IR 05000295/1986008

Forwards Safety Evaluation of Util Use of Negative Flux Rate Trip of -15% Power in 5 Vs -5% Power in 2 S,Per LER 50-295/86-008.Task Initiation Action 86-25HT Closed

ML20212K962
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	08/18/1986
From:	Novak T Office of Nuclear Reactor Regulation
To:	Paperiello C NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
NUDOCS 8608250148
Download: ML20212K962 (3)

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. J August 18, 1985 DISTRIBUTION MEMORANDUM FOR: Carl J. Paperiello, Director 7~ Docket File f Division of Reactor Safety "' NRC'PDR~ '

Region III Local PDR PD#3 Rd FROM: Thomas M. Novak, Acting Director C. Rossi Division of PWR Licensing-A J. Norris C. Vogan SUBJECT: REQUEST FOR ASSISTANCE - ZION NEGATIVE FLUX Tech Branch RATE TRIP (AITS F03020686) (TIA No. 86-25HT)

By memorandum dated May 6, 1986, Region III requested technical assistance to determine the safety significance of Zion's use of a negative flux rate trip of-15% power in five seconds versus the setpoint established by the dropped rods safety analysis of -5% power in two seccads. The issue relates to LER 295/86-008. The results of our analysis are contained in the enclosed Safety Evaluatio This memorandum closes AITS F03U20686 (TIA No. 86-25HT).

Original signed by:

Thomas M. Novak, Acting Director Division of PWR Licensing-A Enclosure:

As stated f /

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SAFETY EVALUATION OF OPERATION WITH A NONCONSERVATIVE NEGATIVE FLUX RATE TRIP SETPOINT

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AITS F03020686

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Introduction By letter dated May 6,1986 to H. L. Thompson from C. J. Paperiello Region III requested assistance on the Zion Negative Flux Rate Trip Issue (AITS F03020686). Both Zion Units operated with the Negative Flux Rate Trip (NFRT)

set nonconservatively, at a value of -15% power in 5 seconds rather than the correct value of -5% power in 2 seconds, from November 1979 to February 1986.

This is delineated in LER 295/86-008 which was furnished with the referenced letter. Because of the complicated nature of the rod drop issue, we will d'scuss some of the ramifications of the event, as well as the central issue r the safety significance of the nonconservative NFRT setpoin Evaluation As a result of a meeting between Westinghouse and NRC on November 19, 1979, an interim solution was adopted to resolve deficiencies in the safety analysis of the rod drop transient. The interim solution required that manual control be used whenever rods were withdrawn less than 216 steps with the reactor power greater than 90% and that the NFRT be set for a -5% in 2 seconds rate tri The requirement for the trip was not made clear as a result of the meeting, although recent discussions with Westinghouse indicate the trip requirement was always in effec There were two concerns with the rod drop accident

The first is a return to i power if the reactor is in automatic control with a dropped rod. This can

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result in the Departure from Nucleate Boiling Ratio (DNBR) falling below the design limits. The scenario requires a failure in the nuclear instrumentation system auctioneering module such that the channel which monitors the quadrant of reduced flux is feeding the automatic rod controller rather than the highest signal. The interim methodology prevented this. The second concern is if the worth of the dropped control rod or rods is high. In that case, if the reactor does not trip there may be a DNBR proble The interim solution for rod drep acciderts was replaced May 13, 1983 when we accepted a new rod drop methodology presented by Westinghouse. The negative flux rate trip requirements remained the same. Since Zion Units 1 and 2 had adopted use of the interim solution in November 1979 and replaced it with the new methodology for Unit 2 on May 30, 1983, and for all subsequent reloads on both units, the reactors have been protected from the return to power concern for rod drop _ _ _ _ _ _ _ _ _ _ _ _ _ _

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The problem area lies with the concern for control rod drops with high wort The negative rate trips at Zion 1 & 2 were set nonconservatively from the time a concern with the rod drop analysis was identified in November 1979 until the nonconservative calibration was discovered and corrected in February 1986. The-15% in 5 second NFRT, according to LER 295/86-088, assures a reactor trip for

.uJ J...r. with a worth greater than approximately 800 pcm. The -5% in 2 seconds NFRT assumes a trip occurs for all rods with worths greater than 400 pcm. Therefore if a drop of control rods worth more than 400 pcm and less than 800 pcm had occurred in either Zion Unit during the period November 1979 to February 1986 the results would have tad unanalyzed consequences. We cannot define the extent of what would occur under these unanalyzed conditions, but must assume that a departure from nucleate boiling would occur, with some enne.nnent fuel failure. The rod drop event is considired an anticipated transient, and fuel failure is an unacceptabe consequence for such events.