ML19274E693

LER 79-007/01T-0 on 790328:non-conservatism Noted in rod-drop Analysis.Corrective Actions Undetermined;Followup LER Will Be Issued

ML19274E693
Person / Time
Site:	Prairie Island
Issue date:	04/12/1979
From:	Hunstad A NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML19274E688	List: ML19274E687 ML19274E693
References
LER-79-007-01T, LER-79-7-1T, NUDOCS 7904170183
Download: ML19274E693 (2)

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o Attachment to:

P-RO-79-7 We have been informed by Westinghouse of a potential non-conservatism in the rod-drop analysis as reported in the FSAR. The previous analysis may not have preperly accounted for t he effect on automatic rod control of indicated core power level. Specifically, the rod control system responds to an inpulse difference between core average power level and turbine setting. If a control rod drops near an excore detector, then the core power level indicated by that detector may be incorrectly low. The resultant average core power signal, which is input to rod control, might also be low. Averaging of all excore detector signals would minimize this efftet at Prairie Island but could still result in a non-conservatively large core power - turbine power mismatch signal to the rod control system. This would cause more outward trotion of control rods than assumed by the Westinghouse analysis.

Our preliminary analynis of this potential non-conservatism agrees with the Westinghouse conclusion that existing safety margins and other conservative featorch of plant dcsign and operation provide high assurance that the single rod drop event does not violate the accepted limiting DNB ratio. At Prairie Island specifically:

1. Sinnie dropped rods have always caused negative rate reactor trips.

2. Prairie Island uses a conservative rate reactor trip setting of less than 1 5% in 2 seconds vs a Technical Specification required setpoint of less than i 15% in 2 seconds.

3. After the analysis reported in the FSAR, Prairie Island began operating with constant axial offset control, which limits core axial offset. As o consequence, control rods are almost fully withdrawn from the core during full power operation. Thus, the amount of reactivity that can be added due to control rod withdrawal is minimal (less than 50 pcm),

regardless of the automatic rod control signal.

Corrective action is undetermined. A followup LER will be issued.