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. DATE OF DOC: DATE REC'D LTR- MEt40 RPT OTHER Syracuse, N.Y. 13202 T. J. Brosnan 6-,16"72 6-16-72 TO t ORIG CC SENT AEC PDR SENT LOCAL PDR Mr. Donald  ; S o holt CLASS: OP INFO No CYS REC'D DOCKET No:

40 ,50"220 DESCRIPTION: Ltr re our 5-18-72 ltr..furn ENCLOSURES: Causes & Corrections for Rod, info re design & administrative procedures a Worth Minimizer Inoperability, at the Nine Mile Station Unit I to preclude occurre ce

• Nine Mile Pt. Nuclear Station Unit I of a control rod drop accident & trans:

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Mr. Donald J. Skovholt 'zetyg~

Assistant Director for Reactor Operations Division of Reactor Licensing United States Atomic. Energy Commission

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'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

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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

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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 =

'. Installed necessary. software to

,require operation of the typer typer which caused a computer only when needed by the computer

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for input or output printing.

Previously,, the typer was required to be contin'uously in service.

2. Stray current influence caused 2. Ground circuitry reconnected ta by improper grounding. eliminate stray current flowing in a loop between the RWM and the process computer.

3. Failure of the cooling fan 3. Vane flow switches and tempera-causing overheating. ture alarms were connected to the process computer to warn of RWM cooling system failure.

4. Internal wiring and circuit 4. Defective wiring and components board component failures. were replaced.

EXTERNAL RWM CIRCUITRY

1. Numerous failures of electri- 1. 129 position probes were replaced cal circuitry within,the rod with new model probes. Like the position indicator probe in- original probes, any failures in side the reactor. These con- these new probes will be uncorrec-sisted of open circuits, grounds table while the reactor is in and shorts. service.

2. Failures of the digital relays 2. In part, these failures are pro-in the position indicator duced by high voltage on the relays circuitry. 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.

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PAGE 2 CAUSES CORRECTIONS EXTERNAL Rl&f CIRCUITRY'CONT'D)

3. Stepping switch failure in the 3. Replaced defective stepping switches rod position indicator scan and intensified maintenance pro-circuitry. gram'to improve reliability of these switches.

SOFTl(ARE

1. Failure to properly transfer 1. Additional instructions were pre-from one rod group to the pared to better accommodate group next. changes up and down.

2. Stalling due to failure to 2. The program was rewritten to reduce execute an instruction within the number of steps required to the design time limit. execute a given instruction.

3. Interface difficulties between 3. It was arranged that the RW will the output buffers of the do the position scanning for itself and process computer and the Rttm the process computer when the computer. 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. Misleading error printouts 4. Improved and simplified programming which resulted in improper should reduce or eliminate these operator corrective action. conflicts.

5. The inability of the program to 5. Program changes permit insertion of accommodate loading new sequences a substitute rod position when a except with all rods fully position indicator has failed in inserted even with the reactor rod notch just entered according to in shutdown or refuel modes. 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.

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