ML20090D581
| ML20090D581 | |
| 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 | |
| NUDOCS 8303020165 | |
| Download: ML20090D581 (4) | |
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NIAGARA MOHAWK POWER CORPORATION NIAGARA b MOHAWK 300 ERIE BOULEVARD, WEST SYRACUSL N. Y.13202 T""
June 16, 1972 g
Ms 197& -n P
43 ru Mr. Donald J. Skovholt Assistant Director for Reactor Operations Division of Reactor Licensing United States Atomic Energy Commission Washington, D. C.
20545
Dear Mr. Skovholt:
Ro: Provisional Operating License: DPR-17 Docket No.: 50-220 Your letter of May 18, 1972 requested Information relating to the desig.i and administrative procedures employed at the Nine Mile Point Nuclear Station Unit No. I to preclude occurrence of a control rod drop accident.
Station procedures require that pre-planned control rod withdrawal sequences be foilowed 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 procedurally by a second licensed operator or qualified station employee. Your May 18, 1972 letter requested replies to four specific questions relating to this Rod Worth Minimizer:
History of RWM Coerabiiity in most instances, i.e.,
more than 95% of the 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 fulIy operational.
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Specific Causes of System Inocerabiiity l
l System inoperability has been caused by combination of hardware arid i
software problems as detailed in the attached tabulation.
I noperab i l ity l
attributable to sof tware appears to be, at least in part, the result of complexity and inflexibility of the initial sof tware program. The principal hardware prob lems l
have been those associated with external circuitry. For exampl e, over 6,400 reed l
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V swit~ches which provide intelligence 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, it has not been possible to mainiain all the reed switches and relays in an operable condition over an extended period.
Current Goerability of the RWM Corrections which have been made to both the hardware and software portions of the RWM system to improve reliabili1y are described in the attachment.
Future operation will determine to what degree this ef fort has been successful, in particular, the ability to substitute rod position for a f ailed 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 veri f ication shoul d be maintained.
Plans and Schedule o Correct Any Deficiencies As discussed above, substantial improvements to the RWM system have al ready been i ncorpo rated. Both Niagara Mohawk and General Electric wil I continue to closely monitor system performance and implement further modifications in design and maintenar:ce wherever practical.
Very tru l y you rs,
A T
J. Brosnan Vice P oident-Chief Engineer
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O CAUSES AND CORRECTIONS FOR R0D WORTil MINIMIZER INOPERABILITY AT Tile NINI MILE POINT NUCLEAR STATION CAUSES CORRECTIONS R)St COMPUTER 1.
Mechanical and electrical fail-1.
Installed necessary software to
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urcs in the input and output require operation of the typer typer which caused a computer only'when needed by the computer
- outage, for input or output printing.
Previously, the typer was required to be continuously in service.
2.
Stray current influence caused 2.
Ground circuitry reconnected to by improper grounding.
eliminate stray current flowing in a loop between the RMI and the process computer.
3.
Failure of the cooling fan 3.
Vane flow switches and ten.pera-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 RIC1 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 tre 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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O CAUSES CORRECTIONS EXTERNAL R101 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.
SOFWARE 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 tine limit.
execute a given instruction..
3.
Interface difficulties between 3.
It was arranged that the Rh'M will the output buffers of the do the position scanning for itself and process computer and the R101 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.
l 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 j
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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