ML19273B409

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Forwards Notification That Several Types of stem-mounted Limit Switches Failed to Meet Environ Qualification Criteria.Situation Will Not Affect Other safety-related Circuits
ML19273B409
Person / Time
Site: Fort Calhoun Omaha Public Power District icon.png
Issue date: 03/29/1979
From: Short T
OMAHA PUBLIC POWER DISTRICT
To: Stello V
Office of Nuclear Reactor Regulation
References
NUDOCS 7904060300
Download: ML19273B409 (3)


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Omaha Public Power District 1623 HARNEY a O M AH A, NEBRASKA 68102 TELEPHONE 530-4000 AREA CODE 402 March 29, 1979 Mr. Vietor Stello, Jr., Director Division of Operating Reactors U. S. Nuclear Regulatcry Commission Washington, D. C. 20555

Reference:

Docket No. 50-235

Dear Mr. Stello:

In accordance with :iRC Inspection and Enforcement Bulletin 79-01, dated February 8,1979, the Craha Public Power District gave 2h-hour notice, by letter dated March 16, 1979, that certain safety-related electrical equipment installed at the Fort Calhoun Staticn had failed to meet environmental qualification criteria. Thic letter provides, by attachment, a detailed written report addressing this subject.

The report clearly demonstrates thr.t the health and safety of the public is not jecpardised by this situation.

Sincerely,

/ e r~~ ) )bj! , /

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NJ Assistant General Manager TES/KJM/EJH:jna Attach.

cc: Mr. Robert W. Reid, Chief Operating Reactors Branch No. k U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Mr. K. V. Seyfrit, Director U. S. Nuclear Regulatory Cocnission Office of Inspectic and Enforcement Region IV 611 Ryan Plaza Drive, Suite 1000 Arlington, Texar 76011 LeEceuf, La b, Leiby & ".acRae 17.57 ":I" Street, :i . W . 1 Washington, D. C. 20036 7

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ATTACHMENT Several types of stem-mounted limit switches were identified in the Fort Calhoun Nuclear Station as being unqualified for service within a LOCA environment. The types identified, as well as the nudber of unquali-fled switches, are listed belev. Note that these limit switches are all located en containment isolation valves.

NAMCO D2h00X 9 x 2 switches / valve = 18 switches NAMCO D12 COG 13 x 2 switches / valve = 20 switches Fisher Type 30k 14 x 2 switches / valve = 28 switches In all cases identified above, the limit switches function to pro-vide valve positien indication (light indication) only. ~n no case vill the failure of these limit switches cause inadvertent valve operation or prohibit the function of other safety-related circuits to perform their designed function. Listed belev are the four possible failure modes of the limit switches in questien. An analysis of the consequences result-ing from each mode of limit switch failure is also given.

Mode 1 Switch shorts - This would result in possible indication of both "open" and " closed". This mode of failure in no way inhibits valve operation or control.

Mode 2 Switch cpen circuit - This would result in a ec=pleue loss of posi-tion indicatien. Valve operability / control, hcVever, vould not be affected.

Mode 3 Grounding of the power supply through switch failure - The valve circuitry is designed such that the Fort Calhoun instrument buses will operate with a ground. The circuit design is such +'-=' or'y one line to ground (the negative base) wou' d ^^^"* The other line is held above ground by the resistance of the indication circuit.

In the highly unlikely event that both lines, positive and negative, are grounded, the control fuse located in each individual contain-ment isolation valve circuit veuld "bicv" and the valve would as-sume its fail safe position.

Mode k Possible tracking o the solenoid circuit - The probability of this mode of failure is extremely unlikely, since the solenoid leads are insulated and separated from the limit switches by the " distance of mounting". Shauld the linit switch track to the solenoid by means of water / saturated steam, +.he conductivity of this vauer/ saturated steam would have to be near that of copper vire. A " bell-park" conductivity figure of 15 horic acid solution (approximately 1800 ppo boric acid) is 15 x 100 thos. This nudber is exceedingly small

as ccmpared to the conductivity of bare copper wire; therefore, the likelihood of tracking to the solenoid circuit from the limit switch is highly unlikely.

Assuming tracking to the solenoid can occur, the current fed to the solenoid vculd be insufficient to take it operate due to the limit-ing function of the light indication circuitry. Analysis has shown that, if tracking did cccur from the linit switch to the solenoid, the smperage which would be supplied to the solenoid would be approxi-nate y .0h amps. This is far below the .13 amps required to operate the solenoid. Therefore, it is concluded that the occurrence of

" tracking" will not cause any malfunction or less of valve operability and vill not inhibit any valve from carrying out its designed or in-tended functions. In addition, tracking would not cause the mal-function of any ather safety related equipment.

It is emphasiced that in no case vill the failure of these limit switches cause inadvertent valve operation or prchibit the function of other safety-related circuits to perform their design function.

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