ML20127L143

Ro:On 730810,discovered That Monitor Reading Lower than Expected for Power Level Due to Local Switch on Sampling Sys in Purge Position.Local Purge Switch Will Be Removed to Allow CR Personnel to Put Stack Gas Monitor in Purge Mode

ML20127L143
Person / Time
Site:	Monticello
Issue date:	08/17/1973
From:	Mayer L NORTHERN STATES POWER CO.
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9211230435
Download: ML20127L143 (2)

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NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55362 vm

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g Mr. J. F. O' Leary, Director r>

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Directorate of Licensing United States Atomic Energy Commissio

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Dear Mr. O' Leary:

hDNflCELLO NUC1 JAR GINLRATING PLANT Docket No. 50-263 License No. DPR 22 Lapse of Off-Gas Stack bbnitoring Due to Switch in Purge Position A condition occurred at the hbnticello Nuclear Generating Plant which we are reporting to your office in accordance with Section 6.7.B.1, Abnormal Occurrence Reports, of the Technical Specifications, of Provisional Opera-ting License DPR-22.

On August 10, 1973, during startup of the plant, it was discovered that the stack gaseous effluent monitor had been in the purge mode and therefore had not been monitoring the radioactive gas release rate from the stack as required by Technical Specification 3.8.A.

The ?l ant had been in hot standby since August 8th, with the reactor isolated.

At about 0000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> on August 10th, the reactor was unisolated and plant start-up was continued. At about 0830, August 10th, with reactor power at 30%, it was observed that the stack monitor reading was lower than expected for that power 1cvel. Immediate investigation revealed that the local switch on the sampling system was in the purge mode. It was immediately restored to the operating mode. Further investigation indicated that the local purge switch had been Icft in the aurge position following system maintenance on August 7th while the plant was sautdown.

The local purge switch being in the , urge position caused the stack monitor be measure radioactivity in the stac < room atmosphere instead of the activity in the stack effluent. There is no indication in the main control room that the local switch is in the purge position. Therefore when the plant was started up, the operators were unaware that the stack monitor was not operable.

There are no automatic functions associated with the stack gas monitoring system. Before the stack release rate would have reached a point when public {

safety would have been endangered, the air-ejector off-gas monitor would have )

isolated the off-gas line to the stack.

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. M4 . J. F. O' Leary August 17, 1973 The following corrective action will be taken:

The local purge switch will be removed. 1his will allow only control room personnel to put the stack gas monitor in the:) urge mode, hhen the stack gas monitor is placed in the purge mode from trie control room it is indi-cated by a red light next to the purge switch.

Yours very truly,

k. $. h kr illf{0 L.' O. Mayer, PE Director of Nuclear Support Services IDVkik cc: B H Grier G Charnoff Minnesota Pollution Control Agency Attn: 4;cn Dzugan

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