United States Nuclear Regulatory Commission - Protecting People and the Environment


ACCESSION #: 9801130266



                                   ABB



                                        January 6, 1998

                                        LD-98-001



Document Control Desk

U.S.  Nuclear Regulatory Commission

Washington, DC 20555



Subject:  Infobulletin 97-08, "Information Update on the Use of the

          DELTSTRAT Code for RCS Flow Rate Determination,"



Gentlemen:



As described more fully below, ABB Combustion Engineering (ABB-CE) has

issued the subject Infobulletin (attached) to its utility customers, and

has recommended that each utility using the DELTSTRAT code evaluate the

matters addressed in the Infobulletin for reportability under 10 CFR 21.

This report is provided to NRC for informational purposes.



The DELTSTRAT code is used at several ABB-CE plants (St. Lucie Units 1 &

2, Waterford Unit 3, San Onofre Nuclear Generating Station Units 2 & 3,

and Palisades) to determine RCS flow rate from data taken with a

calorimetric flow measurement technique.  The code corrects for

temperature stratification effects in the RCS hot legs.  A hot leg

temperature correction is calculated by the code, and is then applied to

the measured hot leg RTD temperatures to determine hot leg bulk coolant

temperature.  The hot leg bulk coolant temperature, along with other

inputs, is then used to calculate the RCS flow rate.



In several instances, the code overestimated the hot leg temperature

correction and produced a calculated flow rate which was substantially

larger than expected.  A review of test data and the underlying

assumptions for the DELTSTRAT code indicated that improvements in the

treatment of certain code inputs could be made which would produce a

calculated flow rate that would be closer to the expected value.

Infobulletin 97-08 discusses ABB-CE's findings and provides

recommendations to DELTSTRAT users for assuring that the code-calculated

RCS flow rates are reasonable and accurate.



              ABB Combustion Engineering Nuclear Operations



Combustion Engineering,  P.O. Box 500            Telephone (203) 688-1911

Inc.                     1000 Prospect Hill Road   Fax (203) 285-9512

                         Windsor, Connecticut

                          06095-0500



                                                                LD-98-001

                                                                   Page 2



ABB-CE has evaluated the matters in the infobulletin with respect to 10

CFR 21 reportability, and has concluded that insufficient information is

available about how individual utilities use the DELTSTRAT code to

determine whether the issue is reportable under 10 CFR 21.  Therefore, in

addition to the recommendation in the Infobulletin, ABB-CE has

recommended that each utility using DELTSTRAT evaluate whether the issues

addressed in Infobulletin are reportable under 10 CFR 21 in light of how

the individual utility uses the code.



Please do not hesitate to call me or George Hess at (860) 285-8405 if

there are questions concerning this matter.



                                        Very truly yours,

                                        COMBUSTION ENGINEERING, Inc.



                                        Ian C. Richard, Director

                                        Operations Licensing



Attachment: As stated



xc: S. Magruder (NRC)



ABB



Combustion Engineering Infobulletin                             No. 97-08

                                                            Dec. 22, 1997



Information Update on the Use of the DELTSTRAT Code for RCS Flow Rate

Determination



Summary: The DELTSTRAT code is used at several ABB-CE plants (St.  Lucie

1&2, Waterford 3, SONGS 28,3, and Palisades) to determine RCS flow rate

from data taken with a calorimetric flow measurement technique.  The code

corrects for temperature stratification effects in the RCS hot legs.  A

hot leg temperature correction is calculated by the code, and is then

applied to the measured hot leg RTD temperatures in order to determine

hot leg bulk coolant temperature.  The hot leg bulk coolant temperature,

along with other inputs, is then used to calculate the RCS flow rate.



In several applications, for Cycles 9 through 11 at Calvert Cliffs 2, the

code overestimated the hot leg temperature correction and produced a

calculated flow rate which was substantially larger than expected.  A

similar application for Calvert Cliffs Unit 1, Cycle 12 produced an RCS

calculated flow rate which appeared to be more reasonable.  The

overestimation of the RCS flow rate for Unit 2 appears to be related to

several factors: 1) unusual temperature trends in some of the hot leg RTD

measured temperatures, 2) the impact of a low leakage fuel management

scheme with a very peaked core radial power distribution, and 3)

isothermal biases for the RTD temperatures.



A review of the Calvert Cliffs test data and the underlying assumptions

for the DELTSTRAT code indicated that improvements in the treatment of

certain code inputs could be made which would produce a calculated flow

rate for Unit 2 that would be closer to the expected value.



This bulletin discusses the findings of the Calvert Cliffs review and

provides recommendations to the users for assuring that the DELTSTRAT

code calculated RCS flow rates are reasonable and accurate.  A listing of

all assumptions inherent in the DELTSTRAT code is included here as

Attachment #1.



Discussion: There are several assumptions inherent in the formulation of

the equations and input for the DELTSTRAT code.  One of the assumptions

is that the flow mixing factors, which relate how the flows exiting from

the core mix with adjoining flows as they proceed into the outlet nozzles

and down the hot legs, are invariant with time, power distributions, and

relative position of the reactor vessel internals.  In the case of

Calvert Cliffs 2, for Cycles 9 through 11, the trend of measured hot leg

temperatures for channels A and C was unusual.  The hot leg temperatures

for channels A and C indicated a constant core AT for these cycles even

though the expected trend, due to the low leakage fuel management, was an

increasing core AT.  Channel B and D hot leg temperatures did show the

expected increasing trends with each cycle.



A second assumption used is that the normalized core radial power

distribution is an accurate proxy for describing the normalized fuel

assembly coolant temperature rise distribution (that is: the ratio of

individual assembly to core average coolant temperature rise).  This

assumption works well for the traditional out-in fuel management schemes

used in the earlier fuel cycles for the ABB-CE plants.  However, with the

more non-uniform radial power distributions inherent in some of today's

low leakage fuel management schemes, a better approach for defining the

fuel assembly coolant temperature rise is to use an open-core thermal-

hydraulic code, such as the ABB-CE TORC code, along with the actual core

radial power distribution.  The open-core code will determine the open-

core effects of mass, momentum, and energy transfers between adjoining

fuel assemblies on the coolant temperature rise in each fuel assembly.

The assembly coolant temperature rise data can be normalized in the form

of assembly to core average temperature rise factors.  The normalized

assembly coolant temperature rise factors can then be used as input to

the DELTSTRAT code (in place of the core radial power distribution).



_________________________

The information contained in this Infobulletin is provided by ABB-CE as a

service to your organization.  Since operation of your plant is

completely within your control and responsibility, and involves many

factors not within ABB-CE's knowledge, this information may be utilized

only with the understanding that ABB-CE makes no warranties or

representations, express or implied, including warranties of fitness for

a particular purpose or merchantability, with respect to the accuracy,

completeness or usefulness of the information contained.  ABB-CE

disclaims, and you assume, all liability, in negligence or otherwise, as

a result of your use of this information.



ABB-CE Infobulletin                                             No. 97-08

Page 2 of 4                                                 Dec. 22, 1997



A third assumption relating to the code inputs involves the measured cold

and hot leg coolant temperatures.  The measured coolant temperatures are

assumed to have any inherent isothermal biases eliminated before being

input to the DELTSTRAT code.  Removing biases will provide a more

accurate set of input to the code and a more consistent picture of the

temperature distribution in the RCS piping.



In the Calvert Cliffs Unit 2 situation, it was found that if the Channel

A and C temperature data were eliminated, and if open-core exit coolant

temperatures were used as input in place of the traditional core radial

power factors, the resulting calculated RCS flow rate would be much

closer to what was expected.  The reasoning for eliminating the Channel A

and C data is as follows:



     1.   The DELTSTRAT code does not have the capability for predicting

          unusual hot leg temperature trends which may be due to special,

          localized effects as seen in the case of Calvert Cliffs Unit 2.

     2.   It is suspected that the measured hot leg temperature trend at

          Calvert Cliffs Unit 2 is real, but local in nature and may not

          be representative of the overall temperature distribution in

          the hot leg.

     3.   By using only Channels B and D hot leg temperatures, a more

          traditional temperature distribution was input to DELTSTRAT and

          a lower calculated RCS flow rate was calculated.



Recommendations:



ABB-CE recommends that the users initiate the following actions in order

to assess if there are any current operability problems in determining

RCS flow rate in their plant(s) when using the DELTSTRAT code with

currently used procedures:



     1.   Review and confirm that the long-term trends (starting from BOC

          1) in measured RCS flow rates, as determined with DELTSTRAT,

          show consistency with any changes in plant hardware on the

          primary side (i.e., with added SG tube plugging, RCP change-

          outs, etc.), within the accuracy of the measurement method.



     2.   Compare current measured RCS flow rates with beginning-of-cycle

          1 (BOC 1) values.  If today's values are larger than the BOC 1

          value by an amount which is greater than the measurement

          uncertainty, without any corresponding justification from

          hardware changes to the primary system, then consideration

          should be given to reducing the RCS flow rate to the BOC 1

          value.  In the case of the digital plants, the flow rate in the

          Core Operating Limit Supervisory System (COLSS) and Core

          Protection Calculators (CPC's) would be reduced.  For the

          analog plants, the flow rate measured at the beginning of the

          current cycle would be reduced.



If recommendation 1 or 2 fails, then proceed with recommendation 3 or 4.



     3.   Substitute normalized fuel assembly coolant temperature rise

          fators, as determined by an open-core thermal-hydraulic

          computer code, in place of a core radial power distribution

          when preparing input to the DELSTRAT code, or else:



     4.   Determine a systematic flow bias that will be applied to the

          flow rate calculated by DELSTRAT with core radial power

          distribution as input.  The flow bias will compensate for the

          fact that the fuel assembly coolant temperature rise factors

          are not being used as input.



_________________________

The information contained in this Infobulletin is provided by ABB-CE as a

service to your organization.  Since operation of your plant is

completely within your control and responsibility, and involves many

factors not within ABB-CE's knowledge, this information may be utilized

only with the understanding that ABB-CE makes no warranties or

representations, express or implied, including warranties of fitness for

a particular purpose or merchantability, with respect to the accuracy,

completeness or usefulness of the information contained.  ABB-CE

disclaims, and you assume, all liability, in negligence or otherwise, as

a result of your use of this information.



ABB-CE Infobulletin                                             No. 97-08

Page 3 of 4                                                 Dec. 22, 1997



It is recommended that for future cycles isothermal RTD temperature

biases be measured at the start of each new cycle for use in correcting

measured RTD temperatures prior to being input to DELTSTRAT.  This step

is intended to improve accuracy, but is not required to support current

operability of the plants.



Applicability: All plants for which the ABB-CE DELTSTRAT code is used to

determine RCS flow rate.



Technical Contacts: R.  P.  Letendre (860) 285-2878

                    P.  F.  Joffre (860) 687-8062



                Attachment #1 to Infobulletin No.  97-08



Assumptions used in the DELSTRAT Methodology



1.   Eight hot leg and eight cold leg RTD's (from the safety channels)

     provide input temperature values.



2.   The average of the cold leg RTD readings in each RCS loop is equal

     to the bulk cold leg temperature in that loop.



3.   Hot leg and cold leg isothermal temperature biases are determined

     and applied to adjust the measured RTD temperatures (individually or

     collectively).



     This is an assumption which the Infobulletin states is important in

     obtaining unbiased temperatures for input to DELTSTRAT.



4.   The flow mixing data from the ABB-CE Palisades flow model tests

     apply to all ABB CE 2-loop reactor designs.  The flow mixing factors

     are invariant with time, core radial power distribution, coolant

     temperatures, and any unusual localized flow conditions in the hot

     legs.



     This is an assumption which appears to be challenged by the Channel

     A and C hot leg temperature data trends at Calvert Cliffs Unit 2

     during Cycles 9 to 11.



5.   The core radial power distribution is a representative proxy for the

     normalized distribution of fuel assembly coolant temperature rise

     factors.



     This is an assumption which the Infobulletin states can be improved

     upon by using a calculated normalized radial distribution of fuel

     assembly coolant temperature rise factors as input to DELTSTRAT in

     place of the radial power distribution.



6.   Flow mixing in the reactor vessel upper plenum between the two loops

     is negligible.



_________________________

The information contained in this Infobulletin is provided by ABB-CE as a

service to your organization.  Since operation of your plant is

completely within your control and responsibility, and involves many

factors not within ABB-CE's knowledge, this information may be utilized

only with the understanding that ABB-CE makes no warranties or

representations, express or implied, including warranties of fitness for

a particular purpose or merchantability, with respect to the accuracy,

completeness or usefulness of the information contained.  ABB-CE

disclaims, and you assume, all liability, in negligence or otherwise, as

a result of your use of this information.



ABB-CE Infobulletin                                             No. 97-08

Page 4 of 4                                                 Dec. 22, 1997



7.   Complete mixing of the hot leg coolant temperatures occurs at a

     distance of 18 hot leg pipe diameters downstream of the reactor

     vessel outlet nozzle exit.



8.   The circumferential distribution of hot leg coolant temperature can

     be described by a cosine variation.



9.   The flow in the hot legs may rotate in a solid-body manner as it

     passes down the length of the hot legs.



10.  The velocity distribution in the cross-section of the hot legs for

     Plant X is the same as existed in the Palisades reactor flow model

     tests, from which the flow mixing factors were determined.



List of recently-issued Infobulletins:



Number         Date                Title

97-07, Rev 01  12/31/97  Tech Spec on Azimuthal Tilt in Analog Plants

97-07          10/28/97  Tech Spec on Azimuthal Tilt in Analog Plants

97-06          10/21/97  Core Snubbers / Blocks

97-05          10/21/97  Core Ledge Loads

97-04          7/11/97   Potential Error in the Energy Redistribution

                         Factor Used in LOCA Analysis

97-03          6/5/97    Use of Non-Safety (non-Q) Components in Reactor

                         Protection System

97-02          5/23/97   Spurious Recirculation Actuation Signal

97-01, Supl 1  4/9/97    CESEC Decay Heat Model

97-01          4/3/97    CESEC Decay Heat Model

96-02          4/24/96   Steam Generator Sleeve-to-Tube Weld Indications



_________________________

The information contained in this Infobulletin is provided by ABB-CE as a

service to your organization.  Since operation of your plant is

completely within your control and responsibility, and involves many

factors not within ABB-CE's knowledge, this information may be utilized

only with the understanding that ABB-CE makes no warranties or

representations, express or implied, including warranties of fitness for

a particular purpose or merchantability, with respect to the accuracy,

completeness or usefulness of the information contained.  ABB-CE

disclaims, and you assume, all liability, in negligence or otherwise, as

a result of your use of this information.



*** END OF DOCUMENT ***





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