ML20008D664

Fluctuation Events of 781104 & 781212, Evaluation & Resultant Revisions to Normal & Test Operating Procedures. Describes Event Resulting from RT-500 Test, Compares All Fluctuation Events & Provides Revised RT-500 Procedure

ML20008D664
Person / Time
Site:	Fort Saint Vrain
Issue date:	01/29/1979
From:	PUBLIC SERVICE CO. OF COLORADO
To:
Shared Package
ML20008D663	List: ML20008D662 ML20008D664
References
NUDOCS 7902010088
Download: ML20008D664 (180)

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W FLUCTUATION EVENTS OF_

NOVEPeER 4 AND DECEMBER 12, 1978

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EVALUATION AND RESULTANT REVISIONS TO NOR"AL AND TEST OPERATING PROCEDURES J

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TABLE OF CONTENTS 1.0 INTRC00 Cit 0N 2.0 SUtPARY AND CONCLUSIONS 3.0 DISCUSSICN

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3.1 November 4,1978 Fluctuation 3.1.1 Sequence of Events 3.1.2 Data Package 3.1. 3 Gas Outlet Temcerature Trends 3.1.4 Evaluation of Temperature Fluctuations 3.1.5 Effect of Region Flow Redistribution on Region Temperatures 3.1. 6 Core Status After November 4 3.2 December 12, 1978 Fluctuation 3.2.1 Sequence of Events and Data Package 3.2.2 New Observations 3.3 Comparison of Fluctuation Events

3. 3.1 Core AP and Resistance 3.3.2 Fluctuation Amplitudes 3.4 Fluctuation Threshold Line and Other Constraints 3.5 Revised RT-500 Test Procedures 3.5.1 Bases for Change to Main Steam Temperature Test Limit 3.5.2 Summary of Monitoring Capabilities Curing Testing 3.5.3 Revised RT-500 (Revision C)

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1.0 INTRODUCTION

The fluctuation event of November 4,1978, which occurred during testing under the approved RT-500 test, exhibited larger amplitudes in temperature fluctuations than had been exhibited in any previous fluctuation event. The localized, small amplitude fluctuation event of Cecember 12, 1978, occurred at a value of AP less than the established threshold value. The purpose of RT-500 testing, which had been suspended due to exceeding a test limit on November 4, is to systematically establish the threshold line.

This report, in Sections 3.1 and 3.2, provides a description of the se-quence of events, a data padk'~ge a and an evaluation including any consequences to plant safety for th.e November 4 and Cecember 12, 1978 fluctuation events, respectively. Section 3.3 provides a comparison of all the fluctuation events regarding operating conditions at the start and stop of the fluctuation and amplitudes of nuclear channel and thermocouple signal changes which took place during the fluctuation. Each event is inoividually identified permitting an assessment of significant changes which may have taken placa with time.

Section 3.4 discusses the fluctuation threshold line and other operating con-straints which can limit plant operation. Section 3.5 is an evaluation of the test limits and monitoring capability previously contained in RT-500 and pro-vides a revised RT-500 as a result of the evaluation.

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f 2.0 SUP19RY MD CONCLUSICNS Since the first occurrence of fluctuations at Fort St. Vrain on October 31, 1977, there have been a total of 30 fluctuation events identified through the last event which occurred en December 12, 1978. A listing of the 30 fluctuation events is provided in Section 3.3.

The latter events, which occurred on November 4,1978, and December 12, 1978, exhibited unique characteristics different than those observed in events. The second event on November 4 resulted in the largest temperature swings for region outlets, steam generator module helium inlet, and main steam outlet observed for any fluctuation event. Analysis of the event has resulted ,

in the conclusion'that the measured temperature swings were real as opposed to changes in region outlet temperatures caused by colder bypass ficw leaking To account for into the graphite tube containing the measuring thermocouple.

the magnitude of the region outlet temperature swings observed, scme ficw One mechanism, which is redistributien into and out of regions must occur.

consistent with the observed data, is ficw inleakage into the plenum element or near the top of the region. This possibility for flow redistribution has been recognized for some time. In prior fluctuation events, however, the observed changes in steam generator module helium inlet temperatures were generally consistent witn displacements of relative cold bypass flows as a result of internal core component movements.

In all probability, some amount of flow redistribution was also occurring althcugh to a lesser degree than for the Evaluations of the flow redistribution theory indicate November 4 event.

there are no detrimental safety consequences nor do they comprcmise or in- .

validate the operating ifmits speciffed in the Technical Specifications.

The December 12 event is unique in that the fluctuation was locali:ed to the northwest quadrant of the core and that it occurred at a lower core aP and core resistance than previously experienced at the 67% thermal power le Prior to the Decemoer 12 event, localized fluctuations had only been observed 4

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at thermal pcwer levels less than 40% of rated. -This fluctuation event most probably represents the fluctuation threshold limit in that-it was locali:ed and was terminated with only a 2% reduction in pcwer. The fluctuation thres-hold limit line for normal pcwer production was reduced as a result of the December 12 event. There are no safety consequences as a result of the event.

An evaluation of fluctuation amplitudes in neutron flux and helium and steam temperatures for the 30 fluctuation events did not reveal any apparent chr6aological trend. The evaluation did indicate the need to revise a limit constituting a reportable incident to the NRC (with further testing suspended until again authori:ed by the Ccemission). Specifically, the medule cutlet main steam temperature limit is revised from 60*F peak to peak to 150*F (Section 3.5). The 60*F con'siraint is retained as an operational ifmit for fluctuation testing. Additionally, the instrumentaticn has been upgraded to provide improved on-line monitoring for both operating and test personnel dur-ing fluctuation testing.

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3.0 DISCUSSION 3.1 November 4. 1978 Fluctuation 3.1.1 Seouence of Events Testing on November 1-3, 1978, succeeded in reproducing the mini-fluctua-tion observed on October 6 at low power and high core resistance.

Near midnight on November 3, the core resistance parameter was reduced

- from a value of 105 to a value of 64 and the fluctuation threshold mapping l portion of the test program was initiated. Power was raised from 38: to 42 with an overshcot to roughly 49 at approximately 0100 on [

, November 4. No fluctuations were observed on region outlet temperatures. Sub-4 sequent review of m system' dita revealed that some slight offsets in nuclear flux channels and fluctuation in gap thermocouple temperatures occurred, but 1:mnediately damped out. A second power increase was made from 42 to 47: power with an overshoot to 54 at 0340. During the power rise, offsets were I observed on nuclear flux channels at 0344 and at 0345 the helium inlet tempera-

ture to steam generator module B-1-6 departed from the expected value indicating -

the start of temperature fluctuations.

l Pewer reached the 47 level at about 0400 and fluctuations were of the l character previously observed.

J At 0413, gap temperature thermocouples and nuclear channel III start new trends, and module 3-2-2 starts a 60 F increase in steam temperature. At 0415

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nuclear channel III drops about 3%, module B-1-4 starts a 50*F increase in steam l temperature, and module 3-1-1 starts a 120*7 decrease in steam camperature.

1 At 0426, a 22 drop in channel IV is followed by the start of a 70*F drop ta modula 3-1-6 steam temperature. Pcwer reduction was initiated at 042S and

. . 'ched 42 at 0432. Fluctuations continued at a lower amplitude. Power was i

th. reduced to 33% from 0445 to 0450.

'y 0525, the fluctuations had apparently stopped. Power remained at the 33" level until 0620, when a return to 40* power was attempted. Fluctuations were again observed at about 0635. They were observed to be of a very regular and medium amplitude, i.e., roughly 20 F a=plitude character as has been previously observed at the 40+: power level. The regulating red was put into =anual control (not moved) from 0715 to 0745, and the fluctuations centinued.

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At 0750, power was reduced to 35% and the fluctuations stopped at 0755. '

Subsequent review of the Di system data indicated roughly 5-10 changes in gap thermocouple temperature prior to the fluctuation at 0630, suggesting that the affects of the previous fluctuatica =ay not have completely damped out. The fluctuation at 0630 occurred at a core .i? and resistance value which was lower than the previously established threshold line and it was therefore presumed that it was a continuation of the previous event.

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d 3.1.2 Data packace This section contains a summary of significant data frem the November 4 fluctuation events. Data resented include 1ccations of initiation points of the threshold curves and indications of what portiens of the core were involved. pCRV displacement probe data, nuclear channel data, steam generator helium and steam temperature data, core region cutlet temperature data, gap  ;

thermocouple temperature data, and average plant parameter data are also presented.

Gap thermocouples 3-6, 7-10, and 15-18 are not located in gaps, but rather are under the core support blocks for regions 37, 36 and 34, respec-tively, for this series of tests.

This data package was 'pFeviously presented and discussed with the NRC in meetings in Bethesda on November 16 and December 14, 1978. Significant portions of the data package pertinent to the evaluation of the event are discussed in other sections of this report.

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