ML20058K817

AO 263/74-15:on 740521,high Voltage Power Supply Fuses in Two of Four Main Steam Line Radiation Monitors Found Blown While Reactor Startup in Progress.Addition of High Voltage Operability Monitor Being Investigated

ML20058K817
Person / Time
Site:	Monticello
Issue date:	06/20/1974
From:	Mayer L NORTHERN STATES POWER CO.
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
References
AO-263-74-15, NUDOCS 9102140463
Download: ML20058K817 (2)

Text

6 50-26C NSD .

NORTHERN STATES POWER COMPANY M m w z A pou s, u n s w r ecTA $ 5401 Mr. J F O' Leary, Director -

Directorate of Licensing ' (/,3' Office of Regulation fYi .. ' ' ':

U S Atomic Energy Co::: mission < :g\

Washington, DC 20545 Q j' . / '

ABNORMAL OCC1'RRENCE REPORT TO THE AEC Inoperability of the Main Steam Line Radiation Monitors

1. Report Number: A0 263/74-15 A. .

2a. Report Date: June 20, 1974 g 2b. Occurrence Date: May 21, 1974 (( [3 n, -

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3. Facility: Monticello Nuclear Generating Plant (DPR-22) i __

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(p Monticello, Minnesota 55362 -3 c

4. Identification of Occurrence:

~his report concerns inoperability of the main steam line radiation s -

=cnitors which constitues an Abnomal Occurrence and is reported in accordance with Section 6.7.B.1, Appendix A Technical Specifications, of Provisional Operating License DPR-22.

5. Conditions Prior to occurrence:

Routine startup operation.

6. Description of Occurrence:

On May 21, 1974, while a reactor startup was in progress, the high voltage power supply fuses in two of the four main steam line radiation monitors were found blosm. As a result of the undetected blosm fuses, both monitors were incapable of performing their protective function.

The four main steam line radiation monitors are connected in a "one-of-two twice" logic scheme to initiate a reactor scram and Group 1 isolation upon detection of high radiation in the vicinity of the main steam lines. Both inoperable monitors were connected in the "B" protection logic channel. The protective function of the monitors was therefore disabled for approximately five hours.

over REGULATORY DOCIET FILE COPY 9102140463 740620 CF ADDCK 05000263 CF

NORTHERN CTATES POWER COMPANY The failed high voltage power supply fuses were replaced and the radiation monitors were functionally tested to demonstrate operability. The significance of the malfunction was not realized until June 10, 1974.

7. Desienation of Apparent Cause of Occurrence:

The cause of the blown fuses could not be determined. The fuses were replaced and the radiation monitors were functionally tested and found to operate properly.

8. Analysis of Occurrence:

The main steam line monitors are installed to initiate a reactor isolation in the event of possible gross fuel failure following a rod drop accident.

Considering the extremely low probability of such an occurrence and the short time during which the monitors were inoperable, there was no signif-icant danger to the health and safety of the public.

9. Corrective Action:

The addition of a high voltage operability monitor, designed to initiate an automatic trip of a channel upon detection of high voltage power supply failure, is being investigated. As a short term preventive measure, the surveillance log has been modified to identify any monitor indication deviation which exceeds established ILmits and consider it as a failure of the lower reading channel.

10. Failure Data:

This is the first instance of such a failure at Monticello.

No defective co=ponent or design deficiency has been identified at this time.

L, O K 9 L 0 Mayer, PE '

/DW Director of Euclear Support Services LQ4/DMM/kn cc: J G Keppler G Charno f f Minnesota Pollution Control Agency Attn. E A Pryzina 1

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