ML20214E040

Reanalysis of Min Required Auxiliary Feedwater Flow Rate

ML20214E040
Person / Time
Site:	Davis Besse
Issue date:	05/25/1984
From:	Carlton J, Tally C BABCOCK & WILCOX CO.
To:
Shared Package
ML20214D825	List: ML20214D821 ML20214E040 ML20214E076
References
86-1152597, 86-1152597-00, TAC-63772, NUDOCS 8611240334
Download: ML20214E040 (17)

Text

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i REANALYSIS OF MINIMUM REQUIRED AUXILIkRY FEEDWATER FLOW

. (B&W MASTER SERVICES CONTRACT 582-7165)

APPLICABLE TO SACRAMENTO MJNICIPAL UTILITY DISTRICT's RANCHO SECO NUCLEAR GENERATING STATION .

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Prepared by:

Oate: difff*f Reviewed by: klti 00l>J_

O i Date: $}II. I4 i

Approved by:

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8611240334 961117 6 DR ADOCK 0500 i

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. 41w4w42: e4 86-1152597-00 o page 3 0 TABLE OF CONTENTS

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P I. INTRODUCTION AND PROBLEM BACKGP.00ND 4 II. ANALYTICAL METHODS AND ACCEPTANCE CRITERIA 4 III. " ANALYTICAL RESULTS 6 IV. CONCLUSIONS .

8 APPENDIX - TRENDS OF KEY PLANT PARAMETERS 9

. A.1 -

RCS Average Temperature vs. Titme (560 gpm AFW Flow with Anticipatory Trip)

A.2 - Pressurizerlevel vs. Time (560 gpm AFW Flow, with Anticipatory Trip) 8.1 -

RCS Average Temperature vs. Time (560 gpm AFWFlow,NoAnticipatoryTrip)

B.2 -

Pressurizer Level vs. Time (560 gpm AFW l Flow, No Anticipatory Trip)

C.1 -

RCSAverageTemperaturevs. Time.(500gpm AFWFlow,withAnticipatoryTrip) l C.2 -

Pressurizer Level vs. Time (500 gpm AFW

. Flow, with Anticipatory Trip) 0' - Adjustment of CADDS Results to Account for RC Pump Heat l

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!. Introduction and Problem Background This report sumarizes the results of B&W Task 407 - Re-Analysis of AFW Minimum Required Flow Rate for Rancho Seco. The de sire by the District to reduce the amount of time spent by the AFW pumps in the runout con-dition led to a proposed reduction in the minimum required AFW flow rate at Rancho Seco. B&W examined the effects of these modifications upon 'the plant prim'ary system following a loss of main feedwater (LOMFW) event.

II. Analytical Methods and Acceptance Criteria B&W utilized the CA005 computer code to detemine the sensitivity of the reactor coolant system to changes in key auxiliary feedwater system para-meters during a LOMFW transient. The specific parameters of interest were the time delay to initiate AFW flow and the flow rate itself. The CADDS code analyzes reactor transients in a heterogeneous pressurized water reactor. It solves the time-dependent neutron kinetics equations in conjunction.with a thermal-hydraulic solution for an average fuel pin which simulates the reactor core. The reactor model is coupled to a simulationofthereactorcoolantloop(hotleg,steamgenerator, cold leg, and reactor core) and the pressurizer.' A region-averaged model of each portion of the loop is utilized to determine the temperature response in the loop, which in turn contributes to the pressurizer model. In the particular version of CA005 used for this analysis, the heat transfer from the primary to the secondary system is detemined by inputting the steam generator heat demand to the secondary side as a function of time.

Key parameters utilized in CADDS for this ' analysis include the following (Table 1).

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TABLE 1 l

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SUMMARY

OF MAJOR CADDS INPUTS 1

Power Level (102%) 2827 MWt RC Pump Heat 16 MWt RC Flow Rate 387.710 gpm Initial Pressurizer Pressure 2,170 psia Initial Pressurizer Level 200 in. indicated /875 ft 3 liquid volume Doppler Cbeffic'ient -

-1.235 x 10-5 ak/k/F Moderator Coefficient 0 ak/k/F Code Safety Valve Satpoint 2,S40 psia Code Safety Valve Flow Rate 690,000 lbm/hr High RCS Pressure Trip Setpoint 2,400 psia TDAFWP Time to Full Flow (MDAFWP Failed) 70 sec. from receipt of initiation signal AFW Temperature 120*F MFW Flow CoastdownTime '(100-0% Flow) 5 sec.

High RCS Pressure Trip Delay 0.4 sec.

The acceptance criteria mutually agreed to by B&W and the District are as follows:

Required Criteria

- Maintain RC pressure at <110% of design pressure

- DNBR greater than minimum allowable

- Acceptable site boundary doses l - Do not go solid in the pressurizer l Desired Criteria

- Prevent SG dryout Do not open pressurizer safety valve No loss of subcooled margin 5

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It was originally intended to run six transient cases as specified by the District and report the results of each case herein. During the course of the evaluation, however, it was observed that a particular characteristic of CADDS impacted the results in the non-conservative direction. This characteristic is the lack of ability to input a constant value of RC' pump power versus time after reactor trip. Since RC pump power respresents a considerable fraction of the total heat that must be removed by. auxiliary feedwater following reactor trip, th'e CA005 results were corrected to account for this phenomenon using Y smsll program called RCPPOWER. Rather than making this correction to all thi CADDS cases, .it was decided that the 5

best course would be to determine th4 minimum acceptable AFW flow rate for two situations: with and without anticipatory reactor trip. The attached figures of reactor coolant system average temperature and pressurizer level (measured from the bottom of the pressurizer) are corrected to account for the additional RC pump power that must be removed (16 MW). Note that the peaks of the corrected Tave and Lpzr do not occur at the same time. This i.

due to RCPPOWER predicting a Ptr level change that is approximately twice the change predicted by CADDS for a given change in Tave. Since RCpP0WER

-cale:.tates increases in Pzr level due to the added 16 MW of pump heat, this is conservative for this analysisy The cause of the phase shift introduced by.RCPPOWER is discussed in Appendix 0.

For situations with and without anticipatory reactor trip, 560 gpm is the minimum AFW flow rate that will satisfy the required acceptance criteria.

Note'that 500 gpM wi,11 not satisfy the requirements since the pressurizer I

will go solid. The results reflect a 20 percent conservatism in the decay heat niodel (ANS5.1). Although an AFW flow rate of 560 gpm will avoid '

l filling'the pressurizer, level indication will be lost. (Levelindication is lost at 34.59 ft. as measured from the pressurizer bottom.)

The sequence of events for the transient is as follows:- ,

6

m.,.

.. I Page 7 Without Anticipatory Trip With Anticipatory Trip Event Time Event Time Steady State 0-8 sec. Steady State 0-8 sec.

LOMFW 8 sec. LOMFW 8 sec.

FW Flow Reaches Zero 13 sec. FW Flow Reaches Zero 13 sec.

Reactor Trip on High 17.45 see Anticipatory Reactor 13 sec.

Pressure -

Trip Without Anticipatsry Trip With Anticipatory Trip Event Time . Event Time AFW Initiation Signal 17.45 sec.' AFW Initiation Signal 13.5 sec.

Full AFW Flow to OTSGs 87.45 sec. Full AFW Flow to OTSGs 83.5 sec.

AFW Heat Removal Matches 550 sec. AFW Heat Removal 545 sec.

Heat Input Matches Heat input The results of the analyses w'ith respect to the acceptance criteria are as follo'ws (AFW flow rata = 560 gpm):

Required Criteria:

- liaintain RC pressure at <110% of design pressure satisfied

- DN8R greater than minimtsn allowable Satisfied

- Acceptable site boundary doses Satisfied

- Do not go solid in the pressurizer Satisfied Desired Criteria:

- Prevent SG dryout Not Satisfied *

- Do not open pressurizer safety valve Not satisfied *

- No' loss of subcooled margin Satisfied l

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• Cannot' be satisfied for an AFW initiation delay of 70 seconds.

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Page 8 IV. Conclusions Based upon the methods, inputs, and assumptions utilized, an AFW flow rate of 560 gpm will satisfy all required acceptance criteria following

. a loss of main feedwater transient for situations with and without anticipatory reactor trip.

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9 APPENDIX TREN05 0F XEY PLANT PARAMETERS e

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• -- ...-n. vu jy Page 16 APPENDIX D Adjustment to CADDS results to account for RC Pump Heat h

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When the CADDS output data was corrected to account for the heat added to the RCS  !

by the RC pumps, a phase shift was introduced. This phase shift causes the peaks l

of the corrected RCS Tave curve and the corrected PZR level curve to occur at different times, even .though the CADDS curves had no such phase shift. The root cause of this phase shift is due to an overprediction (when compared to CADDS) of I

'the response of PZR level to a given change in RCS Tave by RCP0WER. The equatior..

used by RC9 POWER have been checked and the results have been verified with hand calculations. The discrepancy is due to the simplified model used by RCPPOWER.

In any case, the overprediction is conservative in this analysis.

The cause of the phase shift can be illustrated by examination of the 800-900 second period of the 500 gpm case. The pertinent CADDS data is given in the following table:

Time Tave Lpzr 800 591.3 28.81 900 588.7 28.22 The CADOS temperature response was corrected by first interpolating in a proper-ties table for the uncorrected enthalpy and specific volume. then .using the speci-fic volume to calculate the mass of water in the RCS (excluding the pressurizer).

l This mass, the enthalpy, and the time since reactor trip were then used to calcu-late a corrected enthalpy, and thereby a corrected temperature. The correction applied at 800 seconds caused an increase in Tave of 19.2 F, and at 900 seconds v increase o"f 21.65 F. The pump power therefore causes the RCS Tave to increase 2.45 F in 100 seconds. The CADDS output, which accounts for everything but the pump power, predicts a decrease of 2.6 F in Tave from 800 to 900 seconds. The net change in Tave, with' all effects considered, is therefore 2.45 - 2.6 = -0.15 F.

This is reflected in the output from RCPPOWER, which is given below:

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Tir.e Tave Lpzr 800 6i0.5 41.13 900 610.35 42.20 1

The CADDS PZR level response was corrected by suming the calculated changes for each time interval. In any 100 second interval, the correction to PZR level is approximately 1.66 ft (it varies slightly due to variations in the partial deriva-tivedv'Nh). The calculation assumed a constant pressurizer temperatur out the event.

The PZR level correction is practically constant since energy is being added to the RCS at a constant rate. Thle CADDS output predicts a drop of 0.59 ft in PZP. level frem 800 to 900 seconds. Adding the 1.66 ft rise due to the pomp power, the net. change in PZR level is +1.07 ft. The results of.the 1

, corrections to PZR level are shown above. 1 The different response predicted by the simplified " bulb-and-tube" model used in t r 1 RCPPOWER is ' summarized in the following table:

Source Chance in Tave 4

Chance in Lorr Tave/ Lorr l CA005 -2.6 F -0.59 ft 4.407 F/ft RCPP0WER +2.45 +1.66 1.476 Sum of above -0.15 +1.07 --

This table shows how RCPPOWER predicts that it' requires much less change in tem-perature to produce a one foot change in PZR level. The fact that RCPPOWER gives a rise of 1,66 ft fcr a 2.45 F temperature change, and that CA005 give a drop of only 0.59 ft for approximately the same temperature change is the source of the l phase shift. in the corrected curves.

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Docket No. 50-346 License No. NPF-3 Serial No. 1322 November 17, 1986 Attachment 2 AUXILIARY FEEDWATER CALCULATION

SUMMARY

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