SENT AEC PDR SENT LOCAL PDR DOCKET No: ,50"220 DESCRIPTION:

Ltr re our 5-18-72 ltr..furn info re design&administrative procedures a Nine Mile Station Unit I to preclude occurre of a control rod drop accident&trans: PLANT NAMES: Nine Mile Nuclear Station Unit ENCLOSURES:

Causes&Corrections for Rod, Worth Minimizer Inoperability, at the ce*Nine Mile Pt.Nuclear Station Unit I AM!)ILB LEO MIITIKN 3 (1 Orig&39 conf'd cys encl rec d FOR ACTION INFORMATION L 6"19-72 BUTLER L W/, Copies CLARK(L")W/Copies GOLLER(L)W/Copies KNIEL L W/Copies SCHWENCEH(L

)W/Copies STOL@(L)W/Copies VASSALLO L W/Copies H.DENTON W/Copies SCHEMEL(L)W/Copies INTER L DISTRIBUTION IEMANN L W/9Copies'.CHXTWOOD(FM)

W/.Copies DICKER (ENVXRO)W/Copies KNIGHTON ENVIRO'/Copies W/Copies W/Copies FILE LLO-L C PDR 4AOORE-L G OPER (2)LANGE-L C-Rt~i P-506@WLICiCI>>L NTZING 8o SZAFF WOMPSON-L IAMBUSSO>>L WDESCO-L OYD-L-BWR~ONG-L DEYOUNG-L-PWR INAS-L LLER-L-ENVIRO SHAO-L OVHOLT-L-OPER

@NAHOYA-L H-L NORRIS-Ro CCAHY-L DUBE-L HROEDER-L hd.CASE-L OLLt SR-L ENTON-L+GRItSS-L GAMMILL-L KNIGHTON-ENVIRO DICKER-ENVIRO PROJ LDR ENVIHO'ALTZHAN-IND.

McDONALD-PLANS NUSSBAUMER-FM SMILEY-FM P.COLLINS-L EXTERNAL DXSTRIBUTION KARAS-L L A PWR MASON-L L/A BWR BROWN-L L/A PWR WILSON-L L/A PWR.KARX-L L/A BWR SMITH<<L L/A BWR GEARIN-L L/A BWR DIGGS-L L/A TEETS-L L/A WADE-L L/A ENVIRO BRA XTMAN-A/T SS-ENVIHO 0-LOCAL PDR Oswe o N.Y.WDzzE-(LILUGHLIN)

WNSIC-(BUCHANAN) 1-ASLB-YORE/SARYE WOODWARD/H.

ST.1-C.MILES-C-459)

GT 16 CYS ACHS-HOLDING 1-SAN LA NY--PDR 1-CHIEF WATER REACTORS 1-RD.~~.E.HALL F-309 GT 9-NATIONAL LAB'~ANL/ORNL/BNWL 1-R.CARROLL-OCp GT 1-R.CATLIN, A-170'T 1-CONSULTANT'S NEWMARK/BLUME/AGBABIAN 1-DR.'GERALD S.LELLOUCHE BROOKHAVEN NATXONAL LAB E 4i,~~N Regulat~or NIAGARA MOHAWK POWER CORPORATION s'j~l&NIAGARA'I MOHAWK (f)g gQ%gg@+i@ic>%300 ERIE BOULEVARD.

WEST SYRACUSE, N, Y.13202 June l6, l972 Mr.Donald J.Skovholt Assistant Director for Reactor Operations Division of Reactor Licensing United States Atomic.Energy Commission Washington, D.C.20545 DDCI{E~QQ@.'zetyg~@~III'ear Mr, Skovholt: Re: Provisional Operating License: DPR-l7 Docket No.: 50-220 Your letter of May I8, l972 requested information relating io the design and administrative procedures employed at the Nine Mile Point Nuclear Station Unit No.I io preclude occurrence of a control rod drop accident.Station procedures require that pre-planned control rod withdrawal sequences be followed during each startup to minimize the reactivity worth of individual control rods.In addition, conformance with these sequences can be verified in two ways: by use of the Rod Worth Minimizer (RWM)system or procedurall,y by" a second licensed operator or qualified station employee.Your May I8, l972 letter requested replies to four specific questions relating to this Rod Worth Minimizer:

History of RWM 0 erabilit In most instances, i.e., more than 95/of ihe time, verification of conformance to the prescribed control rod withdrawal sequence has been provided procedurally due to inoperability of certain components of the RWM monitoring system as described below.In only two instances out of a total of 55 startups has the RWM system been fully operational.

S ecific Causes of S stem Ino erability System inoperability has been caused by combination of hardware and software problems as detailed in the attached tabulation.

Inoperability attributable to software appears to be, at least in part, the result of complexity and inflexibility of the initial software program.The principal hardware problems have been those associated with external circuitry.

For example, over 6,400 reed n t)I'K hI switches which provide intel I igence to the RWM computer of rod position had to have been operable for the RWM system to perform its intended function.Similarly, 2,200 relays had to actuate properly.'lt has not been possible to maintain al I the reed switches and relays in an operable condition over an extended period.Current 0 erabilit of The RWM Corrections which have been made to both the hardware and software portions of the RWM system to improve reliability are described in the attachment.

Future operation will determine to what degree this effort has been successful.

In particular, the ability to substitute rod position for a failed position switch should enhance the overall reliability of the RWM system.However, until improved reliability can be demonstrated by actual experience, flexibility in rod sequence verification should be maintained.

Plans and Schedule To Correct An Deficiencies As discussed above, substantial improvements to the RWM system have already been incorporated.

Both Niagara Mohawk and General Electric will continue to closely monitor system performance and implement further modifications in design and maintenance wherever practical.

Very truly yours,.J.Brosnan Vice P ident-Chief Engineer l.~~g usa~~CAUSES AND CORRECTIONS FOR ROD WORTH MINIMIZER INOPERABILITY AT THE N NE MILE POINT NUCLEAR STATION CAUSES CORRECTIONS RWM COMPUTER 1.Mechanical and electrical fail-ures in the input and output=typer which caused a computer'utage.'.Installed necessary.

software to ,require operation of the typer only when needed by the computer for input or output printing.Previously,, the typer was required to be contin'uously in service.2.Stray current influence caused by improper grounding.

2.Ground circuitry reconnected ta eliminate stray current flowing in a loop between the RWM and the process computer.3.Failure of the cooling fan causing overheating.

4.Internal wiring and circuit board component failures.3.Vane flow switches and tempera-ture alarms were connected to the process computer to warn of RWM cooling system failure.4.Defective wiring and components were replaced.EXTERNAL RWM CIRCUITRY 1.Numerous failures of electri-cal circuitry within, the rod position indicator probe in-side the reactor.These con-sisted of open circuits, grounds and shorts.1.129 position probes were replaced with new model probes.Like the original probes, any failures in these new probes will be uncorrec-table while the reactor is in service.2.Failures of the digital relays in the position indicator circuitry.

2.In part, these failures are pro-duced by high voltage on the relays which is produced when grounds occur in the position probe cir-cuitry.A voltage supply reduction provides a partial remedy.A better fix is under investigation by General Electric.

lg Ef f V PAGE 2 CAUSES CORRECTIONS EXTERNAL Rl&f CIRCUITRY'CONT'D) 3.Stepping switch failure in the rod position indicator scan circuitry.

3.Replaced defective stepping switches and intensified maintenance pro-gram'to improve reliability of these switches.SOFTl(ARE 1.Failure to properly transfer from one rod group to the next.1.Additional instructions were pre-pared to better accommodate group changes up and down.2.Stalling due to failure to execute an instruction within the design time limit.3.Interface difficulties between the output buffers of the process computer and the Rttm computer.4.Misleading error printouts which resulted in improper operator corrective action.2.The program was rewritten to reduce the number of steps required to execute a given instruction.

3.It was arranged that the RW will do the position scanning for itself and the process computer when the reactor is at lower power.This will eliminate some of the inter-ference problems between the compu-ter.Solution of the remaining interface difficulties is under development.

4.Improved and simplified programming should reduce or eliminate these conflicts.

5.The inability of the program to accommodate loading new sequences except with all rods fully inserted even with the reactor in shutdown or refuel modes.5.Program changes permit insertion of a substitute rod position when a position indicator has failed in rod notch just entered according to the prescribed patterns.The sub-stitution is only made when the operator is assured on the basis of procedure or redundant instrumenta-tion that.the rod is in the position entered.

V'~I I