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{{#Wiki_filter:'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:
{{#Wiki_filter:'1 AEC DISVRIBUTION FOR PART 50 DOCKET MATERXAL TEMPORARY FORM                                       CONTROL   No:
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:
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:
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
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
)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)
* Nine Mile Pt. Nuclear Station Unit I of a control rod drop accident & trans:
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
AM!)ILBLEO (1 Orig & 39 MIITIKN 3 conf'd cys encl rec d PLANT NAMES:        Nine Mile Nuclear Station Unit BUTLER W/, Copies CLARK(L")
@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.
L               KNIEL L W/ Copies SCHWENCEH(L )
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)
FOR ACTION INFORMATION VASSALLO W/ Copies H. DENTON L
WNSIC-(BUCHANAN) 1-ASLB-YORE/SARYE WOODWARD/H.
L IEMANN 6"19-72 L
ST.1-C.MILES-C-459)
W/ 9Copies'.
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.
CHXTWOOD(FM)
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.
KNIGHTON ENVIRO Copies W/ Copies                  W/ Copies              W/ Copies                          W/. Copies               W/  Copies GOLLER(L)                  STOL@(L)                SCHEMEL(L )                        DICKER (ENVXRO)
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.
W/ Copies                 W/ Copies              W/ Copies                         W/     Copies           W/   Copies INTER    L DISTRIBUTION FILE                     LLO-L               OLLtSR-L                          KARAS-L L A      PWR C PDR G OPER     (2) 4AOORE-L LANGE-L ENTON-L GRItSS-L
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.
                                                                +                        MASON-L BROWN-L L/A BWR L/A PWR C-Rt~i P-506             @WLICiCI>>L             GAMMILL-L                          WILSON-L L/A PWR      .
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.
NTZING 8o     SZAFF WOMPSON-L                   KNIGHTON-ENVIRO                    KARX-L L/A BWR IAMBUSSO>>L             WDESCO-L                 DICKER-ENVIRO                      SMITH<<L L/A BWR OYD-L-BWR               ~ONG-L                   PROJ LDR                          GEARIN-L L/A BWR DEYOUNG-L-PWR                 INAS-L                         ENVIHO'ALTZHAN-IND.
DIGGS-L L/A LLER-L-ENVIRO           SHAO-L                                                   TEETS-L L/A OVHOLT-L-OPER           @NAHOYA-L               McDONALD-PLANS                    WADE-L L/A ENVIRO H-L                 NORRIS-Ro               NUSSBAUMER-FM                      BRA XTMAN-A/T CCAHY-L                 DUBE-L                 SMILEY-FM                                  SS-ENVIHO HROEDER-L             hd. CASE-L               P. COLLINS-L                           0 EXTERNAL DXSTRIBUTION
  -LOCAL PDR Oswe o           N.Y.                                                     1-SAN LA NY--PDR WDzzE-(LILUGHLIN)                            9-NATIONAL LAB'                            1-CHIEF WATER REACTORS WNSIC-(BUCHANAN)                              ~
ANL/ORNL/BNWL                          1-RD. ~ ~ .E. HALL F-309 GT 1-ASLB-YORE/SARYE                            1-R. CARROLL-OCp GT WOODWARD/H. ST.                          1-R. CATLIN, A-170'T 1-C. MILES-C-459)          GT              1-CONSULTANT'S 16  CYS ACHS-HOLDING                              NEWMARK/BLUME/AGBABIAN 1-DR. 'GERALD S. LELLOUCHE BROOKHAVEN NATXONAL LAB
 
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Regulat~or (f)
NIAGARA MOHAWK POWER CORPORATION                             gQ% gg@+i@ ic>%
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NIAGARA '         I MOHAWK 300 ERIE BOULEVARD. WEST SYRACUSE, N, Y. 13202 June   l6, l972 DDCI{E~
QQ@.
Mr. Donald J. Skovholt                                                                   'zetyg~
Assistant Director for Reactor Operations Division of Reactor Licensing United States Atomic. Energy Commission
                                                                                  @~ III Washington, D. C.       20545
'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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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.
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.
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.
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.
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,
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.
                                                        . J. Brosnan Vice P     ident-Chief Engineer
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.
l .~
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.
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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.
usa~~
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.
CAUSES AND CORRECTIONS FOR ROD WORTH         MINIMIZER INOPERABILITY AT THE N NE MILE POINT NUCLEAR STATION CAUSES                                                 CORRECTIONS RWM COMPUTER
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.
: 1. Mechanical and electrical fail-ures in the input and output   =
4.Improved and simplified programming should reduce or eliminate these conflicts.
                                                      '.     Installed necessary. software to
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.
                                                            ,''require operation of the typer typer which caused a computer                            only when needed by the computer
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                                      'utage.
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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Latest revision as of 09:02, 30 October 2019

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 Constellation icon.png
Issue date: 06/16/1972
From: Brosnan T
Niagara Mohawk Power Corp
To: Skovholt D
US Atomic Energy Commission (AEC)
References
Download: ML17037C362 (10)
v • d • e


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:

AM!)ILBLEO (1 Orig & 39 MIITIKN 3 conf'd cys encl rec d PLANT NAMES: Nine Mile Nuclear Station Unit BUTLER W/, Copies CLARK(L")

L KNIEL L W/ Copies SCHWENCEH(L )

FOR ACTION INFORMATION VASSALLO W/ Copies H. DENTON L

L IEMANN 6"19-72 L

W/ 9Copies'.

CHXTWOOD(FM)

'/

KNIGHTON ENVIRO Copies W/ Copies W/ Copies W/ Copies W/. Copies W/ Copies GOLLER(L) STOL@(L) SCHEMEL(L ) DICKER (ENVXRO)

W/ Copies W/ Copies W/ Copies W/ Copies W/ Copies INTER L DISTRIBUTION FILE LLO-L OLLtSR-L KARAS-L L A PWR C PDR G OPER (2) 4AOORE-L LANGE-L ENTON-L GRItSS-L

+ MASON-L BROWN-L L/A BWR L/A PWR C-Rt~i P-506 @WLICiCI>>L GAMMILL-L WILSON-L L/A PWR .

NTZING 8o SZAFF WOMPSON-L KNIGHTON-ENVIRO KARX-L L/A BWR IAMBUSSO>>L WDESCO-L DICKER-ENVIRO SMITH<<L L/A BWR OYD-L-BWR ~ONG-L PROJ LDR GEARIN-L L/A BWR DEYOUNG-L-PWR INAS-L ENVIHO'ALTZHAN-IND.

DIGGS-L L/A LLER-L-ENVIRO SHAO-L TEETS-L L/A OVHOLT-L-OPER @NAHOYA-L McDONALD-PLANS WADE-L L/A ENVIRO H-L NORRIS-Ro NUSSBAUMER-FM BRA XTMAN-A/T CCAHY-L DUBE-L SMILEY-FM SS-ENVIHO HROEDER-L hd. CASE-L P. COLLINS-L 0 EXTERNAL DXSTRIBUTION

-LOCAL PDR Oswe o N.Y. 1-SAN LA NY--PDR WDzzE-(LILUGHLIN) 9-NATIONAL LAB' 1-CHIEF WATER REACTORS WNSIC-(BUCHANAN) ~

ANL/ORNL/BNWL 1-RD. ~ ~ .E. HALL F-309 GT 1-ASLB-YORE/SARYE 1-R. CARROLL-OCp GT WOODWARD/H. ST. 1-R. CATLIN, A-170'T 1-C. MILES-C-459) GT 1-CONSULTANT'S 16 CYS ACHS-HOLDING NEWMARK/BLUME/AGBABIAN 1-DR. 'GERALD S. LELLOUCHE BROOKHAVEN NATXONAL LAB

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Regulat~or (f)

NIAGARA MOHAWK POWER CORPORATION gQ% gg@+i@ ic>%

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NIAGARA ' I MOHAWK 300 ERIE BOULEVARD. WEST SYRACUSE, N, Y. 13202 June l6, l972 DDCI{E~

QQ@.

Mr. Donald J. Skovholt 'zetyg~

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

@~ III Washington, D. C. 20545

'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)

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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

'utage.

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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