ML20235E034
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NORTHERN 5TATES POWER COMPANY Minneapolis, Minnesota 55401
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S September 24, 1971 E*P 9
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,8A Dr. Peter A. Morris, Di rector 4
Division of Reactor Licensi N g#
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United States Atomic Energy ommission b
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Dear Dr. Morris:
%! tg NONTICELLO NUCLEAR GENERATitG PLANT Docket No. 50-263 License No. DPR-22 Failure of the ECCS Pump Start Permissive Pressure Switches to Trip Within the Specified Settings.
1 During the performance of a monthly surveillance test on September 15, 1971, it was discovered that the ECCS pump start permissive pressure switches l
tripped below the trip settings specified in the Technical Specifications. This l
occurrence is being reported in accordance with Section 6.6.B.3 of Appendix A, Technical Specifications, of the Provisional Operating License DPR-22. The Region 111 Compliance Office has been notified of the occurrence.
Summarv Descrin+ inn or Occorrence On September 15, 1971, the four trip settings of the low reactor pressure pump start permissive switches for the Core Sprav and LPCI system were found to be approximately 15-25 psi below the trip settings specifled in Table 3.2.2 of the Technical Specifications. An analysis of the calibration history for the switches reveals that during the previous surveillance testing, the calibration of the switches was performed in the increasing pressure di rection.
As a result, the calibrated " trip settings" of the switches were actually the reset positions for the actual trip settings. The use of r, rew surveillance procedure, which specifies that the switches be calibrated in a decreasing pressure direction, led to the discovery of the calibration error on September 15th.
Immediately l
following the discovery of the calibration error, the switches were properly recalibi ated and tested.
To prevent similar occurrences from happening, we are presently reviewing our surveillance procedures and revising them, as necessary, to include more detailed and speci fic calibration ins tructions.
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, 1 Also, during t e, investigation of the switch ' calibration problem 'it j
- was discovered that the permissive pressure switches for the open,ing of the Core. Spray and LPCI valves were both bourdon tube instruments. 'Section l
' 6. 2. 8 - o f s t he M AR s t a tes, "....... in order to preclude ossible corren mode failures, these two switches (valve open permissive, which.are f
connected to too widely separated reactor vessel pressure taps, are c#
di fferent basi: design, i.e., bellows vs. bourdon tube." On September 22, one.bourdo-tube instrument was replaced with a. bellows type to make the 'syste consistent with that described in the FSAR. '
A detailed rep:rt of this occurrence has been written and will'be-available for review by the Region 111 Compliance inspector during his next site visit.
Yours very truly, Roong R.O. Duncanson, dr., P.E.
Gen. Supt. of Power Plants Mechanical Chairman Monticelle Safety Audit Committee FDD/caf
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